Terminology
Absolute Brightness
(Absolute Magnitude)
A measure of the true
brightness of an object. The absolute brightness or magnitude of an object
is the apparent brightness or magnitude it would have if it were located
exactly 32.6 light-years (10 parsecs) away. For example, the apparent
brightness of our Sun is much greater than that of the star Rigel in the
constellation Orion because it is so close to us. However, if both objects
were placed at the same distance from us, Rigel would appear much brighter
than our Sun because its absolute brightness is much larger.
Absolute Zero
The coldest possible
temperature, at which all molecular motion stops. On the Kelvin temperature
scale, this temperature is the zero point (0 K), which is equivalent to
–273° C and –460° F.
Absorption
The process by which light
transfers its energy to matter. For example, a gas cloud can absorb
starlight that passes through it. After the starlight passes through the
cloud, dark lines called absorption lines appear in the star’s continuous
spectrum at wavelengths corresponding to the light-absorbing elements.
Absorption Line
A dark line in a continuous
spectrum caused by absorption of light. Each chemical element emits and
absorbs radiated energy at specific wavelengths, making it possible to
identify the elements present in the atmosphere of a star or other celestial
body by analyzing which absorption lines are present.
Accelerating Universe
A model for the universe in
which a repulsive force counteracts the attractive force of gravity, driving
all the matter in the universe apart at speeds that increase with time.
Recent observations of distant supernova explosions suggest that we may live
in an accelerating universe.
Accretion Disk
A relatively flat, rapidly
rotating disk of gas surrounding a black hole, a newborn star, or any
massive object that attracts and swallows matter. Accretion disks around
stars are expected to contain dust particles and may show evidence of active
planet formation. Beta Pictoris is an example of a star known to have an
accretion disk.
Active Galactic Nucleus
(AGN)
A very bright, compact
region found at the center of certain galaxies. The brightness of an active
galactic nucleus is thought to come from an accretion disk around a
supermassive black hole. The black hole devours matter from the accretion
disk, and this infall of matter provides the firepower for quasars, the most
luminous type of active galactic nucleus.
Active Galaxy
A galaxy possessing an
active galactic nucleus at its center.
Advanced Camera For Surveys
(ACS)
An optical camera aboard
the Hubble Space Telescope that covers twice the area, has twice the
sharpness, and is up to ten times more efficient than the telescope’s Wide
Field and Planetary Camera 2. The camera’s wavelength range spans from
ultraviolet to near-infrared light. The camera’s sharp eye and broader
viewing area enable astronomers to study the life cycles of galaxies in the
remotest regions of the cosmos. Astronauts installed the camera aboard the
telescope in March 2002.
Afterglow
The fading fireball of a
gamma-ray burst — a sudden burst of gamma rays from deep space — that is
observable in less energetic wavelengths, such as X-ray, optical, and radio.
After an initial explosion, an expanding gamma-ray burst slows and sweeps up
surrounding material, generating the afterglow, which is visible for several
weeks or months. The afterglow is usually extremely faint, making it
difficult to locate and study.
Alloy
A mixture of two or more
metals. Brass (a mixture of copper and zinc) and bronze (a mixture of copper
and tin) are common alloys.
Alpha Process
A process by which lighter
elements capture helium nuclei (alpha particles) to form heavier elements.
For example, when a carbon nucleus captures an alpha particle, a heavier
oxygen nucleus is formed.
Altitude-azimuth
A type of telescope
mounting that supports the weight of the telescope and allows it to move in
two directions to locate a specific target. One axis of support is vertical
(called the altitude) and allows the telescope to move up and down. The
other axis is horizontal (called the azimuth) and allows the telescope to
swing in a circle parallel to the ground. This makes it easy to position the
telescope: swing it around in a circle and then lift it to the target.
However, tracking an object as the Earth turns is more complicated. The
telescope needs to be adjusted in both directions while tracking, which
requires a computer to control the telescope.
Amplify
To make larger or more
powerful; increase. Radio signals are amplified because they are very weak.
Amplitude
The size of a wave from the
top of a wave crest to its midpoint.
Angular Momentum
A property that an object,
such as a planet revolving around the Sun, possesses by virtue of its
rotation or circular motion. An object’s angular momentum cannot change
unless some force acts to speed up or slow down its circular motion. This
principle, known as conservation of angular momentum, is why an object can
indefinitely maintain a circular motion around an axis of revolution or
rotation.
Angular Resolution
The ability of an
instrument, such as a telescope, to distinguish objects that are very close
to each other. The angular resolution of an instrument is the smallest
angular separation at which the instrument can observe two neighboring
objects as two separate objects. The angular resolution of the human eye is
about a minute of arc. As car headlights approach from a far-off point, they
appear as a single light until the separation between the lights increases
to a point where they can be resolved as two separate lights.
Angular Size
The apparent size of an
object as seen by an observer; expressed in units of degrees (of arc), arc
minutes, or arc seconds. The moon, as viewed from the Earth, has an angular
diameter of one-half a degree.
Antenna
An electrical device used
to send or receive electromagnetic waves. The aerial (a long piece of metal
attached to the front or rear fender) on a car is the antenna for the radio.
Antimatter
Matter made up of
elementary particles whose masses are identical to their normal-matter
counterparts but whose other properties, such as electric charge, are
reversed. The positron is the antimatter counterpart of an electron, with a
positive charge instead of a negative charge. When an antimatter particle
collides with its normal-matter counterpart, both particles are annihilated
and energy is released.
Apparent Brightness
(Apparent Magnitude)
A measure of the brightness
of a celestial object as it appears from Earth. The Sun is the brightest
object in Earth's sky and has the greatest apparent magnitude, with the moon
second. Apparent brightness does not take into account how far away the
object is from Earth.
Arc Minute
One arc minute is 1/60 of a
degree of arc. The angular diameter of the full moon or the Sun as seen from
Earth is about 30 arc minutes.
Arc Second
One arc second is 1/60 of
an arc minute and 1/3600 of an arc degree. The apparent size of a dime about
3.7 kilometers (2.3 miles) away would be an arc second. The angular diameter
of Jupiter varies from about 30 to 50 arc seconds, depending on its distance
from Earth.
Array
An orderly arrangement or
impressive display. For radio telescopes, an array is a group of individual
l radio dishes that work
together. The VLA (Very Large Array) has 27 telescope dishes arranged in a
“Y” pattern.
Asteroid
A small solar system object
composed mostly of rock. Many of these objects orbit the Sun between Mars
and Jupiter. Their sizes range anywhere from 33 feet (10 meters) in diameter
to less than 620 miles (1,000 kilometers). The largest known asteroid,
Ceres, has a diameter of 579 miles (926 kilometers).
Asteroid Belt
A region of space between
Mars and Jupiter where the great majority of asteroids is found.
Astronomer
A scientist who studies the
universe and the celestial bodies residing in it, including their
composition, history, location, and motion. Many of the scientists at the
Space Telescope Science Institute are astronomers. Astronomers from all over
the world use the Hubble Space Telescope.
Astronomical Unit (AU)
The average distance
between the Earth and the Sun, which is about 150 million kilometers (93
million miles). This unit of length is commonly used for measuring the
distances between objects within the solar system.
Astronomy
Astronomy is the study of
the universe and the celestial bodies that reside in it, including their
composition, history, location, and motion.
Atmosphere
The layer of gases
surrounding the surface of a planet, moon, or star.
Atmospheric distortion
The blurring of an image
due to the layer of gases surrounding the surface of Earth. As starlight
travels through the atmosphere, pockets of air act like little lenses and
bend the light in unpredictable ways. This distortion causes stars to appear
to twinkle.
Atom
The smallest unit of matter
that possesses chemical properties. All atoms have the same basic structure:
a nucleus containing positively charged protons with an equal number of
negatively charged electrons orbiting around it. In addition to protons,
most nuclei contain neutral neutrons whose mass is similar to that of
protons. Each atom corresponds to a unique chemical element determined by
the number of protons in its nucleus.
Atomic Nucleus
The positively charged core
of an atom consisting of protons and (except for hydrogen) neutrons, and
around which electrons orbit.
Aurora
A phenomenon produced when
the solar wind (made up of energized electrons and protons) disturbs the
atoms and molecules in a planet’s upper atmosphere. Some of the energy
produced by these disturbances is converted into colorful visible light,
which shimmers and dances. Auroras have been seen on several planets in our
solar system. On Earth, auroras are also known as the “Northern Lights”
(aurora borealis) or “Southern Lights” (aurora australis), depending on in
which polar region they appear.
Barred Spiral Galaxy
A galaxy with a “bar” of
stars and interstellar matter, such as dust and gas, slicing across its
center. The Milky Way is thought to be a barred spiral galaxy.
Baseline
The distance between two or
more telescopes that are working together as a single instrument to observe
celestial objects. The wider the baseline, the greater the resolving power.
BeppoSAX
A space-based X-ray
observatory built and operated by the Italian Space Agency and the
Netherlands Agency for Aerospace Programs. BeppoSAX has been instrumental in
identifying and locating gamma-ray bursts.
Big Bang
A broadly accepted theory
for the origin and evolution of our universe. The theory says that the
observable universe started roughly 13.7 billion years ago from an extremely
dense and incredibly hot initial state.
Binary Star System
A system of two stars
orbiting around a common center of mass that are bound together by their
mutual gravitational attraction.
Black Hole
A region of space
containing a huge amount of mass compacted into an extremely small volume. A
black hole’s gravitational influence is so strong that nothing, not even
light, can escape its grasp. Swirling disks of material — called accretion
disks — may surround black holes, and jets of matter may arise from their
vicinity.
Blue Star
A massive, hot star that
appears blue in color. Spica in the constellation Virgo is an example of a
blue star.
Blueshift
The shortening of a light
wave from an object moving toward an observer. For example, when a star is
traveling toward Earth, its light appears bluer.
Bolide
Large, brilliant meteors
that enter the Earth’s atmosphere. Friction between a fast-moving meteor and
Earth’s air molecules generates tremendous heat, which causes the meteor to
heat up, glow, and perhaps disintegrate. In some cases, the meteor literally
explodes, leaving a visible cloud that dissipates slowly.
Brown Dwarf
An object too small to be
an ordinary star because it cannot produce enough energy by fusion in its
core to compensate for the radiative energy it loses from its surface. A
brown dwarf has a mass less than 0.08 times that of the Sun.
Bulge
The spherical structure at
the center of a spiral galaxy that is made up primarily of old stars, gas,
and dust. The Milky Way’s bulge is roughly 15,000 light-years across.
Carbonaceous Chondrite
A meteorite with embedded
pebble-sized granules that contain significant quantities of organic
(complex carbon-rich) matter.
Cassegrain telescope
A type of reflecting
telescope whose eyepiece is located behind the primary mirror. The primary
mirror is cast with a hole in the center. When light enters the telescope,
it reflects from the primary mirror to the secondary mirror. The secondary
mirror reflects the light back through the hole in the primary mirror to the
eyepiece.
Celestial
Of or relating to the sky
or visible objects in the sky, like the Moon, Sun, planets, comets,
asteroids, stars, and galaxies.
Celestial Object
An object in the sky –
examples include the Moon, the Sun, planets, comets, asteroids, stars, and
galaxies.
Celestial Sphere
An imaginary sphere
encompassing the Earth that represents the sky. Astronomers chart the sky
using the celestial coordinates of the sphere to locate objects in the
cosmos. This sphere is divided into 88 sections called constellations.
Objects are sometimes named for the major constellation in which they
appear.
Celsius (Centigrade)
Temperature Scale
A temperature scale on
which the freezing point of water is 0° C and the boiling point is 100° C.
Cepheid Variable
A type of pulsating star
whose light and energy output vary noticeably over a set period of time. The
time period over which the star varies is directly related to its light
output or luminosity, making these stars useful standard candles for
measuring intergalactic distances.
Chandra X-Ray Observatory
A space-based X-ray
observatory; also known as the Advanced X-ray Astrophysics Facility (AXAF).
Chandra is designed to observe X-rays from high-energy regions of the
universe, such as hot gas in the remnants of exploded stars. The satellite
was launched and deployed in July 1999.
Charge-Coupled Device (CCD)
An electronic detector that
records visible light from stars and galaxies to make photographs. These
detectors are very sensitive to the extremely faint light of distant
galaxies. They can see objects that are 1,000 million times fainter than the
eye can see. CCDs are electronic circuits composed of light-sensitive
picture elements (pixels), tiny cells that, placed together, resemble mesh
on a screen door. The same CCD technology is used in digital cameras. The
Hubble Space Telescope’s Wide Field and Planetary Camera 2 has four CCDs;
each contains 640,000 pixels. The light collected by each pixel is
translated into a number. These numbers (all 2,560,000 of them) are sent to
ground-based computers, which convert them into an image.
Charge-coupled device (CCD)
An electronic detector that
records visible light from stars and galaxies to make photographs. These
detectors are very sensitive to the extremely faint light of distant
galaxies. They can see objects that are 1,000 million times fainter than the
eye can see. CCDs are electronic circuits composed of light-sensitive
picture elements (pixels), tiny cells that, placed together, resemble mesh
on a screen door. The same CCD technology is used in digital cameras.
Chemical Compound
A pure substance consisting
of atoms or ions of two or more different elements. The elements are in
definite proportions. A chemical compound usually possesses properties
unlike those of its constituent elements. For example, table salt (the
common name for sodium chloride) is a chemical compound made up of the
elements chlorine and sodium.
Chemical Evolution
The chemical (i.e.,
pre-biological) changes that transformed simple atoms and molecules into the
more complex chemicals needed for the origin of life. For example, hydrogen
atoms in the cores of stars combine through nuclear fusion to form the
heavier element helium.
Chromatic aberration
Visible light is made of
different colors. When visible light passes through a glass lens or a prism,
it gets dispersed, or split, into its many colors. A lens focuses each color
at a different point, causing a fringe of color to appear around bright
objects.
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Chromosphere
The middle layer of the
solar atmosphere between the photosphere and the corona. The chromosphere is
roughly 10,000 kilometers (6,200 miles) thick and is composed primarily of
hydrogen. It varies in temperature from below 10,000 Kelvin (18,000° F) to
over 100,000 Kelvin (180,000° F).
Closed Universe
A geometric model of the
universe in which the overall structure of the universe closes upon itself
like the surface of a sphere. The rules of geometry in a closed universe are
like those that would apply on the surface of a sphere.
Coelostat
A system of two moveable
mirrors used in solar telescopes. The mirrors follow the Sun and keep its
image in the same location as Earth rotates.
Collecting Area
The area of a telescope’s
primary light-collecting mirror. A telescope’s light-gathering power rises
with an increase in its collecting area.
Colliding Galaxies
A galactic “car wreck” in
which two galaxies pass close enough to gravitationally disrupt each other’s
shape. The collision rips streamers of stars from the galaxies, fuels an
explosion of star birth, and can ultimately result in both galaxies merging
into one.
Collisional Process
An event involving a
collision of objects; for example, the excitation of a hydrogen atom when it
is hit by an electron.
Color
The visual perception of
light that enables human eyes to differentiate between wavelengths of the
visible spectrum, with the longest wavelengths appearing red and the
shortest appearing blue or violet.
Coma
The cloud of gas and dust
that forms around a comet's nucleus. This cloud is created when the solar
wind strikes the surface of the nucleus.
Comet
A ball of rock and ice,
often referred to as a “dirty snowball.” Typically a few kilometers in
diameter, comets orbit the Sun in paths that either allow them to pass by
the Sun only once or that repeatedly bring them through the solar system (as
in the 76-year orbit of Halley's Comet). A comet’s “signature” long, glowing
tail is formed when the Sun’s heat warms the coma or nucleus, which releases
vapors into space.
Comet Nucleus
The core of a comet, made
up of ice, dirt, and rock.
Comet Shoemaker-Levy 9
(SL-9)
A comet that became
gravitationally bound to Jupiter, colliding with the planet in July 1994.
Prior to entering the planet’s atmosphere, the comet broke into several
distinct pieces, each with a separate coma and tail.
Comet Tail
A tail is made up of dust
and gas from a comet’s coma. A tail forms when the solar wind separates dust
and gas from the coma, pushing it outward and away from the Sun in either a
slightly curved path (for dust) or a straight path (for gas).
Compton Gamma-Ray
Observatory (CGRO)
A space-based observatory
that collected high-energy gamma-ray light from celestial objects. The
Compton satellite consisted of the BATSE, COMPTEL, EGRET, and OSSE
instruments. Astronauts aboard the space shuttle Atlantis deployed the CGRO
into low-Earth orbit in April 1991. The satellite plunged into the Pacific
Ocean in June 2000.
Conservation of Energy And
Mass
A fundamental law of
physics, which states that the total amount of mass and energy in the
universe remains unchanged. However, mass can be converted to energy, and
vice versa.
Constellation
A geometric pattern of
bright stars that appears grouped in the sky. Ancient observers named many
constellations after gods, heroes, animals, and mythological beings. Leo
(the Lion) is one example of the 88 constellations.
Convection
The transfer of heat
through a liquid or gas caused by the physical upwelling of hot matter. The
heat transfer results in the circulation of currents from lower, hotter
regions to higher, cooler regions. An everyday example of this process is
boiling water. Convection occurs in the Sun and other stars.
Convection Zone
The region below a star's
surface where energy flows outward by the rising of hot gas known as
convection.
Core
The central region of a
planet, star, or galaxy.
Corona
The outermost layer of the
atmosphere of a star, including the Sun. The corona is visible during a
solar eclipse or when special adapters or filters are attached to a
telescope to block the light from the star’s central region. The gaseous
corona extends millions of kilometers from the star’s surface and has a
temperature in the millions of degrees.
Coronal Hole
Regions in the corona from
which the high-speed solar wind is known to originate. Coronal holes,
usually found near the Sun's poles, are large regions in the corona that are
less dense and cooler than the surrounding region.
Cosmic Microwave Background
Radiative energy filling
the universe that is believed to be the radiation remaining from the Big
Bang. It is sometimes called the “primal glow.” This radiation is strongest
in the microwave part of the spectrum but has also been detected at radio
and infrared wavelengths. The intensity of the cosmic microwave background
from every part of the sky is almost exactly the same.
Cosmic Rays
High-energy atomic
particles that travel through space at speeds close to the speed of light;
also known as cosmic-ray particles.
Cosmic background radiation
Electromagnetic energy
filling the universe that is believed to be the radiation remaining from the
Big Bang. It is sometimes called the “primal glow.” This radiation is
strongest in the microwave part of the spectrum but has also been detected
at radio and infrared wavelengths. The intensity of the cosmic microwave
background from every part of the sky is almost exactly the same.
Cosmological Principle
This principle states that
the distribution of matter across very large distances is the same
everywhere in the universe and that the universe looks the same in all
directions. According to this principle, our view of the universe is like
the view from a boat on an ocean, which is essentially the same for any
other person on any other boat on any other ocean. Measurements of matter
and energy in the universe on the largest observable scales support the
cosmological principle.
Cosmology
The investigation of the
origin, structure, and development of the universe, including how energy,
forces, and matter interact on a cosmic scale.
Crater
A bowl-shaped depression
caused by a comet or meteorite colliding with the surface of a planet, moon,
or asteroid. On geologically active moons and planets (like Earth), craters
can result from volcanic activity.
Critical Density
The minimum average density
that matter in the universe would need in order for its gravitational pull
to slow the universe’s expansion to a halt.
Crown glass
Originally the main
material used to make flat planes of glass for windows, it is composed of
soda-lime glass. It can be used to make lenses and prisms. Crown glass bends
and disperses, or spreads out, light less than flint glass.
Dark Dust Cloud
A region of interstellar
space that contains a rich concentration of gas and dust. Such a cloud is
often irregular in shape but sometimes has a well-defined edge. Visible
light cannot pass through these clouds, so they obscure the light from stars
beyond them.
Dark Matter
Matter that is too dim to
be detected by telescopes. Astronomers infer its existence by measuring its
gravitational influence. Dark matter makes up most of the total mass of the
universe.
Dark energy
A mysterious force that
seems to work opposite to that of gravity and makes the universe expand at a
faster pace.
Declination (DEC)
One of two celestial
coordinates required to locate an astronomical object, such as a star, on
the celestial sphere. Declination is the measure of angular distance of a
celestial object above or below the celestial equator and is comparable to
latitude. To familiarize yourself with declination, hold out your arm in the
direction of the North Star (Polaris). You are now pointing at plus 90
degrees declination. Move your arm downward by 90 degrees. You are now
pointing at 0 degrees declination.
Degree of Arc
One degree of arc is 1/360
of a full circle. The apparent sizes of objects as seen from Earth can be
measured in degrees of arc. The angular diameter of the full moon or the Sun
as seen from Earth is one-half of a degree.
Density
The ratio of the mass of an
object to its volume. For example, water has a density of one gram of mass
for every milliliter of volume.
Detector
A device used to measure
the amount of electromagnetic radiation emitted by celestial objects.
Frequently, detectors are used to sense light that is not visible.
Deuterium
A special form of hydrogen
(an isotope called “heavy hydrogen”) that has a neutron as well as a proton
in its nucleus.
Diameter
The distance from one side
of a circle to the other measured through the center. For telescopes, the
diameter of a lens or mirror is measured from one side to the opposite side,
passing through the center.
Differentiation
The separation of heavy
matter from light matter, thus causing a variation in density and
composition. Differentiation occurs in an object like a planet as gravity
draws heavier material toward the planet’s center and lighter material rises
to the surface.
Diffraction Grating
A device that splits light
into its component parts or spectrum. A diffraction grating often consists
of a mirror with thousands of closely spaced parallel lines, which spread
out the light into parallel bands of colors or distinct fine lines or bars.
Digital Image
A visible image that is
recorded by an electronic detector and subdivided into small picture
elements (pixels). Each element is assigned a number that corresponds to the
brightness recorded at its physical location on the detector. Computer
software converts the numerical information into a visual image. The Hubble
Space Telescope records digital images.
Dispersion
Visible light is actually
made up of different colors. Each color bends by a different amount when
refracted by glass. That’s why visible light is split, or dispersed, into
different colors when it passes through a lens or prism. Shorter
wavelengths, like purple and blue light, bend the most. Longer wavelengths,
like red and orange light, bend the least.
Doppler Effect
The change in the
wavelength of sound or light waves caused when the object emitting the waves
moves toward or away from the observer; also called Doppler Shift. In sound,
the Doppler Effect causes a shift in sound frequency or pitch (for example,
the change in pitch noted as an ambulance passes). In light, an object’s
visible color is altered and its spectrum is shifted toward the blue region
of the spectrum for objects moving toward the observer and toward the red
for objects moving away.
Double stars
A system of two stars that
are gravitationally bound to each other. They orbit each other around a
common center. They can also be called binary stars.
Dwarf Galaxy
A relatively small galaxy.
The Large and Small Magellanic Clouds, visible in the Southern Hemisphere,
are two dwarf irregular galaxies that are neighbors of the Milky Way.
Dwarf Planet
A celestial body within the
solar system that shares the characteristics of planets. It orbits the Sun,
is not a moon, and has a spherical or nearly spherical shape. Unlike a
planet, however, a dwarf planet has not cleared away any loose cosmic rubble
from its orbit. Dwarf planets include Ceres, Pluto, and Eris.
Earth
The third planet from the
Sun and one of four terrestrial planets in the inner solar system. Earth,
the only planet where water exists in large quantities, has an atmosphere
capable of supporting myriad life forms. The planet is 150 million
kilometers (93 million miles) away from the Sun. Earth has one satellite —
the Moon.
Earth-orbiting
Traveling around Earth, in
the path followed by an object moving in the gravitational field of Earth.
For example, the telescope travels around, or orbits, Earth because Earth’s
gravitational field keeps the telescope in its path, or orbit.
Electromagnetic Force
A fundamental force that
governs all interactions among electrical charges and magnetism.
Essentially, all charged particles attract oppositely charged particles and
repel identically charged particles. Similarly, opposite poles of magnets
attract and like magnetic poles repel.
Electromagnetic Radiation
A form of energy that
propagates through space as vibrations of electric and magnetic fields; also
called radiation or light. All electromagnetic radiation is a form of light.
Electromagnetic Spectrum
The entire range of
wavelengths of electromagnetic radiation, including radio waves, microwaves,
infrared light, visible light, ultraviolet light, X-rays, and gamma rays.
Electromagnetism
The science dealing with
the physical relationship between electricity and magnetism. The principle
of an electromagnet, a magnet generated by electrical current flow, is based
on this phenomenon.
Electron
A negatively charge
elementary particle that typically resides outside the nucleus of an atom
but is bound to it by electromagnetic forces. An electron’s mass is tiny:
1,836 electrons equals the mass of one proton.
Electron Volt (eV)
A unit of energy that is
equal to the energy that an electron gains as it moves through a potential
difference of one volt. This very small amount of energy is equal to
1.602 * 10–19 joules. Because an electron volt is so small,
engineers and scientists sometimes use the terms MeV (mega-million) and GeV
(giga-billion) electron volts.
Element
A substance composed of a
particular kind of atom. All atoms with the same number of protons (atomic
numbers) in the nucleus are examples of the same element and have identical
chemical properties. For example, gold (with 79 protons) and iron (with 26
protons) are both elements, but table salt is not because it is made from
two different elements: sodium and chlorine. The atoms of a particular
element have the same number of protons in the nucleus and exhibit a unique
set of chemical properties. There are about 90 naturally occurring elements
on Earth.
Elementary Particles
Particles smaller than
atoms that are the basic building blocks of the universe. The most prominent
examples are photons, electrons, and quarks.
Ellipse
A special kind of elongated
circle. The orbits of the solar system planets form ellipses.
Elliptical
A special kind of elongated
circle. The orbits of the solar system planets are elliptical.
Elliptical Galaxy
A galaxy that appears
spherical or football-shaped. Elliptical galaxies are comprised mostly of
old stars and contain very little dust and “cool” gas that can form stars.
Emission Line
A bright line in a spectrum
caused by emission of light. Each chemical element emits and absorbs
radiated energy at specific wavelengths. The collection of emission lines in
a spectrum corresponds to the chemical elements contained in a celestial
object.
Erosion
Natural processes that wear
or grind away the surface of an object. On Earth, the major agents of
erosion are water and wind.
Escape Velocity
The minimum velocity
required for an object to escape the gravity of a massive object.
Event Horizon
The spherical outer
boundary of a black hole. Once matter crosses this threshold, the speed
required for it to escape the black hole’s gravitational grip is greater
than the speed of light.
Excited State
A greater-than-minimum
energy state of any atom that is achieved when at least one of its electrons
resides at a greater-than-normal distance from its parent nucleus.
Exposure
The process of allowing
electromagnetic radiation to fall on light-sensitive materials such as
photographic films or plates. An exposure is also the image created by the
process. A long exposure time is needed in order to obtain an image of dim
and distant celestial objects.
Extraterrestrial
An adjective that means
“beyond the Earth.” The phrase “extraterrestrial life” refers to possible
life on other planets.
Eyepiece
The lens or lens group
closest to the eye in an optical instrument such as a telescope or
microscope.
Fahrenheit Temperature
Scale
A temperature scale on
which the freezing point of water is 32° F and the boiling point is 212° F.
Faint Object Camera (FOC)
An instrument aboard the
Hubble Space Telescope that recorded high-resolution images of faint
celestial objects in deep space. Built by the European Space Agency, the
camera collected ultraviolet and visible light from celestial objects. The
camera served as Hubble’s “telephoto lens” — recording the most detailed
images over a small field of view. The FOC’s resolution allowed Hubble to
single out individual stars in distant star clusters. The instrument was
replaced in March 2002 during Servicing Mission 3B.
Faint Object Spectrograph
(FOS)
An instrument aboard the
Hubble Space Telescope that acted like a prism to separate light from the
cosmos into its component colors, providing a wavelength “fingerprint” of
the object being observed. Such information yields clues about an object’s
temperature, chemical composition, density, and motion. Spectrographic
observations also reveal changes in celestial objects as the universe
evolves. The instrument was replaced in February 1997 during the Second
Servicing Mission.
Far-Infrared Spectrum
The region of the infrared
spectrum that exhibits the longest wavelengths and the lowest frequencies
and energies.
Field of View (FOV)
A telescope’s viewing area,
measured in degrees, arc minutes, or arc seconds. A telescope that can just
fit the full moon into its complete viewing area has a field of view of
roughly 30 arc minutes.
Filter Wheels
Rotating wheels in a
telescope instrument that allow specific colors of light from a celestial
object to pass through and form an image on the detector. The Wide Field and
Planetary Camera 2 aboard the Hubble Space Telescope has 12 filter wheels,
each of which holds four filters.
Fission
A nuclear process that
releases energy when heavyweight atomic nuclei break down into lighter
nuclei. Fission is the basis of the atomic bomb.
Flare
A sudden and violent
outburst of solar energy that is often observed in the vicinity of a sunspot
or solar prominence; also known as a solar flare.
Flat Universe
A geometric model of the
universe in which the laws of geometry are like those that would apply on a
flat surface such as a table top.
Flint glass
The lead glass that was
produced in the United States and the United Kingdom prior to the 1860s.
This glass is used to make telescope lenses and prisms. Flint glass bends
and disperses, or spreads out, light more than crown glass.
Flux
The flow of fluid,
particles, or energy through a given area within a certain time. In
astronomy, this term is often used to describe the rate at which light
flows. For example, the amount of light (photons) striking a single square
centimeter of a detector in one second is its flux.
Flyby Spacecraft
A spacecraft that travels
past a celestial object. Frequently, such a spacecraft is unmanned and takes
images of the object.
Focal length
Focal length (shown in red)
is the distance between the center of a convex lens or a concave mirror and
the focal point of the lens or mirror — the point where parallel rays of
light meet, or converge.
Focal point
The focal point of a lens
or mirror is the point in space where parallel light rays meet after passing
through the lens or bouncing off the mirror. A “perfect” lens or mirror
would send all light rays through one focal point, which would result in the
clearest image.
Frequency
Describes the number of
wave crests passing by a fixed point in a given time period (usually one
second). Frequency is measured in Hertz (Hz).
Fusion
A nuclear process that
releases energy when light atomic nuclei combine to form heavier nuclei.
Fusion is the energy source for stars like our Sun.
GRB990123
One of the most energetic
gamma-ray bursts (GRBs) ever detected, occurring at 4:47 a.m. EST, January
23, 1999. The “burst” equaled the power of nearly 10 million billion suns.
It became the first GRB to be viewed simultaneously in both gamma-ray and
optical wavelengths.
Galactic Center
The central hub or nucleus
of a galaxy. The Milky Way’s galactic center is about 28,000 light-years
from Earth.
Galactic Disk
A flattened disk of gas and
young stars in a galaxy. Some galactic disks have material concentrated in
spiral arms (as in a spiral galaxy) or bars (as in barred spirals).
Galactic Halo
Spherical regions around
spiral galaxies that contain dim stars and globular clusters. The radius of
the halo surrounding the Milky Way extends some 50,000 light-years from the
galactic center.
Galactic Nucleus
The central concentration
of matter (stars, gas, dust, and perhaps a black hole) in a galaxy,
typically spanning no more than a few light-years in diameter.
Galactic Plane
The imaginary projection of
the Milky Way’s disk on the sky. Most of the galaxy’s stars and interstellar
matter reside in this disk. Objects in the galaxy are often referred to as
being above, below, or in the galactic plane.
Galaxy
A collection of stars, gas,
and dust bound together by gravity. The smallest galaxies may contain only a
few hundred thousand stars, while the largest galaxies have thousands of
billions of stars. The Milky Way galaxy contains our solar system. Galaxies
are classified or grouped by their shape. Round or oval galaxies are
elliptical galaxies and those showing a pinwheel structure are spiral
galaxies. All others are called irregular because they do not resemble
elliptical or spiral galaxies.
Galaxy Cluster
A collection of dozens to
thousands of galaxies bound together by gravity.
Galaxy Evolution
The study of the birth of
galaxies and how they change and develop over time.
Galaxy Supercluster
A vast collection of galaxy
clusters that may contain tens of thousands of galaxies spanning over a
hundred million light-years of space. Galaxy superclusters are the largest
structures in the universe.
Gamma-Ray Burst (GRB)
A brief, intense, and
powerful burst of gamma rays, the highest-energy, shortest-wavelength
radiation in the electromagnetic spectrum. These bursts emanate from distant
sources outside our galaxy and last only a few seconds. They are the
brightest and most energetic explosions known.
Gamma Rays
Light with the shortest
wavelengths and the highest energies and frequencies in the electromagnetic
spectrum; also called gamma radiation.
Ganymede
One of Jupiter’s largest
moons. Ganymede, the largest satellite in our solar system, is about 5300
kilometers (3300 miles) wide and larger than the planet Mercury.
Gas Giant
A large planet with a
small, rocky core and a deep atmosphere composed mostly of hydrogen and
helium. Our solar system contains four gas giants: Jupiter, Saturn, Uranus,
and Neptune. This group is also known as Jovian planets.
Gaseous Nebula
A glowing cloud of gas in
interstellar space. The cloud of gas may be either an emission nebula, which
absorbs ultraviolet light from nearby stars and re-radiates visible light,
or a reflection nebula, which reflects light off of its dust particles.
General Theory of
Relativity
A theory Einstein developed
to explain how gravity influences space and time.
Geocentric
An adjective meaning
“centered on the Earth.” Most early civilizations had a geocentric view of
the universe.
Geosynchronous Orbit
Also known as
geostationary. An orbit in which an object circles the Earth once every 24
hours, moving at the same speed and direction as the planet’s rotation. The
object remains nearly stationary above a particular point, as observed from
Earth. The International Ultraviolet Explorer (IUE) and some weather
satellites are examples of satellites in geosynchronous orbit.
Giant Star
A dying star that has used
up the hydrogen fuel in its core and has begun to expand. Giant stars are
generally larger than our Sun.
Gigabyte
A measure of computer data
storage capacity equal to approximately a billion bytes. In computer
language, a byte of information represents a letter or digit. So, a billion
bytes is equal to a billion letters.
Globular Cluster
A collection of hundreds of
thousands of old stars held together by gravity. Globular clusters are
usually spherically shaped and are often found in the halos of galaxies.
Each star belonging to a cluster revolves around the cluster’s common center
of mass.
Goddard High Resolution
Spectrograph (GHRS)
A science instrument aboard
the Hubble Space Telescope that made finely detailed spectroscopic
observations of ultraviolet sources. The GHRS was removed from Hubble in
February 1997 and replaced with the Space Telescope Imaging Spectrograph.
Goddard Space Flight Center
(GSFC)
NASA’s flight control
center in Greenbelt, Maryland, which receives data from orbiting
observatories such as the Hubble Space Telescope (HST). HST digital data are
then relayed to the Space Telescope Science Institute in Baltimore,
Maryland, where they are interpreted into pictures. Goddard also conducts
scientific investigations, develops and operates space systems, and works
toward the advancement of space science technologies.
Grand Unified Theory (GUT)
A theory stating that that
strong and weak nuclear forces and electromagnetic forces are varying
aspects of the same fundamental force.
Gravitational Clustering
The process by which a
large-scale structure grows as its gravity attracts smaller building blocks.
Astronomers believe that all the large-scale structures (such as galaxies,
galaxy clusters, and galaxy superclusters) that we see in the universe today
formed through gravitational clustering.
Gravitational Constant (G)
A value used in the
calculation of the gravitational force between objects. In the equation
describing the force of gravity, “G” represents the gravitational constant
and is equal to 6.672 * 10–11 Nm2/kg2.
Gravitational Instability
A condition that occurs
when an object’s inward-pulling gravitational forces exceed the
outward-pushing pressure forces, thus causing the object to collapse on
itself. For example, when the pressure forces within an interstellar gas
cloud cannot resist the gravitational forces that act to compress the cloud,
then the cloud collapses upon itself to form a star.
Gravitational Lens
A massive object that
magnifies or distorts the light of objects lying behind it. For example, the
powerful gravitational field of a massive cluster of galaxies can bend the
light rays from more distant galaxies, just as a camera lens bends light to
form a picture.
Gravitational Redshift
The reddening of light from
a very massive object caused by photons escaping and traveling away from the
object’s strong gravitational field. An example of gravitational redshift is
light escaping from the surface of a neutron star.
Gravity Assist
An effect through which an
orbiting object, such as a spacecraft or a comet, gains or loses speed by
virtue of the gravitational might of a planet or other celestial object that
it passes. For example, the Cassini spacecraft in its journey to Saturn used
a gravity assist from Earth to increase its velocity by about 36,000
kilometers per hour (22,300 miles per hour).
Gravity (Gravitational
Force)
The attractive force
between all masses in the universe. All objects that have mass possess a
gravitational force that attracts all other masses. The more massive the
object, the stronger the gravitational force. The closer objects are to each
other, the stronger the gravitational attraction.
Great Red Spot
A circulating storm located
in Jupiter’s upper atmosphere. The storm, which rotates around the planet in
six days, is the width of two to three Earths. Galileo first observed the
spot in the 17th century.
Greenhouse Effect
The result of a planet’s
atmosphere trapping infrared heat, rather than allowing it to escape into
space. This effect increases the planet’s surface temperature, a phenomenon
known as global warming.
Ground State
The minimum energy state of
an atom that is achieved when all of its electrons have the lowest possible
energy and therefore are as close to the nucleus as possible.
Group of Galaxies
A small collection of
galaxies bound together by gravity. The number of galaxies in a group can
range from a few to dozens. The Milky Way is a member of the Local Group, a
collection of more than 30 galaxies.
Guide Star
A star that a telescope’s
guidance system locks onto to ensure that a celestial object is followed and
observed as the telescope moves, owing either to the Earth’s rotation or the
telescope’s orbital trajectory. The Hubble Space Telescope uses two of its
three Fine Guidance Sensors to detect and lock onto guide stars. The
telescope’s science operations center has more than 15 million guide stars
in its database — the Guide Star Catalogue.
Gyroscope
A spinning wheel mounted on
a non-stationary frame that stabilizes and points a space-based observatory.
This spinning wheel resists applied external forces and tends to retain its
original orientation in space. For example, balancing on a moving bicycle is
easier than balancing on a stationary one because of this tendency.
Gyroscopes are used in navigational instruments for aircraft, satellites,
and ships. The Hubble Space Telescope has six gyroscopes on board for
navigation and sighting purposes.
HDF-N
Hubble Deep Field North
(HDF-N) is a tiny region of the northern sky near the Big Dipper toward
which the Hubble Space Telescope was pointed for ten straight days in 1995.
Because this observation was designed to detect very faint light from the
most distant galaxies Hubble can observe, the field contains few bright
celestial objects. Seemingly devoid of light, this small area provided a
“keyhole” view of the universe’s past, reaching across space and time to see
infant galaxies. By probing these remote regions of space, astronomers are
gaining more information on galaxy development.
HDF-S
Hubble Deep Field South
(HDF-S) is a tiny region of the southern sky near the Southern Cross toward
which the Hubble Space Telescope was pointed for ten straight days in 1998.
Because this observation was designed to detect very faint light from the
most distant galaxies Hubble can observe, the field contains few bright
celestial objects. Seemingly devoid of light, this small area provided a
“keyhole” view of the universe’s past, reaching across space and time to see
infant galaxies. By probing these remote regions of space, astronomers are
gaining more information on galaxy development.
Habitable Zone
A region around a star
where planets with liquid water may be present. A planet on the near edge of
the habitable zone would have a surface temperature slightly lower than the
boiling point of water. A planet on the distant edge of the habitable zone
would have a surface temperature slightly higher than the freezing point of
water.
Heliocentric
An adjective meaning
“centered on the Sun.”
Hemisphere
Half of a spherical or
roughly spherical body; for example, the northern and southern halves of the
Earth, above and below the equator.
Hertzsprung-Russell Diagram
A plot showing the
relationship between the brightness (luminosity) and the surface
temperatures of many stars. Often the spectral class, which is based on the
temperature of the star, is used as a label.
High Speed Photometer (HSP)
An original science
instrument aboard the Hubble Space Telescope that made very rapid
photometric observations of celestial objects in near-ultraviolet to visible
light. The instrument was removed in December 1993 during the First
Servicing Mission.
Host Galaxy
A galaxy in which a cosmic
phenomenon, such as a supernova explosion or a gamma-ray burst, has
occurred.
Hubble’s Law
Mathematically expresses
the idea that the recessional velocities of faraway galaxies are directly
proportional to their distance from us. Hubble’s Law describes the
relationship of velocity and distance by the equation V=Ho * d,
where V is the object’s recessional velocity, d is the distance to the
object, and Ho is the Hubble constant. Essentially, the more
distant two galaxies are from each other, the faster they are traveling away
from each other. American astronomer Edwin Hubble discovered this
relationship in 1929 when he observed that galaxies and clusters of galaxies
were generally moving away from each other.
Hubble Constant (Ho)
A number that expresses the
rate at which the universe expands with time. Ho appears to be
between 60 and 75 kilometers per second per megaparsec.
Hubble Space Telescope
(HST)
An orbiting telescope that
collects light from celestial objects in visible, near-ultraviolet, and
near-infrared wavelengths. The telescope was launched April 24, 1990 aboard
the NASA Space Shuttle Discovery. The 12.5-ton (11,110-kg), tube-shaped
telescope is 13.1 m (43 ft) long and 4.3 m (14 ft) wide. It orbits the Earth
every 96 minutes and is mainly powered by the sunlight collected by its two
solar arrays. The telescope’s primary mirror is 2.4 m (8 ft) wide. The
telescope is operated jointly by the National Aeronautics and Space
Administration (NASA) and the European Space Agency (ESA). HST is one of the
many NASA Origins Missions, which include current satellites such as the Far
Ultraviolet Space Explorer (FUSE) and future space observatories such as the
Next Generation Space Telescope (NGST).
Image Intensifier
A device capable of
intensifying light from a faint source so that it may be more easily
detected.
Impact
When one body strikes
another with great force. Some examples include a meteor colliding with the
Moon or a comet, such as Shoemaker-Levy 9, slamming into Jupiter.
Impact Crater
A large depression on a
moon or a planet. An impact crater is created when an asteroid, a comet, or
a meteorite strikes the moon or the planet with great force.
Impact Event
A collision between two
solar system bodies that releases exceptionally large amounts of energy.
Some examples are the 1908 Siberian Tunguska impact by a comet or an
asteroid and the asteroid that struck Earth 65 million years ago, which may
have led to the extinction of the dinosaurs and other species of the
Cretaceous-Tertiary era.
Impactor
The part of the Deep Impact
spacecraft that crashed into comet 9P/Tempel 1. When launched, the impactor
and the flyby spacecraft were attached to each other. The spacecraft
launched the impactor a day before the crash. As the impactor punched
through the comet’s crust, the flyby craft recorded the event from a safe
distance away.
Inflation
The theory that the
universe expanded very rapidly shortly after the Big Bang.
Infrared
Radiation that has longer
wavelengths and lower frequencies and energies than visible light.
Infrared (IR) Light
A region of the
electromagnetic spectrum that has slightly longer wavelengths and lower
frequencies than visible light, but is not visible to the human eye. This
region of light is comparable to the range of sounds that are too low for
the human ear to hear. Infrared light can be detected as the heat from a
fire or a light bulb.
Infrared (IR) light
A region of the
electromagnetic spectrum not visible to the human eye. This region of light
that cannot be seen is similar to the range of sounds that are too low for
the human ear to hear. Infrared light can be detected as the heat from a
fire or a light bulb.
Infrared Telescope
An instrument that collects
the infrared radiation emitted by celestial objects. There are several
Earth- and space-based infrared observatories. The Infrared Telescope
Facility, an Earth-bound infrared telescope, is the U.S. national infrared
observing facility at the summit of Mauna Kea, Hawaii. A planned space-based
infrared observatory is the Space Infrared Telescope Facility (SIRTF).
Instrument
Any device that measures
and/or records energy from astronomical objects. Some astronomical
instruments include spectrometers, photometers, spectroheliographs, and
charge-coupled devices.
Intensity
The amount, degree, or
quantity of energy passing through a point per unit time. For example, the
intensity of light that Earth receives from the Sun is far greater than that
from any other star because the Sun is the closest star to us.
Interferometer
An instrument that combines
the signal from two or more telescopes to produce a sharper image than the
telescopes could achieve separately.
Interferometry
The process used to combine
the signal from two or more telescopes to produce a sharper image than each
telescope could achieve separately.
International Ultraviolet
Explorer (IUE)
The longest operating
(1978—1996) and most productive ultraviolet space observatory launched into
a high geosynchronous orbit.
Interplanetary Matter
Dust, gas, and other debris
found within the solar system.
Interplanetary Space
The region of space
surrounding our Sun. Asteroids, comets, Earth, and the solar wind are
examples of things occupying interplanetary space.
Interstellar Dust
Small particles of solid
matter, similar to smoke, in the space between stars.
Interstellar Medium (ISM)
The sparse gas and dust
located between the stars of a galaxy.
Interstellar Space
The dark regions of space
located between the stars.
Inverse Square Law
A law that describes any
quantity, such as gravitational force, that decreases with the square of the
distance between two objects. For example, if the distance between two
objects is doubled, then the gravitational force exerted between them is
one-fourth as strong. Likewise, if the distance to a star is doubled, then
its apparent brightness is only one-fourth as great.
Invisible Radiation
Radiation that the eye
cannot detect, such as gamma rays, radio waves, ultraviolet light, and
X-rays.
Io
The innermost of Jupiter’s
four large moons. Due to Jupiter's gravitational might, Io is geologically
active; its surface is peppered with volcanoes that send sulfurous eruptions
into its thin atmosphere. Io appears to have the most active volcanoes in
the solar system.
Io Plasma Torus
A bagel-shaped region of
trapped sulfur ions around Jupiter that originates from the surface of Io,
one of Jupiter’s moons. Gravitational tidal forces between Jupiter, other
Galilean moons, and Io cause tidal friction in Io’s interior, producing
geysers that spew sulfur at tremendous speeds. Some of the sulfur ions leave
Io’s surface and become trapped around Jupiter.
Ion
An atom with one or more
electrons removed (or added), giving the atom a positive (or negative)
charge.
Ionization
The process by which ions
are produced, typically by collisions with other atoms or electrons, or by
absorption of electromagnetic radiation.
Ionosphere
A region of the Earth’s
upper atmosphere where solar radiation ionizes the air molecules. This
region affects the transmission of radio waves and extends from 50 to 400
kilometers (30 to 250 miles) above the Earth's surface.
Irregular Galaxy
A galaxy that appears
disorganized and disordered, without a distinct spiral or elliptical shape.
Irregular galaxies are usually rich in interstellar matter, such as dust and
gas. The Large and Small Magellanic Clouds are examples of nearby irregular
galaxies.
Isotope
An atom of a given element
having a particular number of neutrons in the nucleus. Isotopes of a given
element differ in the numbers of neutrons within the nucleus. Adding or
subtracting a neutron from the nucleus changes an atom’s mass but does not
affect its basic chemical properties.
Jets
Narrow, high-energy streams
of gas and other particles generally ejected in two opposite directions from
some central source. Jets appear to originate in the vicinity of an
extremely dense object, such as a black hole, pulsar, or protostar, with a
surrounding accretion disk. These jets are thought to be perpendicular to
the plane of the accretion disk.
Jovian Atmosphere
The atmosphere surrounding
the giant, massive planet Jupiter. The Jovian atmosphere is composed
primarily of hydrogen (90 percent) and helium (10 percent). Other minor
ingredients include water, hydrogen sulfide, methane, and ammonia.
Jovian Planets
The planets Jupiter,
Saturn, Uranus, and Neptune. They are called Jovian planets because of
similarities in their composition and location. This group is also known as
the “giant planets,” the “gas planets” and, when grouped with the planet
Pluto, the “outer planets.”
Jovian Winds
The hurricane-force,
high-velocity motion of gas molecules in Jupiter’s atmosphere. The wind
speed increases as one travels deeper into Jupiter’s atmosphere. The various
patterns of atmospheric winds are easily identified in Jupiter’s upper cloud
layer.
Jupiter
The fifth planet from the
Sun and the largest planet in our solar system, twice as massive as all the
other planets combined. Jupiter is a gaseous planet with a very faint ring
system. Four large moons and numerous smaller moons orbit the planet.
Jupiter is more than five times the Earth’s distance from the Sun. It
completes an orbit around the Sun in about 12 Earth years.
Keck Observatory
Two telescopes known as the
world's largest optical and infrared telescopes, jointly operated by the
California Institute of Technology and the University of California. The
telescopes comprise the W.M. Keck Observatory and are located on the summit
of Hawaii's dormant Mauna Kea volcano.
Kelvin Scale
The temperature scale most
commonly used in science, on which absolute zero is the lowest possible
value. On this scale, water freezes at 273 K and boils at 373 K.
Kepler’s Laws
Three laws, derived by 17th
century German astronomer Johannes Kepler, that describe planetary motion.
Kepler’s first law: The orbits of planets are ellipses, with the Sun
at one focus. Therefore, each planet moves in an elliptical orbit around the
Sun.
Kepler’s second law: An imaginary line connecting any planet to the
Sun sweeps over equal areas in equal intervals of time.
Kepler’s third law: The square of any planet’s orbital period is
proportional to the cube of its mean distance from the Sun.
Kilometer (km)
A measure of distance in
the metric system equal to 1000 meters or about 0.6 of a mile.
Kinetic Energy
The energy that an object
has by virtue of its motion.
Kitt Peak Observatory
The world’s largest
collection of telescopes, located high above the Sonora Desert in Arizona.
Eight astronomical research institutions share the 22 optical and two radio
telescopes at Kitt Peak. The National Optical Astronomy Observatories
oversee site operations at the observatory.
Kuiper Belt
A region in our outer solar
system where many "short-period" comets originate. The orbits of
short-period comets are less than 200 years. This region begins near
Neptune’s orbit at 30 astronomical units (AU) and extends to about 50 AU
away from the Sun. An astronomical unit is the average distance between
Earth and the Sun. The Kuiper Belt may have as many as 100 million comets.
Lens
A carefully ground or
molded piece of glass, plastic, or other transparent material that causes
light to bend and either come together or spread apart to form an image.
Lens doublet
A set of two lenses, one
concave and one convex, made from different types of glass. Together the
lenses correct both spherical and chromatic aberrations. A single lens alone
cannot correct these aberrations.
Light Curve
A plot showing how the
light output of a star (or other variable astronomical object) changes with
time.
Light-Year
The distance that a
particle of light (photon) will travel in a year — about 10 trillion
kilometers (6 trillion miles). It is a useful unit for measuring distances
between stars.
Lithosphere
The solid part of a
planet’s surface, composed of the crust and upper mantle. On Earth, it
includes the continents and the sea floor.
Local Group
A small cluster of more
than 30 galaxies, including the Andromeda galaxy, the Magellanic Clouds, and
the Milky Way galaxy.
Long-Period Comet
A comet having an orbital
period greater than 200 years and usually moving in a highly elliptical,
eccentric orbit. Comets have orbits that take them great distances from the
Sun. Most long-period comets pass through the inner solar system only once.
Hale-Bopp is an example of a long-period comet.
Luminosity
The amount of energy
radiated into space every second by a celestial object, such as a star. It
is closely related to the absolute brightness of a celestial object.
Lunar Eclipse
A darkening of the Moon, as
viewed from Earth, caused when our planet passes between the Sun and the
Moon.
Lyman Limit
A specific wavelength
(91.2 nm) that corresponds to the energy needed to ionize a hydrogen atom
(13.6 eV). Galactic space is opaque at wavelengths shorter than the Lyman
limit. Subsequently, light from cosmic objects at wavelengths less than the
Lyman limit is exceedingly difficult to detect.
Magellanic Clouds
The Magellanic Clouds are
two dwarf irregular galaxies. Known as the Large Magellanic Cloud (LMC) and
the Small Magellanic Cloud (SMC), the galaxies are in the Local Group. The
closer LMC is 168,000 light-years from Earth. Both galaxies can be observed
with the naked eye in the southern night sky.
Magnetic Field
A region of space in which
magnetic forces may be detected or may affect the motion of an electrically
charged particle. As with gravity, magnetism has a long-range effect and
magnetic fields are associated with many astronomical objects.
Magnetic-Field Lines
Imaginary lines used to
visualize a magnetic field. Magnetic field lines are related to the strength
of the magnetic object’s influence and point in the same direction as a
compass needle would.
Magnetopshere
A region of space above the
Earth’s (or other planet’s) atmosphere where magnetic fields influence the
motions of charged particles. The magnetosphere magnetically deflects or
traps charged particles from space that would otherwise bombard the planet’s
surface.
Magnification
Enlargement in the size of
an optical image. For telescopes, magnification is not as important as the
ability to gather light, which depends on the diameter of the primary lens
or mirror.
Magnify
The process of enlarging
the size of an optical image.
Mantle
The interior region of a
terrestrial (rocky) planet or other solid body that is below the crust and
above the core.
Maria
A dark, flat, large region
on the surface of the Moon. The term is also applied to the less
well-defined areas on Mars. Although maria literally means “seas,” watery
regions do not exist on the Moon or Mars. Marias on the Moon may be evidence
of past volcanic lava flows.
Mars
The fourth planet in the
solar system and the last member of the hard, rocky planets (the inner or
terrestrial planets) that orbit close to the Sun. The planet has a thin
atmosphere, volcanoes, and numerous valleys. Mars has two moons: Deimos and
Phobos.
Marshall Space Flight
Center (MSFC)
NASA center overseeing the
research, development, and implementation of three primary areas essential
to space flight: reusable space transportation systems, generation and
communication of new scientific knowledge, and management of all space lab
activities. Located in Huntsville, Alabama, the center aided in the design,
development, and construction of the Hubble Space Telescope.
Mass
A measure of the total
amount of matter contained within an object.
Matter-Antimatter
Annihilation
A highly efficient
energy-generation process in which equal amounts of matter and antimatter
collide and destroy each other, thus producing a burst of energy.
Megaparsec (MPC)
Equals one million parsecs
(3.26 million light-years) and is the unit of distance commonly used to
measure the distance between galaxies.
Mercury
The closest planet to the
Sun. The temperature range on Mercury’s surface is the most extreme in the
solar system, ranging from about 400° C (750° F) during the day to about
–200° C (–300° F) at night. Mercury, which looks like Earth’s moon, has
virtually no atmosphere, no moons, and no water.
Meteor
A bright streak of light in
the sky caused when a meteoroid enters the Earth’s atmosphere. The streak of
light is produced from heat generated by the meteoroid traveling into the
Earth’s atmosphere.
Meteorite
The remains of a meteoroid
that plunges to the Earth’s surface. A meteorite is a stony or metallic mass
of matter that did not completely vaporize when it entered the Earth’s
atmosphere.
Meteoroid
A small, solid object
moving through space. A meteoroid produces a meteor when it enters the
Earth’s atmosphere.
Methane
A chemical compound
consisting of five atoms: one of carbon and four of hydrogen. On Earth,
methane is a colorless, odorless gas and is the principal ingredient of
natural gas. In the cold vacuum of space, methane is a white solid but, when
hit by sunlight, it can become a gas.
Micrometeoroid
A very small meteoroid with
a diameter of less than a millimeter. Micrometeoroids form the bulk of the
interplanetary solid matter scattered throughout the solar system.
Microwaves
An electromagnetic wave in
the region between infrared and radio wavelengths. Microwave wavelengths
fall between one millimeter and one meter.
Milky Way
The Milky Way, a spiral
galaxy, is the home of Earth. The Milky Way contains more than 100 billion
stars and has a diameter of 100,000 light-years.
Milky Way Galaxy
The Milky Way, a spiral
galaxy, is the home of Earth. The Milky Way contains more than 100 billion
stars and has a diameter of 100,000 light-years.
Minerals
The building blocks of
rocks. They are naturally occurring substances formed through geological
processes, and often have a crystalline form. They can be single elements
(such as gold or silver) or compounds (such as quartz, marble or turquoise).
Modern physics
A group of several theories
developed in the early to mid-20th century that explains how small particles
are affected by light, how measurements change when objects move very fast,
and how gravity affects space and time.
Molecular Cloud
A relatively dense, cold
region of interstellar matter where hydrogen gas is primarily in molecular
form. Stars generally form in molecular clouds. Molecular clouds appear as
dark blotches in the sky because they block all the light behind them.
Molecular Velocity
The average speed of the
molecules in a gas of a given temperature.
Molecule
A tightly knit group of two
or more atoms bound together by electromagnetic forces among the atoms’
electrons and nuclei. For example, water (H2O) is two hydrogen
atoms bound with one oxygen atom. Identical molecules have identical
chemical properties.
Moon
A large body orbiting a
planet. On Earth’s only moon, scientists have not detected life, water, or
oxygen on this heavily cratered body. The Moon orbits our planet in about 28
days.
Mounting
The support structure for a
telescope that bears the weight of the telescope and allows it to be pointed
at a target. The mounting of today’s research telescopes also allows
astronomers to track the object as it appears to move across the sky.
Multi-Layer Insulation
(MLI)
A “skin” or blanket of
insulation covering the Hubble Space Telescope, which protects the
observatory from temperature extremes. This insulation protects the
telescope from the cold of outer space and also reflects sunlight so that
the telescope does not become too warm. The MLI on Hubble is made up of many
layers of aluminized Kapton, with an outer layer of aluminized Teflon.
National Aeronautics And
Space Administration (NASA)
A Federal agency created on
July 29, 1958 after President Dwight Eisenhower signed the National
Aeronautics and Space Act of 1958. NASA coordinates space exploration
efforts as well as traditional aeronautical research functions.
Near-Infrared
The region of the infrared
spectrum that is closest to visible light. Near-infrared light has slightly
longer wavelengths and slightly lower frequencies and energies than visible
light.
Near Infrared Camera And
Multi-Object Spectrometer (NICMOS)
An instrument that sees
objects in near-infrared wavelengths, which are slightly longer than the
wavelengths of visible light. (Human eyes cannot see infrared light.) NICMOS
is actually three cameras in one, each with different fields of view. Many
secrets about the birth of stars, solar systems, and galaxies are revealed
in infrared light, which can penetrate the interstellar gas and dust that
blocks visible light. In addition, light from the most distant objects in
the universe “shifts” into infrared wavelengths due to the universe’s
expansion. By studying objects and phenomena in this spectral region,
astronomers probe our universe’s past, present, and future; and learn how
galaxies, stars, and planetary systems form. Astronauts installed NICMOS
aboard the Hubble Space Telescope in February 1997 during the Second
Servicing Mission.
Nebula
A cloud of gas and dust
located between stars and/or surrounding stars. Nebulae are often places
where stars form.
Nebular Theory
The idea that our solar
system originated in a contracting, rotating cloud of gas that flattened to
form a disk as it contracted. According to this theory, the Sun formed at
the center of the disk and the planets formed in concentric bands of the
disk.
Neptune
The eighth planet and the
most distant giant gaseous planet in our solar system. The planet is 30
times the Earth’s distance from the Sun, and each orbit takes 165 Earth
years. Neptune is the fourth largest planet and has at least eight moons,
the largest of which is Triton. Neptune has a ring system, just like all the
giant gaseous outer planets.
Neutrino
A neutral, weakly
interacting elementary particle having a very tiny mass. Stars like the Sun
produce more than 200 trillion trillion trillion neutrinos every second.
Neutrinos from the Sun interact so weakly with other matter that they pass
straight through the Earth as if it weren’t there.
Neutrino Detector
A device designed to detect
neutrinos.
Neutron
A neutral (no electric
charge) elementary particle having slightly more mass than a proton and
residing in the nucleus of all atoms other than hydrogen.
Neutron Star
An extremely compact ball
of neutrons created from the central core of a star that collapsed under
gravity during a supernova explosion. Neutron stars are extremely dense:
they are only 10 kilometers or so in size, but have the mass of an average
star (usually about 1.5 times more massive than our Sun). A neutron star
that regularly emits pulses of radiation is known as a pulsar.
Newtonian reflector
A type of reflecting
telescope whose eyepiece is located along the side of the telescope. When
light enters the telescope, it reflects from the primary mirror to the
secondary mirror. The secondary mirror reflects the light at a right angle
through the side of the telescope to the eyepiece.
Non-Thermal Radiation
Radiation that is not
produced from heat energy — for example, radiation released when a very
fast-moving charged particle (such as an electron) interacts with a magnetic
force field. Because the electron’s velocity in this case is not related to
the gas temperature, this process has nothing to do with heat.
North Celestial Pole (NCP)
A direction determined by
the projection of the Earth’s North Pole onto the celestial sphere. It
corresponds to a declination of +90 degrees. The North Star, Polaris, sits
roughly at the NCP.
Nova
A binary star system
(consisting of a white dwarf and a companion star) that rapidly brightens,
then slowly fades back to normal.
Nuclear Transformation
The process by which an
atomic nucleus is transformed into another type of atomic nucleus. For
example, by removing an alpha particle from the nucleus, the element radium
is transformed into the element radon.
Nucleus
The core of a comet, made
up of ice, dirt, and rock.
Observable Universe
The portion of the entire
universe that can be seen from Earth.
Observation
In science, an observation
is a fact or occurrence that is noted and recorded. The Hubble Space
Telescope is a tool astronomers use to make observations of celestial
objects.
Observatory
A structure designed and
equipped for making astronomical observations. Observatories are located on
Earth and in space.
Oort Cloud
A vast spherical region in
the outer reaches of our solar system where a trillion long-period comets
(those with orbital periods greater than 200 years) reside. Comets from the
Oort Cloud come from all directions, often from as far away as 50,000
astronomical units.
Opacity
The degree to which light
is prevented from passing through an object or a substance. Opacity is the
opposite of transparency. As an object’s opacity increases, the amount of
light passing through it decreases. Glass, for example, is transparent and
most clouds are opaque.
Open Cluster
Also known as a galactic
cluster, an open cluster consists of numerous young stars that formed at the
same time within a large cloud of interstellar dust and gas. Open clusters
are located in the spiral arms or the disks of galaxies. The Pleiades is an
example of an open cluster.
Open Universe
A geometrical model of the
universe in which the overall structure of the universe extends infinitely
in all directions. The rules of geometry in an open universe are like those
that would apply on a saddle-shaped surface.
Opposition
The point at which a planet
appears opposite the Sun in our sky. During the Martian opposition, for
example, Mars and the Sun are on opposite sides of the Earth.
Optical Telescope
A telescope that gathers
and magnifies visible light. The two basic types of optical telescopes are
refracting (using lenses) and reflecting (using mirrors). The Hubble Space
Telescope is an example of a reflecting telescope.
Optician
A person who grinds lenses
and mirrors.
Optics
The science that deals with
the properties of light; in this case specifically dealing with the way
light changes directions when it is either refracted and dispersed by a lens
or reflected from a mirror.
Orbit
The act of traveling around
a celestial body; or the path followed by an object moving around a
celestial body. For example, the planets travel around, or orbit, the Sun
because the Sun’s gravity keeps them in their paths, or orbits.
Ozone Layer
A region in the upper
atmosphere that has high concentrations of ozone (triatomic oxygen, 03).
The ozone layer protects the Earth by absorbing the Sun’s high-energy
ultraviolet radiation.
Parabola vs. sphere
If cross-sections of a
spherical surface and a parabolic surface were made by slicing each surface
in half, these would be the shapes you would see.
Parallax
The apparent shift of an
object’s position when viewed from different locations. Parallax, also
called trigonometric parallax, is used to determine the distance to nearby
stars. As the Earth’s position changes during its yearly orbit around the
Sun, the apparent locations of nearby stars slightly shift. The stars’
distances can be calculated from those slight shifts with basic
trigonometric methods.
Parsec (PC)
A useful unit for measuring
the distances between astronomical objects, equal to 3.26 light-years and
3.085678 * 1013 kilometers, or approximately 18 trillion miles. A
parsec is also equivalent to 103,132 trips to the Sun
and back.
Perfect lens
The perfect lens does not
exist. Due to the nature of glass, light is dispersed when passing through
glass. In the case of convex lenses, red light bends less than blue light,
so the focal points are in different places, making the image blurry. A
single lens cannot counter this effect.
Period-Luminosity Law
A relationship that
describes how the luminosity or absolute brightness of a Cepheid variable
star depends on the period of time over which that brightness varies.
Periodic Comet
A comet in a closed,
elliptical orbit within our solar system. These comets typically have
orbital periods of less than 200 years. Many comets have orbits that keep
them in the inner solar system and allow their trajectories to be calculated
with great accuracy and precision. Perhaps the best-known periodic comet is
Halley’s comet, whose orbital period is 76 years.
Phases
Regularly occurring changes
in the appearance of the Moon or a planet. Phases of the Moon include new,
full, crescent, first quarter, gibbous, and third quarter.
Photoelectric Effect
The release of electrons
from a solid material when it is struck by radiant energy, such as visible
or ultraviolet light, X-rays, or gamma rays.
Photometer
An instrument that measures
the intensity of light. Astronomers use photometers to measure the
brightness of celestial objects.
Photometry
A technique for measuring
the brightness of celestial objects.
Photon
A packet of electromagnetic
energy, such as light. A photon is regarded as a charge-less, mass-less
particle having an indefinitely long lifetime.
Photosphere
The extremely thin, visible
surface layer of the Sun or a star. The average temperature of the Sun’s
photosphere is about 5800 Kelvin (about 10,000° F). Although the Sun is
completely made up of gas, its gas is so dense that we cannot see through
it. When we look at the Sun, we are seeing the photosphere.
Pickoff Mirror
One of four flat mirrors
inside the Hubble Space Telescope. Each mirror is tilted at a 45-degree
angle to the incoming light, diverting a small portion of it to the optical
detectors or to one of the fine guidance sensors.
Pixel
A light-sensitive picture
element on a charge-coupled device (CCD) or some other kind of digital
camera. A pixel is a tiny cell that, placed together with other pixels,
resembles the mesh on a screen door. The Hubble Space Telescope’s Wide Field
and Planetary Camera 2 has four CCDs, each containing 640,000 pixels. Each
pixel collects light from a celestial object and converts it into a number.
The numbers (all 2,560,000 of them) are sent to ground-based computers,
which convert them into an image. The greater number of pixels, the sharper
the image.
Planck Curve
The graphical
representation of the mathematical relationship between the frequency (or
wavelength) and intensity of radiation emitted from an object by virtue of
its heat energy.
Planet
An object that orbits a
star. Although smaller than stars, planets are relatively large and shine
only by reflected light. Planets are made up mostly of rock or gas, with a
small, solid core. In our solar system, the inner planets — Mercury, Venus,
Earth, and Mars — are the rocky objects, and most of the outer planets —
Jupiter, Saturn, Uranus, and Neptune — are the gaseous ones. Because Pluto
is made largely of ice, like a comet, some astronomers do not consider it a
true planet.
Planetary Nebula
An expanding shell of
glowing gas expelled by a star late in its life. Our Sun will create a
planetary nebula at the end
of its life.
Planetesimal
A small body of rock and/or
ice — under 10 kilometers (6 miles) across — formed during the early stages
of the solar system. Planetesimals are the building blocks of planets, but
many never combined to form large bodies. Asteroids are one example of
planetesimals.
Plasma
A substance composed of
charged particles, like ions and electrons, and possibly some neutral
particles. Our Sun is made of plasma. Overall, the charge of a plasma is
electrically neutral. Plasma is regarded as an additional state of matter
because its properties are different from those of solids, liquids, and
normal gases.
Plume
A column of material that
is shaped like a long feather.
Pluto
A dwarf planet whose small
size and composition of ice and rock resembles the comets in the Kuiper
Belt, a region beyond Neptune’s orbit where Pluto resides. Pluto was
considered the ninth planet until August 2006, when the International
Astronomical Union reclassified it as a dwarf planet. Pluto’s orbit is more
elliptical than those of the eight solar system planets.
Potential Energy
The energy of an object
owing to its position in a force field or its internal condition, as opposed
to kinetic energy, which depends on its motion. Examples of objects with
potential energy include a diver on a diving board and a coiled spring.
Primary Mirror
A large mirror in a
reflecting telescope that captures light from celestial objects and focuses
it toward a smaller secondary mirror. The primary mirror in the Hubble Space
Telescope measures 94.5 inches (2.4 meters) in diameter.
Primary lens
A large convex lens in a
refracting telescope that captures light from celestial objects and focuses
it toward the eyepiece.
Prime Focus
The location where light
reflected from the primary mirror of a reflecting telescope comes into
focus. Placing a secondary mirror in the light path allows the light to be
focused elsewhere, in a more convenient location for the science
instruments.
Primordial Nucleosynthesis
Element building that
occurred in the early universe when the nuclei of primordial matter collided
and fused with one another. Most of the helium in the universe was created
by this process.
Prism
A prism is usually a
triangular-shaped piece of glass used to refract, or bend, light. The shape
of the glass causes the light to disperse, or spread out, as it bends,
producing a rainbow of colors from the white light.
Prominence
An eruption of gas from the
chromosphere of a star. Solar prominences are visible as part of the corona
during a total solar eclipse. These eruptions occur above the Sun’s surface
(photosphere), where gases are suspended in a loop, apparently by magnetic
forces that arch upward into the solar corona and then return to the
surface.
Proper Motion
The apparent motion of a
star across the sky (not including a star’s parallax), arising from the
star’s velocity through space with respect to the Sun.
Protogalaxy
Matter that is beginning to
come together to form a galaxy. It is the precursor of a present-day galaxy
and is sometimes called a “baby galaxy.”
Proton
A positively charged
elementary particle that resides in the nucleus of every atom.
Proton-Proton Chain
A series of nuclear events
occurring in the core of a star whereby hydrogen nuclei (protons) are
converted into helium nuclei. This process releases energy.
Protoplanet
A small body that attracts
gas and dust as it orbits a young star. Eventually, it may form a planetary
body.
Protostar
A collection of
interstellar gas and dust whose gravitational pull is causing it to collapse
on itself and form a star.
Pulsar
A neutron star that emits
rapid and periodic pulses of radiation.
Quark
A basic building block of
protons, neutrons, and other elementary particles.
Quasar
The brightest type of
active galactic nucleus, believed to be powered by a supermassive black
hole. The word “quasar” is derived from quasi-stellar radio source, because
this type of object was first identified as a kind of radio source. Quasars
also are called quasi-stellar objects (QSOs). Thousands of quasars have been
observed, all at extreme distances from our galaxy.
RADAR (Radio Detection and
Ranging)
A method of detecting,
locating, or tracking an object by using beamed, reflected, and timed radio
waves. RADAR also refers to the electronic equipment that uses radio waves
to detect, locate, and track objects.
Radial Motion
The component of an
object’s velocity (speed and direction) as measured along an observer’s line
of sight.
Radiation
The process by which
electromagnetic energy moves through space as vibrations in electric and
magnetic fields. This term also refers to radiant energy and other forms of
electromagnetic radiation, such as gamma rays and X-rays.
Radiative Process
An event involving the
emission or absorption of radiation. For example, a hydrogen atom that
absorbs a photon of light converts the energy of that radiation into
electrical potential energy.
Radio Waves
Radiation with the longest
wavelengths and smallest frequencies and energies in the electromagnetic
spectrum.
Radioactivity
The spontaneous decay of
certain rare, unstable, atomic nuclei into more stable atomic nuclei. A
natural by-product of this process is the release of energy.
Reaction Wheel
One of four spinning
flywheels aboard the Hubble Space Telescope. The flywheels work together to
make the observatory rotate either more rapidly or less rapidly toward a new
target.
Receiver
The part of the radio
telescope that detects long wavelength electromagnetic radiation and
converts it to an electrical signal so that we can sense it.
Recessional Velocity
The velocity at which an
object moves away from an observer. The recessional velocity of a distant
galaxy is proportional to its distance from Earth. Therefore, the greater
the recessional velocity, the more distant the object.
Red Giant Star
An old, bright star, much
larger and cooler than the Sun. Betelgeuse (alpha Orionis) is an example of
a red giant.
Redshift
The lengthening of a light
wave from an object that is moving away from an observer. For example, when
a galaxy is traveling away from Earth, its light shifts to the red end of
the electromagnetic spectrum.
Reflection
Reflection occurs when
light changes direction as a result of "bouncing off" a surface like a
mirror.
Reflector
A type of telescope, also
known as a reflecting telescope, that uses one or more polished, curved
mirrors to gather light and reflect it to a focal point.
Reflector (Reflecting
telescope)
A type of telescope, also
known as a reflecting telescope, that uses one or more polished, curved
mirrors to gather light and reflect it to a focal point.
Refraction
Refraction is the bending
of light as it passes from one substance to another. Here, the light ray
passes from air to glass and back to air. The bending is caused by the
differences in density between the two substances.
Refractor
A telescope, also known as
a refracting telescope, that uses a transparent lens to gather light and
bend it to a focus.
Refractor (Refracting
telescopes)
A type of telescope, also
known as a refracting telescope, that uses a transparent convex lens to
gather the light and bend it to a focal point.
Regolith
The layer of loose rock
resting on bedrock (sometimes called mantle rock), found on the Earth, the
Moon, or a planet. Regolith is made up of soils, sediments, weathered rock,
and hard, near-surface crusts. On the surface of the Moon, regolith is a
fine rocky layer of fragmentary debris (or dust) produced mainly by
meteoroid collisions.
Relativity
A theory of physics that
describes the dynamical behavior of matter and energy. The consequences of
relativity can be quite strange at very high velocities and very high
densities. A direct result of the theory of relativity is the equation
E = mc2, which expresses a relationship between mass (m), energy
(E), and the speed of light (c).
Resolution (Resolving
power)
A measure of the smallest
separation at which a telescope can observe two neighboring objects as two
separate objects.
Resolve
The ability of a telescope
to distinguish objects that are very close to each other as two separate
objects.
Revolution
The orbital motion of one
object around another. The Earth revolves around the Sun in one year. The
moon revolves around the Earth in approximately 28 days.
Right Ascension (RA)
A coordinate used by
astronomers to locate stars and other celestial objects in the sky. Right
ascension is comparable to longitude, but it is measured in hours, minutes,
and seconds because the entire sky appears to pass overhead over a period of
24 hours. The zero hour corresponds to the apparent location of the Sun with
respect to the stars on the day of the vernal (spring) equinox
(approximately March 21).
Rille
A long, narrow depression
on the Moon's surface. A rille can be straight, have a sweeping arc, or
meander, with many curves going in random directions.
Robotic Optical Transient
Search Experiment (ROTSE)
A terrestrial telescope
that searches for the optical counterparts of gamma-ray bursts. When
orbiting satellites detect a gamma-ray burst, ROTSE begins searching for its
visible-light afterglow. ROTSE-I (an array of four electronic telephoto
cameras) and ROTSE-II (a set of identical telescopes) are located in Los
Alamos, New Mexico.
Roche Limit
The smallest distance at
which two celestial bodies can remain in a stable orbit around each other
without one of them being torn apart by tidal forces. The distance depends
on the densities of the two bodies and their orbit around
each other.
Rocky Planet
A planet located in the
inner solar system and made up mostly of rock. The rocky planets are
Mercury, Venus, Earth, and Mars. This group is also known as terrestrial
planets.
Satellite
A man-made object that
orbits Earth, the Moon, or another celestial object.
Saturn
The sixth planet in the
solar system, noted for its obvious ring structure. Saturn is almost ten
times the Earth’s distance from the Sun. The planet completes a circuit
around the Sun in about 30 Earth years. Saturn is the second largest and the
least dense planet in our solar system. The planet has more than 21 moons,
including Titan, the second largest known moon in our solar system.
Schwarzchild Radius
The distance from the
“center” of a black hole to its “edge” (called an event horizon). If the
Earth became a black hole, all of its mass would be squeezed into a sphere
with a Schwarzschild radius of 0.03 cm, about the size of a bacterium.
Scintillation
A flash of light produced
when gamma rays strike a certain material. The high energy of gamma rays
makes them hard to capture but they can be detected using scintillation.
Secondary Atmosphere
A gas or gases, such as
helium, that a planet discharges from its interior after having lost its
primary or primordial atmosphere.
Secondary mirror
A small mirror in a
reflecting telescope that redirects light from the larger primary mirror
toward the light-sensitive scientific instruments. In a Cassegrain-type
telescope like the Hubble Space Telescope, the secondary mirror is slightly
convex and directs light from the primary mirror back through a hole in the
center of the primary mirror.
Seismic Wave
The transfer of energy
throughout a celestial object, such as a planet, resulting from an external
impact or an internal event. On Earth, seismic waves are generated primarily
by earthquakes.
Seyfert Galaxy
A galaxy characterized by a
moderately bright, compact active galactic nucleus, presumably powered by a
black hole.
Shock Wave
A high-pressure wave that
travels at supersonic speeds. Shock waves are usually produced by an
explosion.
Shock Wave
A high-pressure wave that
travels at supersonic speeds. Shock waves are usually produced by an
explosion.
Short-Period Comet
A comet that orbits mainly
in the inner solar system. Short-period comets usually orbit the Sun in less
than 200 years. Halley’s comet is an example of a short-period comet.
Singularity
A black hole’s center,
where the matter is thought to be infinitely dense, the volume is infinitely
small, and the force of gravity is infinitely large.
Solar Arrays
Two rigid, wing-like arrays
of solar panels that convert sunlight directly into electricity to operate
the Hubble Space Telescope’s scientific instruments, computers, and radio
transmitters. Some of the energy generated is stored in onboard batteries so
the telescope can operate while in Earth’s shadow (which is about 36 minutes
out of each 97-minute orbit). The solar arrays are designed for replacement
by visiting astronauts during servicing missions.
Solar Constant
The average amount of solar
radiation reaching a planet; usually expressed in watts (energy per unit
time) per square meter. For Earth, the solar constant equals 1,372 W/m2.
Each planet has a unique solar constant depending on its distance from the
Sun.
Solar Cycle
The periodic changing of
the Sun’s magnetic field, which determines the number of sunspots and the
amount of particles emitted in the solar wind. The period of the cycle is
about 11 years.
Solar Eclipse
A phenomenon in which the
Moon’s disk passes in front of the Sun, blocking sunlight. A total eclipse
occurs when the Moon completely obscures the Sun’s disk, leaving only the
solar corona visible. A solar eclipse can only occur during a new phase of
the Moon.
Solar Maximum
The midpoint in the solar
cycle where the amount of sunspot activity and the output of cosmic
particles and solar radiation is highest.
Solar Minimum
The beginning and the end
of a sunspot cycle when only a few sunspots are usually observed, and the
output of particles and radiation is normal.
Solar System
The Sun and its surrounding
matter, including asteroids, comets, planets and moons, held together by the
Sun’s gravitational influence.
Solar Wind
Streams of charged
particles flowing from the Sun at millions of kilometers an hour. The
composition of this high-speed solar wind may vary, but it always streams
away from the Sun. The solar wind is responsible for the Northern and
Southern Lights on Earth and causes the tails of comets to point away from
the Sun.
Solar panels
Two rigid, wing-like
structures that convert sunlight directly into electricity to operate a
space telescope’s scientific instruments, computers, and radio transmitters.
Some of the energy generated is stored in onboard batteries so the telescope
can operate while in Earth’s shadow.
Solar telescope
A special reflecting
telescope designed to study our closest star, the Sun. Solar telescopes
differ from normal telescopes in that they are stationary and use small
tracking mirrors to direct sunlight into the primary mirror. This is
necessary because the Sun appears to move across the sky due to Earth’s
rotation.
South Celestial Pole (SCP)
A direction determined by
the projection of the Earth's South Pole onto the celestial sphere. The SCP
is exactly 180 degrees from the North Celestial Pole and corresponds to a
declination of –90 degrees.
Southern Hemisphere
Half of a spherical or
roughly spherical body; for example, the Southern Hemisphere of Earth is the
half below the
equator.
Space Infrared Telescope
Facility (SIRTF)
A space-borne infrared
telescope that will study planets, comets, stars, galaxies, and other
celestial objects. NASA plans to launch SIRTF in December 2002 on a Delta
rocket. SIRTF represents the fourth and final satellite in NASA’s Great
Observatories program, which includes the Hubble Space Telescope and the
Chandra X-Ray Observatory.
Space Shuttle
A reusable U.S. spacecraft
operated by astronauts and used to transport cargo, such as satellites, into
space. The spacecraft uses rockets to launch into space, but it lands like
an airplane. A space shuttle carried the Hubble Space Telescope into space
in 1990. Astronauts aboard subsequent space shuttles have visited the
telescope to service it.
Space Telescope Imaging
Spectrograph (STIS)
An instrument that acts
like a prism to separate light from the cosmos into its component colors,
providing a wavelength “fingerprint” of the object being observed. The
information yields clues about an object’s temperature, chemical
composition, density, and motion. Spectrographic observations also reveal
changes in celestial objects as the universe evolves. Astronauts installed
STIS aboard the Hubble Space Telescope in February 1997 during the Second
Servicing Mission. STIS spans ultraviolet, visible, and near-infrared
wavelengths. The spectrograph can sample 500 points along a celestial object
simultaneously.
Space Telescope Science
Institute (STScI)
The astronomical research
center responsible for operating the Hubble Space Telescope as an
international scientific observatory. Located in Baltimore, Maryland, STScI
is managed by AURA (Association of Universities for Research in Astronomy)
under contract to the National Aeronautics and Space Administration (NASA)
and the National Science Foundation (NSF).
Spacetime
The four-dimensional
coordinate system (three dimensions of space and one of time) in which
physical events are located.
Spectral Class (Spectral
Type)
A classification scheme
that groups stars according to their surface temperatures and spectral
features.
Spectral Line
In a spectrum, an emission
(bright) or absorption (dark) at a specific frequency or wavelength.
Spectrograph (Spectrometer)
An instrument that spreads
electromagnetic radiation into its component frequencies and wavelengths for
detailed study. A spectrograph is similar to a prism, which spreads white
light into a continuous rainbow.
Spectrograph
(Spectrometer/spectroscope)
An instrument that spreads
electromagnetic radiation into its component frequencies and wavelengths for
detailed study. A spectrograph is similar to a prism, which spreads white
light into a continuous rainbow.
Spectroheliograph
An instrument used in solar
telescopes to photograph the Sun in a single wavelength of light. Different
wavelengths reveal different features of the Sun’s surface.
Spectroscopy
The study and
interpretation of a celestial object’s electromagnetic spectrum. A
spectrograph or spectrometer is used to analyze an object’s electromagnetic
spectrum.
Spectrum
The entire range of
electromagnetic rays from the longest radio waves to the shortest gamma
rays. Arranged from longest to shortest wavelengths, the spectrum of
electromagnetic radiation includes radio waves, microwaves, infrared light,
visible light, ultraviolet light, X-rays and gamma rays.
Speed Of Light (c)
The speed at which light
(photons) travels through empty space is roughly 3 * 108 meters
per second or 300 million meters per second.
Spherical aberration
Spherical aberration is an
optical defect of a lens or mirror caused by its rounded shape. Spherical
lenses and mirrors produce a distorted (blurry) image.
Spherical aberration in
lenses
The shape of a spherical
lens causes a problem called spherical aberration.
In spherical aberration,
parallel light rays that pass through the central region of the lens focus
farther away than light rays that pass through the edges of the lens. The
result is many focal points, which produce a blurry image. To get a clear
image, all rays need to focus at the same point.
Spherical aberration in
mirrors
The shape of a spherical
telescope mirror causes a problem called spherical aberration.
In spherical aberration,
parallel light rays that bounce off the central region of a spherical mirror
focus farther away than light rays that bounce off the edges. The result is
many focal points, which produce a blurry image. To get a clear image, all
rays need to focus at the same point.
Spiral Arms
A pinwheel structure,
composed of dust, gas, and young stars, that winds its way out from the core
of a normal spiral galaxy and from the ends of the bar in a barred spiral
galaxy.
Spiral Galaxy
A spiral-shaped system of
stars, dust, and gas clouds. A typical spiral galaxy has a spherical central
bulge of older stars surrounded by a flattened galactic disk that contains a
spiral pattern of young, hot stars, as well as interstellar matter.
Sprites
Gamma-ray flashes produced
in Earth’s atmosphere by severe lightning storms and upper atmospheric
events.
Standard Candle
An object whose properties
allow us to measure large distances through space. The absolute brightness
of a standard candle can be determined without a measurement of its apparent
brightness. Comparing the absolute brightness of a standard candle to its
apparent brightness therefore allows us to measure its distance. For
example, the distinct variations of Cepheid variable stars in other galaxies
tell us their absolute brightness. By accurately measuring the apparent
brightness of these stars, astronomers can precisely determine the distance
to the galaxy in which they reside.
Star
A huge ball of gas held
together by gravity. The central core of a star is extremely hot and
produces energy. Some of this energy is released as visible light, which
makes the star glow. Stars come in different sizes, colors, and
temperatures. Our Sun, the center of our solar system, is a yellow star of
average temperature and size.
Star Cluster
A group of stars born at
almost the same time and place, capable of remaining together for billions
of years because of their mutual gravitational attraction.
Starburst Galaxy
A galaxy undergoing an
extremely high rate of star formation. Starburst galaxies contain massive,
deeply embedded stars that are among the youngest stars observed.
Static
Random noise in a radio
receiver. It can also be heard in telephone lines and cell phones.
Stellar Black Hole
A black hole formed from
the death of a massive star during a supernova explosion. A stellar black
hole, much like a supermassive black hole, feeds off of nearby material — in
this case, the dead star. As it gains mass, its gravitational field
increases.
Stellar Evolution
The process of change that
occurs during a star’s lifetime from its birth to its death.
Stellar Parallax
The apparent change in the
position of a nearby star when observed from Earth due to our planet’s
yearly orbit around the Sun. This method allows astronomers to calculate
distances to stars that are less than 100 parsecs from Earth.
Stellar nursery
A region in space where
stars are forming from a cloud of gas and dust.
Strong Force
The force that binds
protons and neutrons within atomic nuclei and is effective only at distances
less than 10—13 centimeters.
Sun
The star at the center of
our solar system. An average star in terms of size and mass, the Sun is a
yellow dwarf of spectral type G2. It is about 5 billion years old, contains
2 * 1030 kilograms of material, and has a diameter more than 100
times that of Earth.
Sunspot
A region on the Sun’s
photosphere that is cooler and darker than the surrounding material.
Sunspots often appear in pairs or groups with specific magnetic polarities
that indicate electromagnetic origins.
Sunspot Cycle
The change in strength of
the Sun’s magnetic field, which determines the number of sunspots and the
amount of particles emitted in the solar wind. The period of the cycle is
about 11 years.
Sunspots
A sunspot is a region on
the Sun’s photosphere that is cooler and darker than the surrounding
material. Sunspots often appear in pairs or groups with specific magnetic
polarities that indicate electromagnetic origins.
Supermassive Black Hole
A black hole possessing as
much mass as a million or a billion stars. Supermassive black holes reside
in the centers of galaxies and are the engines that power active galactic
nuclei and quasars.
Supernova
The explosive death of a
massive star whose energy output causes its expanding gases to glow brightly
for weeks or months. A supernova remnant is the glowing, expanding gaseous
remains of a supernova explosion.
Supernova Remnant
The glowing, expanding
gaseous remains of a supernova explosion.
T-Tauri Star
A class of very young,
flaring stars on the verge of becoming normal stars fueled by nuclear
fusion.
Tail
A tail is made up of dust
and gas from a comet’s coma. A tail forms when the solar wind separates dust
and gas from the coma, pushing it outward and away from the Sun in either a
slightly curved path (for dust) or a straight path (for gas).
Telescope
An instrument used to
observe distant objects by collecting and focusing their electromagnetic
radiation. Telescopes are usually designed to collect light in a specific
wavelength range. Examples include optical telescopes that observe visible
light and radio telescopes that detect radio waves.
Temperature
A measure of the amount of
heat energy in a substance, such as air, a star, or the human body. Because
heat energy corresponds to motions and vibrations of molecules, temperature
provides information about the amount of molecular motion occurring in a
substance.
Terabyte
A measure of computer data
storage capacity equal to approximately a thousand billion bytes (or a
thousand gigabytes). In computer language, a byte of information represents
a letter or digit. So, a thousand billion bytes is equal to a thousand
billion letters.
Terrestrial
Planets whose density and
chemical makeup are similar to those of Earth.
Terrestrial Planets
The four planets of the
inner solar system (Mercury, Venus, Earth, and Mars) are called terrestrial
planets because
they are made up mostly of
rock.
Theory
An accepted idea used to
explain nature. Theories not only explain an observed event, they can also
be used to predict what will happen. Sometimes, an idea that is really a
hypothesis is incorrectly called a theory. A true scientific theory is a
hypothesis that makes predictions. Those predictions have been tested and
have proven to be accurate.
Thermal Radiation
Radiation released by
virtue of an object’s heat, namely, the transfer of heat energy into the
radiative energy of electromagnetic waves. Examples of the
rmal radiation are
sunlight, the orange glow of an electric range, and the light
from in incandescent light
bulb.
Titanium Oxides
Minerals composed of oxygen
and the metal titanium. Titanium oxides frequently contain other metals. One
such titanium oxide is the mineral ilmenite, which contains titanium,
oxygen, and iron. Ilmenite is found in both lunar rock and Earth rocks.
Tracking and Data Relay
Satellite System (TDRSS)
A network of four
communication satellites used to relay data and commands to and from U.S.
spacecraft, including the Hubble Space Telescope. The Goddard Space Flight
Center provides the day-to-day management and operations of TDRSS, the first
space-based global tracking system.
Triton
The largest of Neptune’s
satellites. Triton has an atmosphere and is roughly the size of Earth's
moon. It has an “ice cap” of frozen nitrogen and methane with “ice
volcanoes” that erupt liquid nitrogen, dust, and methane compounds from
beneath its frozen surface.
Turbulence
Unstable and disorderly
motion, as when a smooth, flowing stream becomes a churning rapid.
Ultraviolet (UV)
Electromagnetic radiation
with shorter wavelengths and higher energies and frequencies than visible
light. UV light is lower in frequency than X-rays.
Ultraviolet (UV) Light
The part of the
electromagnetic spectrum that has slightly higher energy than visible light,
and is not visible to the human eye. Just as there are high-pitched sounds
that cannot be heard, there is high-energy light that cannot be seen. Too
much ultraviolet light causes sunburns.
Universe
The totality of space and
time, along with all the matter and energy in it. Current theories assert
that the universe is expanding and that all its matter and energy was
created during the Big Bang.
Uranus
The third largest planet in
the solar system and the seventh from the Sun. Uranus is 19 times the
Earth’s distance from the Sun and completes a circuit around the Sun in
about 84 Earth years. This gaseous, giant outer planet has a visible ring
system and over 20 moons, the largest of which is Titania. Uranus is tipped
on its side, with a rotation axis in nearly the same plane as its orbit.
Van Allen Belt
A region containing charged
particles trapped in the Earth’s magnetic force field (magnetosphere). The
belt’s lower boundary begins at about 800 kilometers (496 miles) above the
Earth’s surface and extends thousands of kilometers into space.
Variable Star
A star whose luminosity
(brightness) changes with time.
Vela Satellite
Launched by the U.S. in the
1960s to monitor the Limited Nuclear Test Ban Treaty. The satellite’s
mission was to detect the gamma rays produced during nuclear blasts.
Although not intended for astronomical studies, the Vela satellite provided
useful celestial data, detecting an unexpected blast of cosmic gamma
radiation in 1967. The satellite discovered several other gamma-rays bursts
during the years of the Vela project, which ceased operation in 1979.
Venus
An inner, terrestrial
(rocky) planet that is slightly smaller than Earth. Located between the
orbits of Mercury and Earth, Venus has a very thick atmosphere that is
covered by a layer of clouds that produce a “greenhouse effect” on the
planet. Venus’s surface temperature is roughly 480° C (900° F), making it
the hottest planet in the solar system.
Very Large Array (VLA)
One of the world’s premier
radio observatories, consisting of 27 antennas arranged in a huge “Y”
pattern. The VLA spans up to 22 miles (36 km) across, which is roughly one
and a half times the size of Washington, D.C. Each antenna is 81 feet (25
meters) in diameter. Located in Socorro, New Mexico, the telescopes work in
tandem to produce a sharper image than any single telescope could record.
Visible Light
The part of the
electromagnetic spectrum that human eyes can detect; also known as the
visible spectrum. The colors of the rainbow make up visible light. Blue
light has more energy than red light.
Wave
A vibration in some media
that transfers energy from one place to another. Sound waves are vibrations
passing in air. Light waves are vibrations in electromagnetic fields.
Wavelength
The distance between two
wave crests. Radio waves can have lengths of several feet; the wavelengths
of X-rays are roughly the size of atoms.
Wavelength and frequency
Light is measured by its
wavelength (in nanometers) or frequency (in Hertz).
One wavelength
equals the distance between two successive wave crests or troughs.
Weak Force
The force that governs the
change of one kind of elementary particle into another. This force is
associated with radioactive processes that involve neutrons.
White Dwarf Star
The hot, compact remains of
a low-mass star like our Sun that has exhausted its sources of fuel for
thermonuclear fusion. White dwarf stars are generally about the size of the
Earth.
Wide Field / Planetary
Camera (WF/PC)
A collection of eight
separate, yet interconnected, cameras originally used as the main optical
instrument on the Hubble Space Telescope. Four cameras were used in tandem
to observe in either wide-field, low-resolution mode or narrow-field,
high-resolution (“planetary”) mode. The Wide Field and Planetary Camera 2
replaced the WF/PC during the December 1993 servicing mission.
X-Rays
Electromagnetic radiation
with very short wavelengths and very high energies and frequencies. X-rays
fall between gamma rays and ultraviolet radiation; also called X-radiation
or Roentgen ray.
X-power
Identifies the magnifying
power of a lens or mirror. For example, a 50-power telescope makes the image
50 times larger than it is when viewed without the telescope.
X-ray sources
Celestial objects that give
off X-rays. These exotic objects are producing very energetic radiation and
include black holes, neutron stars (pulsars), supernovae remnants, and the
centers of galaxies.
X-ray telescope
A special telescope used to
detect X-rays – high-energy electromagnetic radiation. The high energy of
X-rays means they will go through rather than bounce off a “normal”
telescope mirror. Instead, the mirrors are arranged so the X-rays skip
across them much like a stone skips across the surface of a lake.
Zenith
The point on the celestial
sphere that is directly above the observer. Holding a balloon overhead
places the balloon at your zenith. Although celestial objects appear to rise
and set as they move across the sky, they rarely reach the zenith point.