The universe is a large place full of wondrous and unimaginable things. The amateur astronomy hobby contains words, labels, and technical terms that may seem be a little confusing for both beginners and the more experienced hobbyists. This glossary of astronomy terms contains definitions for some of the most common words used in astronomy, cosmology, astrophysics, and space exploration. You may click on any yellow highlighted word to jump directly to the definition for that astronomy word. You can also click on any letter of the alphabet below to jump directly to that section in the astronomy glossary listing.
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.
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.
A dark or light marking on the surface of an object that may or may not be a geological or topographical feature.
The angular distance of an object above the horizon.
Matter consisting of particles with charges opposite that of ordinary matter. In antimatter, protons have a negative charge while electrons have a positive charge.
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.
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.
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.
Astronomical Unit (AU)
A unit of measure equal to the average distance between the Earth and the Sun, approximately 93 million miles.
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.
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.
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.
The angular distance of an object around or parallel to the horizon from a predefined zero point.
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.
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.
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.
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.
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.
A hollow, irregular depression.
A distinctive area of broken terrain.
Another name used to describe a canyon.
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.
A torus or ring-shaped accumulation of gas, dust, or other debris in orbit around a star in different phases of its life cycle.
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.
An event that occurs when two or more celestial objects appear close close together in the sky.
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.
A branch of science that deals with studying the origin, structure, and nature of the universe.
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.
The surface of the Sun or other celestial body projected against the sky.
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.
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.
The total or partial blocking of one celestial body by another.
An imaginary line in the sky traced by the Sun as it moves in its yearly path through the sky.
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.
The full range of frequencies, from radio waves to gamma waves, that characterizes light.
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.
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.
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.
A term used to describe anything that does not originate on Earth.
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.
A faint red star that appears to change in brightness due to explosions on its surface.
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.
The name given to Jupiter's four largest moons, Io, Europa, Callisto & Ganymede. They were discovered independently by Galileo Galilei and Simon Marius.
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.
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.
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.
A mutual physical force of nature that causes two bodies to attract each other.
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.
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.
A term used to describe water or a number of gases such as methane or ammonia when in a solid state.
International Astronomical Union (IAU)
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.
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.
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.
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.
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.
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.
A disk-shaped galaxy that contains no conspicuous structure within the disk. Lenticular galaxies tend to look more like elliptical galaxies than spiral galaxies.
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.
An astronomical unit of measure equal to the distance light travels in a year, approximately 5.8 trillion miles.
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.
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.
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.
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.
Either of two limited regions in a magnet at which the magnet's field is most intense.
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.
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.
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.
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.
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.
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.
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.
A term used to describe a point directly underneath an object or body.
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.
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.
The angle between a body's equatorial plane and orbital plane.
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.
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.
The path of a celestial body as it moves through space.
The apparent change in position of two objects viewed from different locations.
A particle of light composed of a minute quantity of electromagnetic energy.
The bright visible surface of the Sun.
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.
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.
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.
A low plain.
A high plain or plateau.
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.
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.
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.
The apparent angular motion across the sky of an object relative to the Solar System.
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.
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.
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.
The movement of an object either towards or away from a stationary observer.
Energy radiated from an object in the form of waves or particles.
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.
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.
The orbit of a satellite where the satellite travels in a direction opposite to that direction of the planet's rotation.
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.
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.
A line of cliffs produced erosion or by the action of faults.
A main-sequence star that rotates rapidly, causing a loss of matter to an ever-expanding shell.
A satellite that constrains the extent of a planetary ring through gravitational forces. Also known as a shepherd moon.
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.
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
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.
The approximately 11-year quasi-periodic variation in frequency or number of solar active events.
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.
The cloud of dust and gas out of which the Solar System was believed to have formed about 5 billion years ago.
A flow of charged particles that travels from the Sun out into the Solar System.
The technique of observing the spectra of visible light from an object to determine its composition, temperature, density, and speed.
The range of colors that make up visible white light. A spectrum is produced when visible light passes through a prism.
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.
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.
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.
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.
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.
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.
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.
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.
A term used to describe anything originating on the planet Earth.
The differential gravitational pull exerted on any extended body within the gravitational field of another body.
Trans-Neptunian Object (TNO)
Any one of a number of celestial objects that orbit the Sun at a distance beyond the orbit of the planet Neptune.
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.
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.
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.
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.
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.
A point directly overhead from an observer.
An imaginary belt across the sky in which the Sun, moon, and all of the planets can always be found.
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.