Astronomy glossary

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A[edit | edit source]

Absolute Magnitude[edit | edit source]

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[edit | edit source]

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[edit | edit source]

A process by where the atmosphere melts away and removes the surface material of an incoming meteorite.

Accretion[edit | edit source]

The process by where dust and gas accumulated into larger bodies such as stars and planets.

Accretion Disk[edit | edit source]

A disk of gas that accumulates around a center of gravitational attraction, such as a white dwarf, neutron star, or black hole. As the gas spirals in, it becomes hot and emits light or even X-radiation.

Achondrite[edit | edit source]

A stone meteorite that contains no chondrules.

Albedo[edit | edit source]

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[edit | edit source]

A dark or light marking on the surface of an object that may or may not be a geological or topographical feature.

Altitude[edit | edit source]

The angular distance of an object above the horizon.

Am star[edit | edit source]

Am star is a chemically peculiar star belonging to the more general class of A-type stars. The spectrum of the Am stars shows abnormal enhancements and deficiencies of certain metals.

Antimatter[edit | edit source]

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[edit | edit source]

A point that is on the direct opposite side of a planet.

Apastron[edit | edit source]

The point of greatest separation of two stars, such as in a binary star system.

Aperture[edit | edit source]

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[edit | edit source]

The point in the orbit of a planet or other celestial body where it is farthest from the Sun.

Apoapsis[edit | edit source]

Apoapsis is the point of furthest excursion, or separation, between two orbiting objects.

Apogee[edit | edit source]

The point in the orbit of the Moon or other satellite where it is farthest from the Earth.

Apparent Magnitude[edit | edit source]

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.

Asterism[edit | edit source]

Asterism is a pattern of stars recognized on Earth's night sky. It may form part of an official constellation, or be composed of stars from more than one.

Asteroid[edit | edit source]

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[edit | edit source]

The branch of science that explores the chemical interactions between dust and gas interspersed between the stars.

Astrometric binary[edit | edit source]

Astrometric binary is a type of binary system where evidence for an unseen orbiting companion is revealed by its periodic gravitational perturbation of the visible component. See also spectroscopic binary.

Astronomical Unit (AU)[edit | edit source]

A unit of measure equal to the average distance between the Earth and the Sun, approximately 93 million miles.

Atmosphere[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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[edit | edit source]

Also known as the poles, this is an imaginary line through the center of rotation of an object.

Azimuth[edit | edit source]

The angular distance of an object around or parallel to the horizon from a predefined zero point.

B[edit | edit source]

Bar[edit | edit source]

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[edit | edit source]

The theory that suggests that the universe was formed from a single point in space during a cataclysmic explosion about 13.7 billion years ago. This is the current accepted theory for the origin of the universe and is supported by measurements of background radiation and the observed expansion of space.

Binary[edit | edit source]

A system of two stars that revolve around a common center of gravity.

Black Hole[edit | edit source]

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[edit | edit source]

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[edit | edit source]

A term used to describe an exceptionally bright meteor. Bolides typically will produce a sonic boom.

Brown dwarf[edit | edit source]

Brown dwarf is a substellar object that is too low in mass to sustain the nuclear fusion of hydrogen-1 in its core, which is a characteristic of stars on the main sequence. Brown dwarfs can still generate energy from gravitational contraction and by the fusion of deuterium.

C[edit | edit source]

Caldera[edit | edit source]

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[edit | edit source]

A series or chain of craters.

Cavus[edit | edit source]

A hollow, irregular depression.

Celestial Equator[edit | edit source]

An imaginary line that divides the celestial sphere into a northern and southern hemisphere.

Celestial Poles[edit | edit source]

The North and South poles of the celestial sphere.

Celestial Sphere[edit | edit source]

An imaginary sphere around the Earth on which the stars and planets appear to be positioned.

Cepheid Variable[edit | edit source]

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[edit | edit source]

A distinctive area of broken terrain.

Chasma[edit | edit source]

Another name used to describe a canyon.

Chondrite[edit | edit source]

A meteorite that contains chondrules.

Chondrule[edit | edit source]

Small, glassy spheres commonly found in meteorites.

Chromosphere[edit | edit source]

The part of the Sun's atmosphere just above the surface.

Circumpolar Star[edit | edit source]

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.

Circumstellar Disk[edit | edit source]

A torus or ring-shaped accumulation of gas, dust, or other debris in orbit around a star in different phases of its life cycle.

Color index[edit | edit source]

A numeric value that is used to compare the brightness of a star measured from two different frequency bands of the electromagnetic spectrum. Because the energy output of a star varies by frequency as a function of temperature, the color index can be used to indicate the star's temperature.

Coma[edit | edit source]

An area of dust or gas surrounding the nucleus of a comet.

Comet[edit | edit source]

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.

Commensurability[edit | edit source]

The property of two objects orbiting the same body whose periods are in a rational proportion. For example, the orbital period of Saturn around the Sun is very nearly 5/2 the orbital period of Jupiter.

Conjunction[edit | edit source]

An event that occurs when two or more celestial objects appear close together in the sky.

Constellation[edit | edit source]

A grouping of stars that make an imaginary picture in the sky.

Corona[edit | edit source]

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[edit | edit source]

Atomic nuclei (mostly protons) that are observed to strike the Earth's atmosphere with extremely high amounts of energy.

Cosmic String[edit | edit source]

A tube-like 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.

==y====Cosmogony The study of celestial systems, including the Solar System, stars, galaxies, and galactic clusters. This is the definition of cosmology, not cosmogony.

Cosmology[edit | edit source]

A branch of science that deals with studying the origin, structure, and nature of the universe. This is the definition of the word 'cosmogony', not cosmology.

Crater[edit | edit source]

A bowl-shaped depression formed by the impact of an asteroid or meteoroid. Also the depression around the opening of a volcano.

D[edit | edit source]

Dark Matter[edit | edit source]

A term used to describe matter in the universe that cannot be seen, but can be detected by its gravitational effects on other bodies.

Debris Disk[edit | edit source]

A ring-shaped circumstellar disk of dust and debris in orbit around a star. Debris disks can be created as the next phase in planetary system development following the protoplanetary disk phase. They can also be formed by collisions between planetesimals.

Declination[edit | edit source]

The angular distance of an object in the sky from the celestial equator.

Density[edit | edit source]

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[edit | edit source]

The surface of the Sun or other celestial body projected against the sky.

Double Asteroid[edit | edit source]

Two asteroids that revolve around each other and are held together by the gravity between them. Also called a binary asteroid.

Doppler Effect[edit | edit source]

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[edit | edit source]

A grouping of two stars. This grouping can be apparent, where the stars seem close together, or physical, such as a binary system.

Dwarf Planet[edit | edit source]

A celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite. It has to have sufficient mass to overcome rigid body forces and achieve hydrostatic equilibrium. Pluto is considered to be a dwarf planet.

E[edit | edit source]

Eccentricity[edit | edit source]

The measure of how an object's orbit differs from a perfect circle. Eccentricity defines the shape of an object's orbit.

Eclipse[edit | edit source]

The total or partial blocking of one celestial body by another.

Eclipsing Binary[edit | edit source]

A binary system where one object passes in front of the other, cutting off some or all of its light.

Ecliptic[edit | edit source]

An imaginary line in the sky traced by the Sun as it moves in its yearly path through the sky.

Effective temperature[edit | edit source]

Effective temperature of a star or planet is the temperature of an ideal black body that would emit the same total amount of electromagnetic radiation.

Ejecta[edit | edit source]

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 appear as a lighter color than the surrounding surface.

Electromagnetic Radiation[edit | edit source]

Another term for light. Light waves created by fluctuations of electric and magnetic fields in space.

Electromagnetic Spectrum[edit | edit source]

The full range of frequencies, from radio waves to gamma waves, that characterizes light.

Ellipse[edit | edit source]

An ellipse is an oval shape. Johannes Kepler discovered that the orbits of the planets were elliptical in shape rather than circular.

Elliptical Galaxy[edit | edit source]

A galaxy whose structure shaped like an ellipse and is smooth and lacks complex structures such as spiral arms.

Elongation[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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[edit | edit source]

The speed required for an object to escape the gravitational pull of a planet or other body.

Event Horizon[edit | edit source]

The invisible boundary around a black hole past which nothing can escape the gravitational pull - not even light.

Evolved Star[edit | edit source]

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[edit | edit source]

The apparent dimming of star or planet when low on the horizon due to absorption by the Earth's atmosphere.

Extragalactic[edit | edit source]

A term that means outside of or beyond our own galaxy.

Extraterrestrial[edit | edit source]

A term used to describe anything that does not originate on Earth.

Eyepiece[edit | edit source]

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.

F[edit | edit source]

Faculae[edit | edit source]

Bright patches that are visible on the Sun's surface, or photosphere.

Filament[edit | edit source]

A strand of cool gas suspended over the photosphere by magnetic fields, which appears dark as seen against the disk of the Sun.

Finder[edit | edit source]

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[edit | edit source]

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[edit | edit source]

A faint red star that appears to change in brightness due to explosions on its surface.

G[edit | edit source]

Galactic Halo[edit | edit source]

The name given to the spherical region surrounding the center, or nucleus of a galaxy.

Galactic Nucleus[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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[edit | edit source]

The highest energy, shortest wavelength form of electromagnetic radiation.

Geosynchronous Orbit[edit | edit source]

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)[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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[edit | edit source]

A mutual physical force of nature that causes two bodies to attract each other.

Greenhouse Effect[edit | edit source]

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.

H[edit | edit source]

Heliopause[edit | edit source]

The point in space at which the solar wind meets the interstellar medium or solar wind from other stars.

Heliosphere[edit | edit source]

The space within the boundary of the heliopause containing the Sun and the Solar System.

Hydrogen[edit | edit source]

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[edit | edit source]

The law of physics that states that the farther a galaxy is from us, the faster it is moving away from us.

Hydrostatic equilibrium[edit | edit source]

A state that occurs when compression due to gravity is balanced by a pressure gradient which creates a pressure gradient force in the opposite direction. Hydrostatic equillibrium is responsible for keeping stars from imploding and for giving planets their spherical shape.

Hypergalaxy[edit | edit source]

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.

I[edit | edit source]

Ice[edit | edit source]

A term used to describe water or a number of gases such as methane or ammonia when in a solid state.

Inclination[edit | edit source]

A measure of the tilt of a planet's orbital plane in relation to that of the Earth.

Inferior Conjunction[edit | edit source]

A conjunction of an inferior planet that occurs when the planet is lined up directly between the Earth and the Sun.

Inferior Planet[edit | edit source]

A planet that orbits between the Earth and the Sun. Mercury and Venus are the only two inferior planets in our solar system.

International Astronomical Union (IAU)[edit | edit source]

An international organization that unites national astronomical societies from around the world and acts as the internationally recognized authority for assigning designations to celestial bodies and their surface features.

Interplanetary Magnetic Field[edit | edit source]

The magnetic field carried along with the solar wind.

Interstellar Medium[edit | edit source]

The gas and dust that exists in open space between the stars.

Ionosphere[edit | edit source]

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[edit | edit source]

A meteorite that is composed mainly of iron mixed with smaller amounts of nickel.

Irregular Galaxy[edit | edit source]

A galaxy with no spiral structure and no symmetric shape. Irregular galaxies are usually filamentary or very clumpy in shape.

Irregular Satellite[edit | edit source]

A satellite that orbits a planet far away with an orbit that is eccentric and inclined. They also tend to have retrograde orbits. Irregular satellites are believed to have been captured by the planet's gravity rather than being formed along with the planet.

J[edit | edit source]

Jansky[edit | edit source]

A unit used in radio astronomy to indicate the flux density (the rate of flow of radio waves) of electromagnetic radiation received from outer space. A typical radio source has a spectral flux density of roughly 1 Jy. The jansky was named to honor Karl Gothe Jansky who developed radio astronomy in 1932.

Jet[edit | edit source]

A narrow stream of gas or particles ejected from an accretion disk surrounding a star or black hole.

K[edit | edit source]

Kelvin[edit | edit source]

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[edit | edit source]

A planet orbits the Sun in an ellipse with the Sun at one focus.

Kepler's Second Law[edit | edit source]

A ray directed from the Sun to a planet sweeps out equal areas in equal times.

Kepler's Third Law[edit | edit source]

The square of the period of a planet's orbit is proportional to the cube of that planet's semi major axis; the constant of proportionality is the same for all planets.

Kiloparsec[edit | edit source]

A distance equal to 1000 parsecs.

Kirkwood Gaps[edit | edit source]

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[edit | edit source]

A large ring of icy, primitive objects beyond the orbit of Neptune. Kuiper 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.

L[edit | edit source]

Lagrange Point[edit | edit source]

French mathematician and astronomer Joseph Louis Lagrange showed that three bodies could 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[edit | edit source]

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[edit | edit source]

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[edit | edit source]

An astronomical unit of measure equal to the distance light travels in a year, approximately 5.8 trillion miles.

Limb[edit | edit source]

The outer edge or border of a planet or other celestial body.

Local Group[edit | edit source]

A small group of about two dozen galaxies of which our own Milky Way galaxy is a member.

Luminosity[edit | edit source]

The amount of light emitted by a star.

Lunar Eclipse[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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.

M[edit | edit source]

Magellanic Clouds[edit | edit source]

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[edit | edit source]

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[edit | edit source]

Either of two limited regions in a magnet at which the magnet's field is most intense.

Magnetosphere[edit | edit source]

The area around a planet most affected by its magnetic field. The boundary of this field is set by the solar wind.

Magnitude[edit | edit source]

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[edit | edit source]

The area between Mars and Jupiter where most of the asteroids in our solar system are found.

Major Planet[edit | edit source]

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[edit | edit source]

A term used to describe a large, circular plain. The word mare means "sea". On the Moon, the maria are the smooth, dark-colored areas.

Mass[edit | edit source]

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[edit | edit source]

A word used to describe anything that contains mass.

Meridian[edit | edit source]

An imaginary circle drawn through the North and South poles of the celestial equator.

Metal[edit | edit source]

A term used by astronomers to describe all elements except hydrogen and helium, as in "the universe is composed of hydrogen, helium and traces of metals". This astronomical definition is quite different from the traditional chemistry definition of a metal.

Meteor[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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[edit | edit source]

A small, rocky object in orbit around the Sun, smaller than an asteroid.

Millibar[edit | edit source]

A measure of atmospheric pressure equal to 1/1000 of a bar. Standard sea-level pressure on Earth is about 1013 millibars.

Minor Planet[edit | edit source]

A term used since the 19th century to describe objects, such as asteroids, that are in orbit around the Sun but are not planets or comets. In 2006, the International Astronomical Union reclassified minor planets as either dwarf planets or small solar system bodies.

Molecular Cloud[edit | edit source]

An interstellar cloud of molecular hydrogen containing trace amounts of other molecules such as carbon monoxide and ammonia.

N[edit | edit source]

Nadir[edit | edit source]

A term used to describe a point directly underneath an object or body.

Nebula[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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[edit | edit source]

In a system where no external forces are present, every action force is always opposed by an equal and opposite reaction.

Nova[edit | edit source]

A star that flares up to several times its original brightness for some time before returning to its original state.

Nuclear Fusion[edit | edit source]

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.

O[edit | edit source]

Obliquity[edit | edit source]

The angle between a body's equatorial plane and orbital plane.

Oblateness[edit | edit source]

A measure of flattening at the poles of a planet or other celestial body.

Occultation[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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[edit | edit source]

The position of a planet when it is exactly opposite the Sun in the sky as seen from Earth. A planet at opposition is at its closest approach to the Earth and is best suitable for observing.

Orbit[edit | edit source]

The path of a celestial body as it moves through space.

P[edit | edit source]

Parallax[edit | edit source]

The apparent change in position of two objects viewed from different locations.

Parsec[edit | edit source]

A large distance often used in astronomy. A parsec is equal to 3.26 light-years.

Patera[edit | edit source]

A shallow crater with a complex, scalloped edge.

Penumbra[edit | edit source]

The area of partial illumination surrounding the darkest part of a shadow caused by an eclipse.

Perigee[edit | edit source]

The point in the orbit of the Moon or other satellite at which it is closest to the Earth.

Perihelion[edit | edit source]

The point in the orbit of a planet or other body where it is closest to the Sun.

Perturb[edit | edit source]

To cause a planet or satellite to deviate from a theoretically regular orbital motion.

Phase[edit | edit source]

The apparent change in shape of the Moon and inferior planets as seen from Earth as they move in their orbits.

Photon[edit | edit source]

A particle of light composed of a minute quantity of electromagnetic energy.

Photosphere[edit | edit source]

The bright visible surface of the Sun.

Planemo[edit | edit source]

A large planet or planetary body that does not orbit a star. Planemos instead wander cold and alone through the cosmos. It is believed that most planemos once orbited their mother star but were ejected from the star system by gravitational interaction with another massive object.

Planet[edit | edit source]

A celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighboring region of planetesimals.

Planetary Nebula[edit | edit source]

A shell of gas surrounding a small, white star. The gas is usually illuminated by the star, producing a variety of colors and shapes.

Planetesimal[edit | edit source]

A solid object that is believed to exist in protoplanetary disks and in debris disks. Planetesimals are formed from small dust grains that collide and stick together and are the building blocks that eventually form planets in new planetary systems.

Planitia[edit | edit source]

A low plain.

Planum[edit | edit source]

A high plain or plateau.

Plasma[edit | edit source]

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[edit | edit source]

The apparent shift of the celestial poles caused by a gradual wobble of the Earth's axis.

Prominence[edit | edit source]

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.

Prograde Orbit[edit | edit source]

In reference to a satellite, a prograde orbit means that the satellite orbits the planet in the same direction as the planet's rotation. A planet is said to have a prograde orbit if the direction of its orbit is the same as that of the majority of other planets in the system.

Proper Motion[edit | edit source]

The apparent angular motion across the sky of an object relative to the Solar System.

Protoplanetary Disk[edit | edit source]

A rotating circumstellar disk of dense gas surrounding a young newly formed star. It is thought that planets are eventually formed from the gas and dust within the protoplanetary disk.

Protostar[edit | edit source]

Dense regions of molecular clouds where stars are forming.

Pulsar[edit | edit source]

A spinning neutron star that emits energy along its gravitational axis. This energy is received as pulses as the star rotates.

Q[edit | edit source]

Quadrature[edit | edit source]

A point in the orbit of a superior planet where it appears at right angles to the Sun as seem from Earth.

Quasar[edit | edit source]

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[edit | edit source]

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.

R[edit | edit source]

Radial Velocity[edit | edit source]

The movement of an object either towards or away from a stationary observer.

Radiant[edit | edit source]

A point in the sky from which meteors in a meteor shower seem to originate.

Radiation[edit | edit source]

Energy radiated from an object in the form of waves or particles.

Radiation Belt[edit | edit source]

Regions of charged particles in a magnetosphere.

Radio Galaxy[edit | edit source]

A galaxy that gives off large amounts of energy in the form of radio waves.

Red Giant[edit | edit source]

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[edit | edit source]

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.

Regular Satellite[edit | edit source]

A satellite that orbits close to a planet in a nearly circular, equatorial orbit. Regular satellites are believed to have been formed at the same time as the planet, unlike irregular satellites which are believed to have been captured by the planet's gravity.

Resonance[edit | edit source]

A state in which an orbiting object is subject to periodic gravitational perturbations by another.

Retrograde Motion[edit | edit source]

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.

Retrograde Orbit[edit | edit source]

The orbit of a satellite where the satellite travels in a direction opposite to that direction of the planet's rotation.

Right Ascension[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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[edit | edit source]

The spin of a body about its axis.

S[edit | edit source]

Satellite[edit | edit source]

A natural or artificial body in orbit around a planet.

Scarp[edit | edit source]

A line of cliffs produced erosion or by the action of faults.

Seyfert Galaxy[edit | edit source]

A main-sequence star that rotates rapidly, causing a loss of matter to an ever-expanding shell.

Shell Star[edit | edit source]

A type of star which is believed to be surrounded by a thin envelope of gas, which is often indicated by bright emission lines in its spectrum.

Shepherd Satellite[edit | edit source]

A satellite that constrains the extent of a planetary ring through gravitational forces. Also known as a shepherd moon.

Sidereal[edit | edit source]

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[edit | edit source]

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[edit | edit source]

The period of revolution of a planet around the Sun or a satellite around its primary.

Singularity[edit | edit source]

The center of a black hole, where the curvature of space time is maximal. At the singularity, the gravitational tides diverge. Theoretically, no solid object can survive hitting the singularity.

Small Solar System Body[edit | edit source]

A term defined in 2006 by the International Astronomical Union to describe objects in the Solar System that are neither planets or dwarf planets. These include most of the asteroids, comets, and other small bodies in the Solar System.

Solar Cycle[edit | edit source]

The approximately 11-year quasi-periodic variation in frequency or number of solar active events.

Solar Eclipse[edit | edit source]

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[edit | edit source]

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[edit | edit source]

The cloud of dust and gas out of which the Solar System was believed to have formed about 5 billion years ago.

Solar Wind[edit | edit source]

A flow of charged particles that travels from the Sun out into the Solar System.

Solstice[edit | edit source]

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[edit | edit source]

The instrument connected to a telescope that separates the light signals into different frequencies, producing a spectrum.

Spectroscopy[edit | edit source]

The technique of observing the spectra of visible light from an object to determine its composition, temperature, density, and speed.

Spectrum[edit | edit source]

The range of colors that make up visible white light. A spectrum is produced when visible light passes through a prism.

Spicules[edit | edit source]

Grass-like patterns of gas seen in the atmosphere of the Sun.

Spiral Galaxy[edit | edit source]

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[edit | edit source]

A giant ball of hot gas that creates and emits its own radiation through nuclear fusion.

Star Cluster[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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[edit | edit source]

A meteorite that resembles a terrestrial rock and is composed of similar materials.

Stony Iron[edit | edit source]

A meteorite that contains regions resembling both a stone meteorite and an iron meteorite.

Sunspot[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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[edit | edit source]

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 is in synchronous rotation around the Earth.

Synodic Period[edit | edit source]

The interval between points of opposition of a superior planet.

T[edit | edit source]

Tektite[edit | edit source]

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[edit | edit source]

An instrument that uses lenses and sometimes mirrors to collect large amounts of light from distant objects and enable direct observation and photography. A Telescope can also include any instrument designed to observe distant objects by their emissions of invisible radiation such as x-rays or radio waves.

Terminator[edit | edit source]

The boundary between the light side and the dark side of a planet or other body.

Terrestrial[edit | edit source]

A term used to describe anything originating on the planet Earth.

Terrestrial Planet[edit | edit source]

A name given to a planet composed mainly of rock and iron, similar to that of Earth.

Tidal Force[edit | edit source]

The differential gravitational pull exerted on any extended body within the gravitational field of another body.

Tidal Heating[edit | edit source]

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[edit | edit source]

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.

Trans-Neptunian Object (TNO)[edit | edit source]

Any one of a number of celestial objects that orbit the Sun at a distance beyond the orbit of the planet Neptune.

Trojan[edit | edit source]

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.

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U[edit | edit source]

Ultraviolet[edit | edit source]

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[edit | edit source]

The area of total darkness in the shadow caused by an eclipse.

Universal Time (UT)[edit | edit source]

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.

V[edit | edit source]

Van Allen Belts[edit | edit source]

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[edit | edit source]

A star that fluctuates in brightness. These include eclipsing binaries.

Visible Light[edit | edit source]

Wavelengths of electromagnetic radiation that are visible to the human eye.

Virgo Cluster[edit | edit source]

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[edit | edit source]

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.

W[edit | edit source]

Wavelength[edit | edit source]

The distance between consecutive crests of a wave. This serves as a unit of measure of electromagnetic radiation.

White Dwarf[edit | edit source]

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[edit | edit source]

X-ray[edit | edit source]

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[edit | edit source]

The field of astronomy that studies celestial objects by the x-rays they emit.

X-ray Star[edit | edit source]

A bright celestial object that gives off x-rays as a major portion of its radiation.

Y[edit | edit source]

Yellow Dwarf[edit | edit source]

An ordinary star such as the Sun at a stable point in its evolution.

Z[edit | edit source]

Zenith[edit | edit source]

A point directly overhead from an observer.

Zodiac[edit | edit source]

An imaginary belt across the sky in which the Sun, moon, and all of the planets can always be found.

Zodiacal Light[edit | edit source]

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.