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Below are the definitions for some of
the most common terms used in space exploration and astronomy. You can click on
any letter below to jump directly to that section of the listing. You can also click on any
underlined word to see its definition.
A
Absolute Magnitude
A scale for measuring the actual brightness of a celestial object without
accounting for the distance of the object. Absolute magnitude measures how bright an
object would appear if it were exactly 10 parsecs (about 33 light years) away from Earth. On this scale, the Sun has an
absolute magnitude of +4.8 while it has an apparent
magnitude of -26.7 because it is so close.
Absolute
Zero
The temperature at which the motion of all atoms and molecules stops and
no heat is given off. Absolute zero is reached at 0 degrees Kelvin
or -273.16 degrees Celsius.
Ablation
A process by where the atmosphere melts away and
removes the surface material of an incoming meteorite.
Accretion
The process by where dust and gas accumulated into larger bodies such as stars and planets.
Achondrite
A stone meteorite which contains no chondrules.
Albedo
The reflective property of a non-luminous object. A perfect mirror
would have an albedo of 100% while a black hole would have an
albedo of 0%.
Albedo
Feature
A dark or light marking on the surface of an object that may or may not be
a geological or topographical feature.
Altitude
The angular distance of an object above the horizon.
Antimatter
Matter consisting of particles with charges opposite that of ordinary
matter. In antimatter, protons have a negative charge while electrons have a
positive charge.
Antipodal
Point
A point that is on the direct opposite side of a planet.
Apastron
The point of greatest separation of two stars, such as
in a binary star system.
Aperture
The size of the opening through which light passes in an optical
instrument such as a camera or telescope. A higher number
represents a smaller opening while a lower number represents a larger opening.
Aphelion
The point in the orbit of a planet
or other celestial body where it is farthest from the Sun.
Apogee
The point in the orbit of the Moon or other satellite where it is farthest from the Earth.
Apparent Magnitude
The apparent brightness of an object in the sky as it appears to an
observer on Earth. Bright objects have a low apparent magnitude while dim objects
will have a higher apparent magnitude.
Asteroid
A small planetary body in orbit around the Sun,
larger than a meteoroid but smaller than a planet. Most
asteroids can be found in a belt between the orbits of Mars and Jupiter. The orbits
of some asteroids take them close to the Sun, which also takes them across the paths of
the planets.
Astrochemistry
The branch of science that explores the chemical interactions between dust
and gas interspersed between the stars.
Astronomical Unit (AU)
A unit of measure equal to the average distance between the Earth and the
Sun, approximately 93 million miles.
Atmosphere
A layer of gases surrounding a planet, moon, or star. The Earth's atmosphere is 120 miles thick and is composed
mainly of nitrogen, oxygen, carbon dioxide, and a few other trace gases.
Aurora
A glow in a planet's ionosphere caused by the
interaction between the planet's magnetic field and charged
particles from the Sun. This phenomenon is known as the Aurora Borealis in the Earth's
northern hemisphere and the Aurora Australis in the Earth's Southern Hemisphere.
Aurora Australis
Also known as the southern lights, this is an atmospheric phenomenon that
displays a diffuse glow in the sky in the southern hemisphere. It is caused by
charged particles from the Sun as they interact with the Earth's magnetic field. Known as
the Aurora Borealis in the northern hemisphere.
Aurora
Borealis
Also known as the northern lights, this is an atmospheric phenomenon that
displays a diffuse glow in the sky in the northern hemisphere. It is caused by
charged particles from the Sun as they interact with the Earth's magnetic field. Known as
the Aurora Australis in the southern hemisphere.
Axis
Also known as the poles, this is an imaginary line through the center of rotation of an object.
Azimuth
The angular distance of an object around or parallel to the horizon from a
predefined zero point.
Bar
A unit of measure of atmospheric pressure. One bar is equal to 0.987
atmospheres, 1.02 kg/cm2, 100 kilopascal, and 14.5 lbs/square inch.
Big
Bang
The theory that suggests that the universe was formed from a single point
in space during a cataclysmic explosion about 18 billion years ago. The force of the
explosion accounts for the current expansion of the universe.
Binary
A system of two stars that revolve around a common
center of gravity.
Black
Hole
The collapsed core of a massive star. Stars that
are very massive will collapse under their own gravity when their
fuel is exhausted. The collapse continues until all matter is crushed out of
existence into what is known as a singularity. The
gravitational pull is so strong that not even light can escape.
Blueshift
A shift in the lines of an object's spectrum
toward the blue end. Blueshift indicates that an object is moving toward the
observer. The larger the blueshift, the faster the object is moving.
Bolide
A term used to describe an exceptionally bright meteor.
Bolides typically will produce a sonic boom.
Caldera
A type of volcanic crater that is extremely large, usually formed by the
collapse of a volcanic cone or by a violent volcanic explosion. Crater Lake is one
example of a caldera on Earth.
Catena
A series or chain of craters.
Cavus
A hollow, irregular depression.
Celestial Equator
An imaginary line that divides the celestial
sphere into a northern and southern hemisphere.
Celestial
Poles
The North and South poles of the celestial
sphere.
Celestial Sphere
An imaginary sphere around the Earth on which the stars
and planets appear to be positioned.
Cepheid Variable
This is a variable star whose light pulsates
in a regular cycle. The period of fluctuation is linked to the brightness of the
star. Brighter cepheids will have a longer period.
Chaos
A distinctive area of broken terrain.
Chasma
Another name used to describe a canyon.
Chondrite
A meteorite which contains chondrules.
Chondrule
Small, glassy spheres commonly found in meteorites.
Chromosphere
The part of the Sun's atmosphere just above the
surface.
Circumpolar Star
A star that never sets but always stays above the
horizon. This depends on the location of the observer. The further South you go
the fewer stars will be circumpolar. Polaris, the North Star, is circumpolar in most
of the northern hemisphere.
Coma
An area of dust or gas surrounding the nucleus of a comet.
Comet
A gigantic ball of ice and rock that orbit the Sun in a
highly eccentric orbit. Some comets have an orbit that brings them close to the Sun where
they form a long tail of gas and dust as they are heated by the Sun's rays.
Conjunction
An event that occurs when two or more celestial objects appear close close
together in the sky.
Constellation
A grouping of stars that make an imaginary picture in
the sky.
Corona
The outer part of the Sun's atmosphere. The
corona is visible from Earth during a total solar eclipse. It
is the bright glow seen in most solar eclipse photos.
Cosmic
Ray
Atomic nuclei (mostly protons) that are observed to strike the Earth's atmosphere with extremely high amounts of energy.
Cosmic
String
A tubelike configuration of energy that is believed to have existed in the
early universe. A cosmic string would have a thickness smaller than a trillionth of an
inch but its length would extend from one end of the visible universe to the other.
Cosmogony
The study of celestial systems, including the solar system, stars, galaxies, and galactic clusters.
Cosmology
A branch of science that deals with studying the origin, structure, and
nature of the universe.
Crater
A bowl-shaped depression formed by the impact of an asteroid
or meteoroid. Also the depression around the opening of a
volcano.
Dark
Matter
A term used to describe matter in the universe that
cannot be seen, but can be detected by its gravitational effects on other bodies.
Declination
The angular distance of an object in the sky from the celestial equator.
Density
The amount of matter contained within a given
volume. Density is measured in grams per cubic centimeter (or kilograms per liter). The
density of water is 1.0, iron is 7.9, and lead is 11.3.
Disk
The surface of the Sun or other celestial body projected against the sky.
Double
Asteroid
Two asteroids that revolve around each other and
are held together by the gravity between them. Also called a binary
asteroid.
Doppler
Effect
The apparent change in wavelength of sound or
light emitted by an object in relation to an observer's position. An object approaching
the observer will have a shorter wavelength (blue) while an object moving away will have a
longer (red) wavelength. The Doppler effect can be used to estimate an object's speed and
direction.
Double
Star
A grouping of two stars. This grouping can be
apparent, where the stars seem close together, or physical, such as a binary
system.
Eccentricity
The measure of how an object's orbit differs from a
perfect circle. Eccentricity defines the shape of an object's orbit.
Eclipse
The total or partial blocking of one celestial body by another.
Eclipsing Binary
A binary system where one object passes in front of
the other, cutting off some or all of its light.
Ecliptic
An imaginary line in the sky traced by the Sun as it moves in its yearly
path through the sky.
Ejecta
Material from beneath the surface of a body such as a moon or planet that is ejected by an impact such as a meteor
and distributed around the surface. Ejecta usually appears as a lighter color than the
surrounding surface.
Electromagnetic Radiation
Another term for light. Light waves created by fluctuations of electric
and magnetic fields in space.
Electromagnetic Spectrum
The full range of frequencies, from radio waves to gamma waves, that
characterizes light.
Ellipse
An ellipse is an oval shape. Johannes Kepler discovered that the orbits of
the planets were elliptical in shape rather than circular.
Elliptical Galaxy
A galaxy whose structure shaped like an ellipse and
is smooth and lacks complex structures such as spiral arms.
Elongation
The angular distance of a planetary body from the Sun as seen from Earth.
A planet at greatest eastern elongation is seen in the evening sky and a planet at
greatest western elongation will be seen in the morning sky.
Ephemeris
A table of data arranged by date. Ephemeris tables are typically to list
the positions of the Sun, Moon, planets and other solar system objects.
Equinox
The two points at which the Sun crosses the celestial
equator in its yearly path in the sky. The equinoxes occur on or near March 21 and
September 22. The equinoxes signal the start of the Spring and Autumn seasons.
Escape
Velocity
The speed required for an object to escape the gravitational pull of a planet or other body.
Event
Horizon
The invisible boundary around a black hole past
which nothing can escape the gravitational pull - not even light.
Evolved
Star
A star that is near the end of its life cycle where
most of its fuel has been used up. At this point the star begins to loose mass
in the form of stellar wind.
Extinction
The apparent dimming of star or planet
when low on the horizon due to absorption by the Earth's atmosphere.
Extragalactic
A term that means outside of or beyond our own galaxy.
Extraterrestrial
A term used to describe anything that does not originate on Earth.
Eyepiece
The lens at the viewing end of a telescope. The
eyepiece is responsible for enlarging the image captured by the instrument.
Eyepieces are available in different powers, yielding differing amounts of
magnification.
Faculae
Bright patches that are visible on the Sun's surface, or photosphere.
Filament
A strand of cool gas suspended over the photosphere
by magnetic fields, which appears dark as seen against the disk of the Sun.
Finder
A small, wide-field telescope attached to a
larger telescope. The finder is used to help point the larger telescope to the
desired viewing location.
Fireball
An extremely bright meteor. Also known as bolides, fireballs can be several times brighter than the full Moon.
Some can even be accompanied by a sonic boom.
Flare
Star
A faint red star that appears to change in brightness due to explosions on
its surface.
Galactic
Halo
The name given to the spherical region surrounding the center, or nucleus
of a galaxy.
Galactic Nucleus
A tight concentration of stars and gas found at the
innermost regions of a galaxy. Astronomers now believe that massive black holes may exist in the center of many galaxies.
Galaxy
A large grouping of stars. Galaxies are found in
a variety of sizes and shapes. Our own Milky Way galaxy is spiral in shape and
contains several billion stars. Some galaxies are so distant the their light takes
millions of years to reach the Earth.
Galilean
Moons
The name given to Jupiter's four largest moons, Io, Europa, Callisto &
Ganymede. They were discovered independently by Galileo Galilei and Simon Marius.
Gamma-ray
The highest energy, shortest wavelength form of electromagnetic radiation.
Geosynchronous Orbit
An orbit in which a satellite's
orbital velocity is matched to the rotational velocity of the planet.
A spacecraft in geosynchronous orbit appears to hang motionless above one position of a
planet's surface.
Giant Molecular Cloud (GMC)
Massive clouds of gas in interstellar space composed primarily of hydrogen molecules. These clouds have enough mass to produce
thousands of stars and are frequently the sites of new star formation.
Globular Cluster
A tight, spherical grouping of hundreds of thousands of stars. Globular
clusters are composed of older stars, and are usually found around the central regions of
a galaxy.
Granulation
A pattern of small cells that can be seen on the surface of the Sun. They
are caused by the convective motions of the hot gases inside the Sun.
Gravitational
Lens
A concentration of matter such as a galaxy or
cluster of galaxies that bends light rays from a background object. Gravitational lensing
results in duplicate images of distant objects.
Gravity
A mutual physical force of nature that causes two bodies to attract each
other.
Greenhouse Effect
An increase in temperature caused when incoming solar radiation
is passed but outgoing thermal radiation is blocked by the atmosphere.
Carbon dioxide and water vapor are two of the major gases responsible for this effect.
Heliopause
The point at which the solar wind meets the interstellar medium or solar wind from other stars.
Heliosphere
The space within the boundary of the heliopause
containing the Sun and the solar system.
Hydrogen
An element consisting of one electron and one proton. Hydrogen is the
lightest of the elements and is the building block of the universe. Stars
form from massive clouds of hydrogen gas.
Hubble's
Law
The law of physics that states that the farther a galaxy
is from us, the faster it is moving away from us.
Hypergalaxy
A system consisting of a spiral galaxy surrounded by
several dwarf white galaxies, often ellipticals. Our galaxy and the Andromeda galaxy are
examples of hypergalaxies.
Ice
A term used to describe water or a number of gases such as methane or
ammonia when in a solid state.
Inclination
A measure of the tilt of a planet's orbital plane in
relation to that of the Earth.
Inferior Conjunction
A conjunction of an inferior
planet that occurs when the planet is lined up directly between the Earth and the Sun.
Inferior
Planet
A planet that orbits between the Earth and the
Sun. Mercury and Venus are the only two inferior planets in our solar
system.
Interplanetary Magnetic Field
The magnetic field carried along with the solar wind.
Interstellar Medium
The gas and dust that exists in open space between the stars.
Ionosphere
A region of charged particles in a planet's upper atmosphere. In Earth's atmosphere, the ionosphere begins at an
altitude of about 25 miles and extends outward about 250.
Iron
Meteorite
A meteorite which is composed mainly of iron
mixed with smaller amounts of nickel.
Irregular Galaxy
A galaxy with no spiral structure and no symmetric
shape. Irregular galaxies are usually filamentary or very clumpy in shape.
[none]
Kelvin
A temperature scale used in sciences such as astronomy to measure
extremely cold temperatures. The Kelvin temperature scale is just like the Celsius scale
except that the freezing point of water, zero degrees Celsius, is equal to 273 degrees
Kelvin. Absolute zero, the coldest known temperature, is
reached at 0 degrees Kelvin or -273.16 degrees Celsius.
Kepler's First
Law
A planet orbits the Sun in an ellipse
with the Sun at one focus.
Kepler's
Second Law
A ray directed from the Sun to a planet sweeps out equal
areas in equal times.
Kepler's Third
Law
The square of the period of a planet's orbit is proportional
to the cube of that planet's semimajor axis; the constant of
proportionality is the same for all planets.
Kiloparsec
A distance equal to 1000 parsecs.
Kirkwood Gaps
Regions in the main belt of asteroids
where few or no asteroids are found. They were named after the scientist who first noticed
them.
Kuiper Belt
A large ring of icy, primitive objects beyond the orbit of Neptune. Kupier Belt
objects are believed to be remnants of the original material that formed the
solar system.
Some astronomers believe Pluto and Charon are Kuiper Belt objects.
Lagrange
Point
French mathematician and astronomer Joseph Louis Lagrange showed that
three bodies can lie at the apexes of an equilateral triangle which rotates in its plane.
If one of the bodies is sufficiently massive compared with the other two, then the
triangular configuration is apparently stable. Such bodies are sometimes referred to as Trojans. The leading apex of the triangle is known as the leading
Lagrange point or L4; the trailing apex is the trailing Lagrange point or L5.
Lenticular Galaxy
A disk-shaped galaxy that contains no conspicuous structure within the
disk. Lenticular galaxies tend to look more like elliptical galaxies than spiral galaxies.
Libration
An effect caused by the apparent wobble of the Moon as it orbits the
Earth. The Moon always keeps the same side toward the Earth, but due to libration, 59% of
the Moon's surface can be seen over a period of time.
Light
Year
An astronomical unit of measure equal to the distance light travels in a
year, approximately 5.8 trillion miles.
Limb
The outer edge or border of a planet or other
celestial body.
Local
Group
A small group of about two dozen galaxies of which
our own Milky Way galaxy is a member.
Luminosity
The amount of light emitted by a star.
Lunar
Eclipse
A phenomenon that occurs when the Moon passes into the shadow of the
Earth. A partial lunar eclipse occurs when the Moon passes into the penumbra, or partial shadow. In a total lunar eclipse, the Moon
passes into the Earth's umbra, or total shadow.
Lunar
Month
The average time between successive new or full moons. A lunar month is
equal to 29 days 12 hours 44 minutes. Also called a synodic month.
Lunation
The interval of a complete lunar cycle, between one new Moon and the next.
A lunation is equal to 29 days, 12 hours, and 44 minutes.
Magellanic Clouds
Two small, irregular galaxies found just outside our own Milky Way galaxy. The Magellanic clouds are visible in the skies of the
southern hemisphere.
Magnetic
Field
A condition found in the region around a magnet or an electric current,
characterized by the existence of a detectable magnetic force at every point in the region
and by the existence of magnetic poles.
Magnetic
Pole
Either of two limited regions in a magnet at which the magnet's field is
most intense.
Magnetosphere
The area around a planet most affected by its magnetic field. The boundary of this field is set by the solar wind.
Magnitude
The degree of brightness of a star or other object in
the sky according to a scale on which the brightest star has a magnitude -1.4 and the
faintest visible star has magnitude 6. Sometimes referred to as apparent magnitude. In
this scale, each number is 2.5 times the brightness of the previous number. Thus a star
with a magnitude of 1 is 100 times brighter than on with a visual magnitude of 6.
Main
Belt
The area between Mars and Jupiter where most of the asteroids
in our solar system are found.
Major
Planet
A name used to describe any planet that is
considerably larger and more massive than the Earth, and contains large quantities of hydrogen and helium. Jupiter and Neptune are examples of major
planets.
Mare
A term used to describe a large, circular plain. The word mare means
"sea". On the moon, the mare are the smooth, dark-colored areas.
Mass
A measure of the total amount of material in a body, defined either by the
inertial properties of the body or by its gravitational influence on other bodies.
Matter
A word used to describe anything that contains mass.
Meridian
An imaginary circle drawn through the North and South poles of the celestial equator.
Metal
A term used by astronomers to describe all elements except hydrogen and helium.
Meteor
A small particle of rock or dust that burns away in the Earth's atmosphere. Meteors are also referred to as shooting stars.
Meteor
Shower
An event where a large number of meteors enter the Earth's
atmosphere from the same direction in space at nearly the same time. Most meteor
showers take place when the Earth passes through the debris left behind by a comet.
Meteorite
An object, usually a chunk or metal or rock, that survives entry through
the atmosphere to reach the Earth's surface. Meteors become meteorites if they reach the ground.
Meteoroid
A small, rocky object in orbit around the Sun, smaller than an asteroid.
Millibar
A measure of atmospheric pressure equal to 1/1000 of a bar.
Standard sea-level pressure on Earth is about 1013 millibars.
Minor
Planet
Another name used to describe a large asteroid.
Molecular
Cloud
An interstellar cloud of molecular hydrogen
containing trace amounts of other molecules such as carbon monoxide and ammonia.
Nadir
A term used to describe a point directly underneath an object or body.
Nebula
A cloud of dust and gas in space, usually illuminated by one or more stars. Nebulae represent the raw material the stars are made of.
Neutrino
A fundamental particle produced by the nuclear reactions in stars. Neutrinos are very hard to detect because the vast majority of
them pass completely through the Earth without interacting.
Neutron
Star
A compressed core of an exploded star made up almost
entirely of neutrons. Neutron stars have a strong gravitational field and some emit pulses
of energy along their axis. These are known as pulsars.
Newton's
First Law of Motion
A body continues in its state of constant velocity (which may be zero) unless it is
acted upon by an external force.
Newton's
Second Law of Motion
For an unbalanced force acting on a body, the acceleration produced is proportional
to the force impressed; the constant of proportionality is the inertial mass
of the body.
Newton's
Third Law of Motion
In a system where no external forces are present, every action force is always
opposed by an equal and opposite reaction.
Nova
A star that flares up to several times its original
brightness for some time before returning to its original state.
Nuclear
Fusion
The nuclear process whereby several small nuclei are combined to make a
larger one whose mass is slightly smaller than the sum of the small
ones. Nuclear fusion is the reaction that fuels the Sun, where hydrogen
nuclei are fused to form helium.
Obliquity
The angle between a body's equatorial plane and orbital plane.
Oblateness
A measure of flattening at the poles of a planet or
other celestial body.
Occultation
An event that occurs when one celestial body conceals or obscures another.
For example, a solar eclipse is an occultation of the Sun by
the Moon.
Oort
Cloud
A theoretical shell of comets that is believed to
exist at the outermost regions of our solar system. The oort cloud was named after the
Dutch astronomer who first proposed it.
Open
Cluster
A collection of young stars that formed together. They
may or may not be still bound by gravity. Some of the youngest open
clusters are still embedded in the gas and dust from which they formed.
Opposition
The position of a planet when it is exactly opposite
the Sun as seen from Earth. A planet at opposition is at its closest approach to the Earth
and is best suitable for observing.
Orbit
The path of a celestial body as it moves through space.
Parallax
The apparent change in position of two objects viewed from different
locations.
Parsec
A large distance often used in astronomy. A parsec is equal to 3.26 light years.
Patera
A shallow crater with a complex, scalloped edge.
Penumbra
The area of partial illumination surrounding the darkest part of a shadow
caused by an eclipse.
Perigee
The point in the orbit of the Moon or other satellite at
which it is closest to the Earth.
Perihelion
The point in the orbit of a planet or other body
where it is closest to the Sun.
Perturb
To cause a planet or satellite
to deviate from a theoretically regular orbital motion.
Phase
The apparent change in shape of the Moon and inferior
planets as seen from Earth as they move in their orbits.
Photon
A particle of light composed of a minute quantity of electromagnetic
energy.
Photosphere
The bright visible surface of the Sun.
Planet
A very large body in orbit around a star. Planets can be composed mainly of rock or of dense gases.
Planetary Nebula
A shell of gas surrounding a small, white star. The
gas is usually illuminated by the star, producing a variety of colors and shapes.
Planitia
A low plain.
Planum
A high plain or plateau.
Plasma
A form of ionized gas in which the temperature is too high for atoms to
exist in their natural state. Plasma is composed of free electrons and free atomic
nuclei.
Precession
The apparent shift of the celestial poles caused by a gradual wobble of
the Earth's axis.
Prominence
An explosion of hot gas that erupts from the Sun's surface. Solar
prominences are usually associated with sunspot activity and can
cause interference with communications on Earth due to their electromagnetic effects on
the atmosphere.
Proper
Motion
The apparent angular motion across the sky of an object relative to the
solar system.
Protostar
Dense regions of molecular clouds where stars are forming.
Pulsar
A spinning neutron star that emits energy
along its gravitational axis. This energy is received as pulses as the star rotates.
Quadrature
A point in the orbit of a superior planet
where it appears at right angles to the Sun as seem from Earth.
Quasar
An unusually bright object found in the remote areas of the universe.
Quasars release incredible amounts of energy and are among the oldest and farthest objects
in the known universe. They may be the nuclei of ancient, active galaxies.
Quasi-Stellar Object
Sometimes also called quasi-stellar source, this is a star-like object
with a large redshift that gives off a strong source of radio waves. They are highly
luminous and presumed to be extragalactic.
Radial
Velocity
The movement of an object either towards or away from a stationary
observer.
Radiant
A point in the sky from which meteors in a meteor
shower seem to originate.
Radiation
Energy radiated from an object in the form of waves or particles.
Radiation
Belt
Regions of charged particles in a magnetosphere.
Radio
Galaxy
A galaxy that gives off large amounts of energy in
the form of radio waves.
Red
Giant
A stage in the evolution of a star when the fuel
begins to exhaust and the star expands to about fifty times its normal size. The
temperature cools, which gives the star a reddish appearance.
Redshift
A shift in the lines of an object's spectrum
toward the red end. Redshift indicates that an object is moving away from the
observer. The larger the redshift, the faster the object is moving.
Resonance
A state in which an orbiting object is subject to periodic gravitational perturbations by another.
Retrograde Motion
The phenomenon where a celestial body appears to slow down, stop, them
move in the opposite direction. This motion is caused when the Earth overtakes the body in
its orbit.
Right
Ascension
The amount of time that passes between the rising of Aries and another
celestial object. Right ascension is one unit of measure for locating an object in
the sky.
Ring
Galaxy
A galaxy that has a ring-like appearance. The ring
usually contains luminous blue stars. Ring galaxies are believed to
have been formed by collisions with other galaxies.
Roche
Limit
The smallest distance from a planet or other body at which
purely gravitational forces can hold together a satellite or
secondary body of the same mean density as the primary. At a lesser distance the tidal forces of the primary would break up the secondary.
Rotation
The spin of a body about its axis.
Satellite
A natural or artificial body in orbit around a planet.
Scopulus
A lobate or irregular scarp.
Scarp
A line of cliffs produced erosion or by the action of faults.
Seyfert
Galaxy
A main-sequence star which rotates rapidly, causing
a loss of matter to an ever-expanding shell.
Shell
Star
A satellite which constrains the extent of a planetary ring through
gravitational forces. Also known as a shepherd moon.
Shepherd Satellite
A satellite which constrains the extent of a planetary ring through
gravitational forces. Also known as a shepherd moon.
Sidereal
Of, relating to, or concerned with the stars. Sidereal rotation is that
measured with respect to the stars rather than with respect to the Sun or the primary of a
satellite.
Sidereal
Month
The average period of revolution of the Moon around the
Earth in reference
to a fixed star, equal to 27 days, 7 hours, 43 minutes in units of
mean solar time.
Sidereal
Period
The period of revolution of a planet around the Sun
or a satellite around its primary.
Singularity
The center of a black hole, where the curvature of
spacetime is maximal. At the singularity, the gravitational tides diverge. Theoretically,
no solid object can survive hitting the singularity.
Solar Cycle
The approximately 11-year quasi-periodic variation in frequency or number
of solar active events.
Solar
Eclipse
A phenomenon that occurs when the Earth passes into the shadow of the
Moon. A total solar eclipse occurs when the Moon is close enough to completely block
the Sun's light. An annular solar eclipse occurs when the Moon is farther away and is
not able to completely block the light. This results in a ring of light around the
Moon.
Solar
Flare
A bright eruption of hot gas in the Sun's photosphere.
Solar prominences are usually only detectable by specialized instruments but can be
visible during a total solar eclipse.
Solar
Nebula
The cloud of dust and gas out of which the solar system was believed to
have formed about 5 billion years ago.
Solar
Wind
A flow of charged particles that travels from the Sun out into the
solar system.
Solstice
The time of the year when the Sun appears furthest north or south of the celestial equator. The solstices mark the beginning of the
Summer and Winter seasons.
Spectrometer
The instrument connected to a telescope that separates the
light signals into different frequencies, producing a spectrum.
Spectroscopy
The technique of observing the spectra of visible light from an object to
determine its composition, temperature, density, and speed.
Spectrum
Grass-like patterns of gas seen in the atmosphere
of the Sun.
Spicules
The range of colors produced when visible light passes through a prism.
Spiral
Galaxy
A galaxy that contains a prominent central bulge and
luminous arms of gas , dust, and young stars that wind out from the central nucleus in a
spiral formation. Our galaxy, the Milky Way, is a spiral galaxy.
Star
A giant ball of hot gas that creates and emits its own radiation through nuclear fusion.
Star
Cluster
A large grouping of stars, from a few dozen to a few
hundred thousand, that are bound together by their mutual gravitational attraction.
Steady State Theory
The theory that suggests the universe is expanding but exists in a
constant, unchanging state in the large scale. The theory states that new matter is
being continually being created to fill the gaps left by expansion. This theory has
been abandoned by most astronomers in favor of the big bang
theory.
Stellar
Wind
The ejection of gas from the surface of a star. Many different
types of stars, including our Sun, have stellar winds. The stellar wind of our Sun is also
known as the Solar wind. A star's stellar wind is strongest near the end of its life when
it has consumed most of its fuel.
Stone
Meteorite
A meteorite which resembles a terrestrial rock and is composed of similar materials.
Stony
Iron
A meteorite which contains regions resembling
both a stone meteorite and an iron
meteorite.
Sunspot
Areas of the Sun's surface that are cooler than surrounding
areas. The usually appear black on visible light photographs of the
Sun. Sunspots are usually associated disturbances in the Sun's electromagnetic field.
Supergiant
The stage in a star's evolution where the core
contracts and the star swells to about five hundreds times its original size. The
star's temperature drops, giving it a red color.
Superior Conjunction
A conjunction that occurs when a superior planet passes behind the Sun and is on the
opposite side of the Sun from the Earth.
Superior
Planet
A planet that exists outside the orbit of the
Earth. All of the planets in our solar system are superior except for Mercury and
Venus. These two planets are inferior planets.
Supernova
A supernova is a cataclysmic explosion caused when a star
exhausts its fuel and ends its life. Supernovae are the most powerful forces in the
universe. All of the heavy elements were created in supernova explosions.
Supernova Remnant
An expanding shell of gas ejected at high speeds by a supernova
explosion. Supernova remnants are often visible as diffuse gaseous nebulae
usually with a shell-like structure. Many resemble "bubbles" in space.
Synchronous Rotation
A period of rotation of a satellite about its axis that is the same as the period of its orbit
around its primary. This causes the satellite to always keep the same face to the primary.
Our Moon in in synchronous rotation about the Earth.
Synodic
Period
The interval between points of opposition of a superior planet.
Tektite
A small, glassy material formed by the impact of a large body, usually a meteor or asteroid. Tektites are commonly found
at the sites of meteor craters.
Telescope
An instrument used to collect large amounts of light from far away objects
and increase their visibility to the naked eye. Telescopes can also enlarge objects
that are relatively close to the Earth.
Terminator
The boundary between the light side and the dark side of a planet or other body.
Terrestrial
A term used to describe anything originating on the planet Earth.
Terrestrial Planet
A name given to a planet composed mainly of rock and
iron, similar to that of Earth.
Tidal
Force
The differential gravitational pull exerted on any extended body within
the gravitational field of another body.
Tidal
Heating
Frictional heating of a satellite's interior due
to flexure caused by the gravitational pull of its parent planet
and/or other neighboring satellites.
Transit
The passage of a celestial body across an observer's meridian;
also the passage of a celestial body across the disk of a larger one.
Trojan
An object orbiting in the Lagrange points of
another (larger) object. This name derives from a generalization of the names of some of
the largest asteroids in Jupiter's Lagrange points. Saturn's moons
Helene, Calypso and Telesto are also sometimes called Trojans.
Ultraviolet
Electromagnetic radiation at wavelengths
shorter than the violet end of visible light. The atmosphere of the Earth effectively blocks the transmission of most
ultraviolet light, which can be deadly to many forms of life.
Umbra
The area of total darkness in the shadow caused by an eclipse.
Universal Time (UT)
Also known as Greenwich Mean Time, this is local time on the Greenwich
meridian. Universal time is used by astronomers as a standard measure of time.
Van
Allen Belts
Radiation zones of charged particles that surround the Earth. The
shape of the Van Allen belts is determined by the Earth's magnetic field.
Variable
Star
A star that fluctuates in brightness. These
include eclipsing binaries.
Visible
Light
Wavelengths of electromagnetic radiation that are visible to the
human eye.
Virgo
Cluster
A gigantic cluster of over 2000 galaxies that is located mainly within the
constellation of Virgo. This cluster is located about 60 million light years from Earth.
Visual Magnitude
A scale used by astronomers to measure the brightness of a star
or other celestial object. Visual magnitude measures only the visible light from the
object. On this scale, bright objects have a lower number than dim objects.
Wavelength
The distance between consecutive crests of a wave. This serves as a unit of
measure of electromagnetic radiation.
White
Dwarf
A very small, white star formed when an
average sized star uses up its fuel
supply and collapses. This process often produces a planetary
nebula, with the white dwarf star at its center.
X-ray
Electromagnetic radiation of a very short wavelength and very high-energy. X-rays have shorter wavelengths than ultraviolet light but
longer wavelengths than cosmic rays.
X-ray
Astronomy
The field of astronomy that studies celestial objects by the x-rays
the emit.
X-ray
Star
A bright celestial object that gives off x-rays as a major
portion of its radiation.
Yellow
Dwarf
An ordinary star such as the Sun at a stable point in its
evolution.
Zenith
A point directly overhead from an observer.
Zodiac
An imaginary belt across the sky in which the Sun, Moon, and all of the planets
can always be found.
Zodiacal
Light
A faint cone of light that can sometimes be seen above the horizon after sunset
or before sunrise. Zodiacal light is caused by sunlight reflecting off small
particles of material in the plane of the solar system.
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