This section provides definitions of selected astronomical and astrophysical terms in alphabetical order

Asteroid. A small, rocky object that orbits the Sun. Tens of thousands of these objects make up the Asteroid Belt, located between the orbits of Mars and Jupiter.

Asteroid Belt. The region of the Solar System between the orbits of Mars and Jupiter where most of the asteroids are found.

Astrology. The study of the positions and aspects of celestial bodies in the false belief that they have an influence on the course of natural earthly occurrences and human affairs.

Astronomical Unit. Unit of length based on the distance from the Earth to the Sun, nearly 150 million kilometers.

Big Bang Theory. A theory that describes the origin of the universe as an enormous explosion at the beginning of time. The explosion was followed by a cooling and thinning out of the universe. The universe is still expanding.

Binary Star. Two stars that are paired by virtue of being gravitationally attracted to each other. They orbit about a common center of mass.

Black Hole. An object predicted to exist by the theory of general relativity. It is maximally gravitationally collapsed, and not even light can escape.

Black Hole Singularity. The object of zero radius into which the matter of a black hole is comprised.

Blazar. A very compact and highly variable energy source associated with a supermassive black hole at the center of a host galaxy. Blazars are among the most violent phenomena in the universe.

Celestial Mechanics. The branch of astrophysics that deals with the motions of celestial objects.

Celestial Sphere. An imaginary sphere around the Earth on which the Sun, Moon, stars, and planets appear to be placed.

Cepheid. A member of a particular class of variable stars, notable for a fairly tight correlation between their period of variability and absolute luminosity.

Chromosphere. A layer of the Sun’s atmosphere lying above the photosphere. It has a width of about 2000 km. It has a lower gas density than the photosphere, but a higher temperature. The temperature is sufficiently high to ionize hydrogen gas and produce emission lines, notably the red Balmer line that gives the chromosphere a pinkish color.

Comet. An object composed of rock, ice, dust, and gases moving about the Sun in an elliptical orbit. A comet has three distinct components—the nucleus, made up of rock and ice; the coma, consisting of gases and dust; and the tail, formed when gases and dust spread out from the nucleus or coma. Short period comets complete their orbits in less than 200 years; long-period comets may take thousands of years to revolve around the Sun. Comets are thought to originate in the Oort cloud, a region or space that lies outside our Solar System.

Constellation. An arbitrary formation of stars perceived as a figure or design. One of 88 recognized groups named after characters from classical mythology and various common animals and objects.

Core Collapse. Catastrophic gravitational infall of the center of a star when it no longer can generate sufficient pressure to maintain hydrostatic equilibrium.

Corona. The atmosphere of the Sun, composed of hot, very thin gas and extending out from the Sun for a substantial distance. This gas emits light, but normally that light can’t be seen against the direct glare of the Sun. During a total eclipse this direct light is blocked by the Moon and the white glow of the corona becomes visible.

Cosmic Background Radiation. The blackbody radiation, mostly in the microwave band, which consists of photons left over from the very hot, early phase of the Big Bang.

Cosmic Rays. Very high energy atomic particles (mostly protons) traveling through space at high speeds close to that of light. When they hit atoms in the upper atmosphere of the Earth they generate short-lived exotic particles in much the same way that experimental particle accelerators do.

Cosmological Principle. The principle that there is no center to the universe, and that the universe is the same in all directions. This principle means that what we observe of the universe from our specific location is representative of the true nature of the universe.

Cosmology. The study of the universe, its origin and evolution, the distribution and behavior of the matter and energy in it, and the laws that govern these factors.

Dark Matter. Matter that is thought to exist in the universe but has not yet been observed. Its existence is based on measurements of unexplained gravitational effects on visible matter.

Declination. The angular distance to a point on a celestial object, measured north or south from the celestial equator.

Distance Ladder. The chain of overlapping methods by which astronomers establish a distance scale for objects in the universe. At every step of the distance ladder, errors and uncertainties creep in. Each step inherits all the problems of the ones below.

Doppler Effect. The change in frequency of wave motion resulting from motion of the sender or the receiver. Things moving toward you have their emitted wavelengths shortened. Things moving away have their emitted wavelengths lengthened.

Dwarf Planet. A celestial body that orbits the sun and is large enough to assume a nearly round shape, but that does not clear the neighborhood around its orbit and is not a satellite of a planet.

Dwarf Star. White dwarfs are collapsed stars that are still hot and shining; black dwarfs are cold, dead stars; and brown dwarfs are not massive enough to be able to fuse hydrogen in their cores.

Ecliptic. The annual path of the Sun in a circular path in the sky.

Event Horizon. In the case of a black hole, an event horizon is that surface surrounding the region out of which light itself cannot escape.

Exosphere. The fifth atmospheric layer above the Earth’s surface, extending from the thermosphere upward and out into interplanetary space.

Full Moon. The phase of the Moon in which its sunlit side is the side facing Earth.

Fusion, Nuclear. The release of nuclear energy by the fusing of light elements to form a heavier element. The Sun obtains its central power from the fusing of four hydrogen atoms into one helium atom.

Galaxy. A large system of stars, usually containing between a million and a trillion stars, along with clouds of gas and dust. Galaxies are sometimes classified according to their shapes as elliptical, irregular, or spiral.

Galaxy Cluster. Pertains to a group of more than one galaxy.

Galaxy Super Cluster. A group of an enormous number of galaxies.

Galaxy, Elliptical. Elliptical galaxies range in size from the relatively rare giant ellipticals, which can have a trillion stars, to the very common dwarf ellipticals, which can be as small as a million stars.

Galaxy, Irregular. A galaxy with a chaotic appearance and with large clouds of gas and dust, but without spiral arms.

Galaxy, Spiral. A galaxy consisting of a flattened, rotating disk of stars; a central bulge; and a surrounding halo. The disk is prominent due to the presence of young, hot stars which are often arrayed in spiral patterns. The Milky Way is a spiral galaxy.

Giant Molecular Cloud. A region of dense interstellar medium that is sufficiently cold that molecules can form. The molecules in these molecular clouds emit radio radiation which can be detected on Earth. These regions are believed to be where new stars can form.

Globular Cluster. A nearly spherical, dense cluster of hundreds of thousands to millions of stars.

Gravitational Collapse. The contraction of a star when the pressure of thermonuclear reactions can no longer sustain the force of self-gravitation. Collapse occurs at the end of a star’s life when its fuel of hydrogen and other elements is depleted. Depending on its original mass, the star may evolve into a white dwarf, a neutron star, or a black hole, or it may explode as a supernova.

Gravitational Radiation. The theory of general relativity predicts that if one changes the distributions of masses (which generate gravitational fields) in certain ways one can get propagating waves of gravity in a manner analogous to the propagating waves of electric and magnetic fields. Gravitational radiation carries energy and travels at the speed of light.

H II region. A cloud of glowing gas and plasma, sometimes several hundred light-years across, in which star formation is taking place.

H-R (Hertzsprung-Russell) Diagram. A graph that plots star luminosity versus star surface temperature. When so plotted, most stars fall about a main sequence line, with exotic stars above or below these.

Hubbles’s Law. The farther a galaxy, the faster it is receding from Earth. V = H_oD, where V is velocity, D is distance, and H_o is Hubble’s constant.

Interstellar Extinction. As light from a star travels through interstellar space it encounters some amount of dust. This dust scatters some of the light, causing the total intensity of the light to diminish. The more dust, the dimmer the star will appear.

Interstellar Medium. The name given to the material that floats in space between the stars. It consists of gas (mostly hydrogen) and dust. Even at its densest, the interstellar medium is emptier than the best vacuum we can create in the laboratory. Because space is so vast, the interstellar medium adds up to a huge amount of mass.

Interstellar Reddening. As light from a star travels through interstellar space it encounters some amount of dust. This dust scatters some of the light, mainly the short wavelength (blue) components. The spectrum of the light that remains is increasingly dominated by the long wavelength (red) end of the spectrum. Hence the light is “reddened” as it travels through space.

Kuiper Belt. Disk-shaped region of the sky beyond Neptune, populated by many icy bodies. A source of short-period comets.

Lunar Eclipse. The phenomenon whereby the shadow of the Earth falls upon the Moon, producing relative darkness of the full Moon.

Mesophere. The third atmospheric layer above the Earth’s surface, extending from the top of the Stratosphere to a distance of 80 km.

Meteor. A meteoroid that produces a streak of light as it enters the Earth’s atmosphere and is vaporized by the resulting friction. Meteors are commonly called shooting stars.

Meteorite. A meteor that passes through the outer layers of the Earth’s atmosphere and strikes the planet’s surface. Meteorites are classified as siderites (containing only metals, chiefly nickel and iron), aerolites (stony objects consisting of a variety of mineral elements), and siderolites (meteorites composed of both metal and stone).

Meteoroid. A small rock in interplanetary space.

Milky Way. The spiral galaxy in which our Solar System is located. It contains about 150 billion stars, has a diameter of 500,000 light-years, and is about 13.6 billion years old.

Moon Phases. The cycle of change of the face of the Moon, changing from new to waxing, to full, to waning, and back to new.

Near Earth Object (NEO). Asteroids, comets, and large meteoroids whose orbit intersects Earth’s orbit, and which may therefore pose a collision danger.

Nebula. A concentration of gas and dust in the galaxy.

Nebula, Absorption. A nebula seen in silhouette because it is absorbing or blocking light from behind it (also called a dark nebula).

Nebula, Emission. A glowing cloud of hot interstellar gas. The gas is energized by nearby or embedded hot young stars. The gas is mainly hydrogen, and the light mainly hydrogen emission, but other elements are also present and give off their own emission lines.

Nebula, Planetary. A gaseous shell thrown off by a dying star just before the star settles down to become a degenerate white dwarf.

Nebula, Reflection. A nebula composed of dust particles that scatter and reflect incident light (rather than glowing from their own intrinsic emission). Dust preferentially scatters short wavelengths, so reflection nebulae have a characteristic blue appearance.

Neutrino. Any of three species of very weakly-interacting leptons with an extremely small, possibly zero, mass. Electron neutrinos are generated in the interior of the Sun and other stars. Generally, such neutrinos do not interact with matter and stream out through the Sun. In 1987, neutrinos from a Supernova in the Large Magellanic Cloud were detected in terrestrial neutrino experiments.

Neutron Star. A dead star supported by neutron degeneracy pressure. A neutron star is the core remnant left over after a supernova explosion.

Nova. A star which suddenly increases in brightness thousands of      times, then fades back to near its original intensity.

Nuclear Fusion. The combining of nuclei of light atoms to form heavier nuclei, with the release of much energy.

Nucleosynthesis. The process by which heavier chemical elements are synthesized from hydrogen nuclei in the interiors of stars or in the Big Bang.

Nucleus, Galactic. The central region of a galaxy, characterized by high densities of stars. The nucleus may also contain a supermassive black hole and may be the source of considerable high-energy, nonstellar luminosity.

Nutation. A slight irregular motion in the axis of rotation of a largely axially symmetric object, such as a planet.

Oort Cloud. The region beyond the Kuiper Belt populated by trillions of icy bodies, and a source of long-period comets.

Parallax. An apparent shift in the position of an object, such as a star, caused by a change in the observer’s position that provides a new line of sight. In astronomy, the term is used for several techniques for determining distance.

Perturbation. A local gravitational disturbance in the uniform motion of a body because of the gravitational influence of another object.

Photosphere. The surface layer of the Sun where the continuous blackbody-type spectrum is produced that we directly observe when we look at the Sun. It is as far into the Sun as we can directly see.

Planetoid. A mostly obsolete term, usually used to describe the larger remnants of rocks left over from the formation of the Solar System.

Planets. The major bodies orbiting the Sun, massive enough for their gravity to make them spherical, but small enough to avoid nuclear fusion in their cores.

Plasma. Matter in the form of electrically charged particles; the state in which most of the universe exists.

Proper Motion. The measurement of a star’s change in position in the sky over time after improper motions are accounted for. Improper motion is the change of a star’s coordinates on the sky not originating from the motion of the star itself.

Protostar. A forming star prior to settling down to the main sequence and burning hydrogen in its core.

Pulsar. A rotating, magnetized neutron star that produces regular pulses of radiation when observed from a distance. A pulse is produced every time the rotation brings the magnetic pole region of the neutron star into view. In this way the pulsar acts much as a light house, sweeping a short precisely-timed beam of radiation through space.

Quasar. A contraction of the word quasi-stellar used to describe celestial objects with a star-like appearance. Quasars are the most distant objects known. They have large red shifts, indicating great recessional velocities, and emit energy that is more than a thousand times that of an average galaxy.

Radial Velocity. The speed with which a star moves toward or away from the Earth. It is determined from the red or blue shift in the star’s spectrum.

Radio Galaxy. A galaxy that is a powerful source of radio waves.

Radio Jets. Narrow, collimated beams of plasma that are producing radio (synchrotron) emission. These jets emerge from the cores of radio galaxies and can extend outward across regions of space larger than the size of the galaxy itself. These jets are believed to be powered and launched from an accretion disk orbiting a supermassive black hole at the galaxy’s core.

Red Giant. A star with low surface temperature (thus red) and large size (giant). These stars are found on the upper-right hand side of the H-R diagram. The red giant phase in a star’s life occurs after it has left the main sequence. The Sun will become a red giant in about five billion years.

Redshift. Shift of a spectrum of light toward long, red wavelengths.
Due to the Doppler effect resulting from the recession of a star. The faster an object recedes from Earth, the greater the shift of its light toward the red end of the spectrum.

Satellite. A projectile or small celestial body that orbits a larger celestial body.

Solar Wind. A stream of particles, primarily protons and electrons, that constantly flow outward from the Sun.

Space-time. A four-dimensional way of describing events and locations with three units of distance and one of time. Under the influence of gravity, space-time can actually warp and bend.

Spectroscope. An instrument used to determine the spectrum or wavelength of a ray of light emanating from an object.

Spectrum. Radiation (usually visible light) broken into its component wavelengths.

Star. A spherical celestial body consisting of a large mass of hot gas held together by its own gravity. It is self-luminating because of extensive internal nuclear fusion. Our Sun is a typical star.

Star Cluster. Groups of stars held together by mutual gravitational attraction.

Steady-state Theory. Proposes that matter is being continuously created to allow the density of the universe to remain constant as it expands.

Stratosphere. The region of the atmosphere above the troposphere and below the mesosphere.

Sunspots. Temporary, relatively cool and dark regions of the Sun’s surface. Due to intense magnetic fields.

Supercluster. A large conglomeration of galaxy clusters and galaxy groups, typically more than 100 million light-years in size and containing tens of thousands to hundreds of thousands of galaxies.

Supermassive Black Hole. An extremely large black hole that is believed to be at the center of many large galaxies. Has a mass ranging from a few million to more than several billion solar masses.

Supernova. Supernova come in two types: Type I is caused by sudden nuclear burning in a white dwarf star. Type II is caused by the collapse of the core of a super-massive star at the end of its nuclear-burning life. In either case, the star is destroyed and the light given off in its explosion briefly rivals the total light given off by a whole galaxy.

Supernova Remnant. The material blown off during a supernova, seen as a great glowing cloud expanding into space.

Telescope. (a) Reflector uses a single or combination of curved mirrors that reflect light and form an image. (b) Refractor uses a lens as its objective to form an image.

Troposphere. The atmospheric layer closest to the Earth’s surface, containing 90 percent of the atmosphere’s mass and essentially all of its water vapor and clouds.

T Tauri Star. A member of a class of very young, optically visible, solar-mass stars with peculiarities such as variability and evidence for mass loss.

Universe. The entirety of all that is known to exist. The size of the observable universe is limited to the distance light has traveled since the Big Bang.

Variable Star. A star that undergoes significant periodic variation in its luminosity.