Astronmy Glossary
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Aberration
An effect caused by the Earth's motion which slightly changes the positions of stars. Aberration of starlight was discovered by James Bradley in 1728. This was the first direct proof of the Earth's movement around the Sun.

Absolute Magnitude
A measurement that is used to compare the total light output of stars. Astronomers use this to determine how far away a star is.

Altitude
The angle between a celestial object and the horizon.

Alpha Beta Gamma Theory
An explanation of how primordial hydrogen was partly converted into helium during the Big Bang to provide the raw material from which stars were made. This theory predicts the existence of background radiation, which was later discovered to fill the universe.

Alpha decay
The process in which the nucleus of an atom emits an alpha particle and is transformed into a nucleus with four less atomic mass units and two fewer units of charge.

Alpha Particle
Two protons and two neutrons held together by nuclear interactions to make a stable nucleus.

Andromeda
A constellation in the northern hemisphere of the sky, located next to Pegasus. Andromeda is 5,000 light-years away. At the heart of the constellation is the Andromeda galaxy.

Andromeda galaxy
The most distant object in space that is visible to the naked eye. It is a separate spiral galaxy in space, twice the diameter of the Milky Way. This galaxy appears as a fuzzy oval patch to the naked eye, but telescopes show that it is composed of approximately 300 billion stars. It is about 2.2 million light-years away, making it the closest major galaxy to our own.

Antimatter
Matter which consists of antiparticles, elementary particles of ordinary matter, such as protons and electrons, but have an opposite electrical charge. For example, an antiproton has a negative charge while a proton has a positive charge. When a particle of matter and a particle of antimatter come into contact with each other they are destroyed, releasing energy.

Aperture synthesis
A technique used in astronomy to make radio telescopes simulate the power of much larger instruments. Antennae are linked together to observe the same part of the sky. The information received by all the telescopes are combined to produce a final image of the sky.

Apollo program
An American space project that landed 12 astronauts on the Moon between 1966 and 1972.

Apparent magnitude
How bright a star appears to be from Earth.

Asteroid
A small, rocky body that moves in an elliptical orbit around the Sun. They are too small to have atmospheres, and occasionally fall to Earth as Meteorites. On January 1, 1801, an Italian astronomer, Giuseppe Piazzi, became the first person to discover an asteroid which was later named Ceres.

Astrometry
The measurement of objects' positions in space.

Astronomy
The science of the celestial bodies: the Sun, the Moon, the planets, the stars and galaxies, and every other object in the universe. Astronomy includes the positions, motions, distances, physical conditions, and the origin and evolution of the celestial bodies.

Astrophysics
The application of physics to the study of the universe.

Baby Universes
Regions of space time that are connected to one another by wormholes.

Background radiation
Radiation that comes from space at radio and infrared wavelengths.

Barnard's star
The second-closest star to the Sun, about 5.9 light-years away. It is named after Edward E. Barnard who discovered it in 1916. This star has the fastest proper motion of any star that has been discovered.

Beta decay
The process in which a neutron emits an electron and an anti-neutrino as it transforms into a proton. When this happens inside a nucleus, the charge of the nucleus is increased by one unit.

Big Bang
The event which is believed to have started the universe's expansion. It is believed that the Big Bang occurred 18 billion years ago when all the matter in the universe was squeezed into a fireball. The fireball exploded, rapidly expanding the universe and sending particles into the newly created space.

Black hole
A theoretical object which has a gravitational pull so strong that nothing can escape, not even light.

BL Lac objects
These are bright, compact sources of intense energy located in the middle of some galaxies.

Bursters
Very powerful gamma-ray sources.

Celestial mechanics
The study of the motion and gravitational interaction of bodies in space. It is used to calculate the orbits of objects in space and predict their motion.

Cepheid variable
A star which enlarges and contracts in size, varying in brightness as it does so. They can be used to measure distances to objects in space.

Ceres
The largest and the first asteroid to be discovered, by Guiseppe Piazzi in 1801. It is one of the few asteroids to be visible to the naked eye.

Chromosphere
A layer of gas about 10,000 miles (16,000 kilometers) thick above the Sun's surface. It is a pinkish-red color because of the light emitted from hydrogen atoms.

Comet
A small, icy body embedded in a cloud of gas which orbits around the Sun. When they orbit close to the Sun they heat up, releasing gas, which appears as a tail always pointing away from the Sun.

Cosmic rays
The nuclei of atoms which are stripped of all their electrons that move through space at speeds close to that of light.

Cosmology
The study of the origin of the Universe.

Crab Nebula
An expanding cloud of gas that was ejected by a star that exploded in July, 1054 AD. It is abut 6,300 light-years away from the Sun and emits powerful radiation at all of the wavelengths.

Craters
Bowl-shaped depressions formed by the striking of Meteorites.

Curvature of space
A distortion of space that is caused by the presence of matter. For example, the Earth is spherical, but on a map it is shown to be flat with the Polar regions "stretched out."

Degenerate Matter
Matter that is at such a high density that quantum effects dominate its behavior. The outward pressure is much greater than the pressure appropriate to that density of material.

Dirty Snowball
A term used to describe the core (nucleus) of a comet which is made of ice and dust.

Doppler effect
The change in frequency of waves emitted by an object as it moves. For example, as an emergency vehicle approaches the sirens get louder and as the vehicle moves away the sirens fade out.

Double star
Also called a binary star, two stars that are linked together by gravity. The first double star was identified in 1650. About 46 percent of all stars are double stars, 15 percent are single stars, and 39 percent are multiple systems of three or more stars.

Dwarf stars
Ordinary stars, like the Sun, that are in the prime of their lives, converting hydrogen into helium in their centers.

Eclipse
The passage of one astronomical body into the shadow of another, but this term is usually applied to the passage of the Moon in front of the Sun, called a solar eclipse. A Lunar eclipse is when the Moon passes into the Earth's shadow, which can only happen during a full Moon.

Electromagnetic radiation
The range of radiation, from gamma rays through to the spectrum of visible light, and to radio waves.

Ellipse
A shape that looks like a squashed circle that is produced by cutting a cone at an angle.

Emission
The production of Electromagnetic Radiation. Radiation is emitted when atomic particles accelerate or decelerate.

Fermions
Elementary particles that are conserved during particle interactions. The archetypal fermion is the electron because an electron cannot be created or destroyed in a particle interaction.

Fireball
A meteor that has a magnitude of -5 or brighter. Occasionally they are as bright as magnitude -25. The Moon has a magnitude of -12.7.

Fission
See Nuclear fission.

Flare
A burst of light, lasting only a few hours, that occurs near a Sunspot.

Frequency
The number of waves that pass through a specific point in a certain period of time.

Fusion
See Nuclear fusion.