Glossary items by letter: a — b

Absolute Brightness (Absolute Magnitude)

A measure of the true brightness of an object. The absolute brightness or magnitude of an object is the apparent brightness or magnitude it would have if it were located exactly 32.6 light-years (10 parsecs) away. For example, the apparent brightness of our Sun is much greater than that of the star Rigel in the constellation Orion because it is so close to us. However, if both objects were placed at the same distance from us, Rigel would appear much brighter than our Sun because its absolute brightness is much larger.

Absolute Zero

The coldest possible temperature, at which all molecular motion stops. On the Kelvin temperature scale, this temperature is the zero point (0 K), which is equivalent to –273° C and –460° F.


The process by which light transfers its energy to matter. For example, a gas cloud can absorb starlight that passes through it. After the starlight passes through the cloud, dark lines called absorption lines appear in the star’s continuous spectrum at wavelengths corresponding to the light-absorbing elements.

Absorption Line

A dark line in a continuous spectrum caused by absorption of light. Each chemical element emits and absorbs radiated energy at specific wavelengths, making it possible to identify the elements present in the atmosphere of a star or other celestial body by analyzing which absorption lines are present.

Accelerating Universe

A model for the universe in which a repulsive force counteracts the attractive force of gravity, driving all the matter in the universe apart at speeds that increase with time. Recent observations of distant supernova explosions suggest that we may live in an accelerating universe.

Accretion Disk

A relatively flat, rapidly rotating disk of gas surrounding a black hole, a newborn star, or any massive object that attracts and swallows matter. Accretion disks around stars are expected to contain dust particles and may show evidence of active planet formation. Beta Pictoris is an example of a star known to have an accretion disk.

Active Galactic Nucleus (AGN)

A very bright, compact region found at the center of certain galaxies. The brightness of an active galactic nucleus is thought to come from an accretion disk around a supermassive black hole. The black hole devours matter from the accretion disk, and this infall of matter provides the firepower for quasars, the most luminous type of active galactic nucleus.

Active Galaxy

A galaxy possessing an active galactic nucleus at its center.

Advanced Camera For Surveys (ACS)

An optical camera aboard the Hubble Space Telescope that uses CCD detectors to make images. The camera covers twice the area, has twice the sharpness, and is up to 10 times more efficient than the telescope’s Wide Field and Planetary Camera 2. The ACS wavelength range spans from ultraviolet to near-infrared light. The camera’s sharp eye and broader viewing area allow astronomers to study the life cycles of galaxies in the remotest regions of the universe. Astronauts installed the camera aboard the telescope in March 2002, but the camera experienced an electrical short in 2007 that shut down all but one data channel. During Servicing Mission 4 in 2009, astronauts replaced the failed circuit boards and added a new power supply box to restore power to the camera.


The fading fireball of a gamma-ray burst — a sudden burst of gamma rays from deep space — that is observable in less energetic wavelengths, such as X-ray, optical, and radio. After an initial explosion, an expanding gamma-ray burst slows and sweeps up surrounding material, generating the afterglow, which is visible for several weeks or months. The afterglow is usually extremely faint, making it difficult to locate and study.


A mixture of two or more metals. Brass (a mixture of copper and zinc) and bronze (a mixture of copper and tin) are common alloys.

Alpha Process

A process by which lighter elements capture helium nuclei (alpha particles) to form heavier elements. For example, when a carbon nucleus captures an alpha particle, a heavier oxygen nucleus is formed.


A type of telescope mounting that supports the weight of the telescope and allows it to move in two directions to locate a specific target. One axis of support is vertical (called the altitude) and allows the telescope to move up and down. The other axis is horizontal (called the azimuth) and allows the telescope to swing in a circle parallel to the ground. This makes it easy to position the telescope: swing it around in a circle and then lift it to the target. However, tracking an object as the Earth turns is more complicated. The telescope needs to be adjusted in both directions while tracking, which requires a computer to control the telescope.


To make larger or more powerful; increase. Radio signals are amplified because they are very weak.


The size of a wave from the top of a wave crest to its midpoint.

Angular Momentum

A property that an object, such as a planet revolving around the Sun, possesses by virtue of its rotation or circular motion. An object’s angular momentum cannot change unless some force acts to speed up or slow down its circular motion. This principle, known as conservation of angular momentum, is why an object can indefinitely maintain a circular motion around an axis of revolution or rotation.

Angular Resolution

The ability of an instrument, such as a telescope, to distinguish objects that are very close to each other. The angular resolution of an instrument is the smallest angular separation at which the instrument can observe two neighboring objects as two separate objects. The angular resolution of the human eye is about a minute of arc. As car headlights approach from a far-off point, they appear as a single light until the separation between the lights increases to a point where they can be resolved as two separate lights.

Angular Size

The apparent size of an object as seen by an observer; expressed in units of degrees (of arc), arc minutes, or arc seconds. The moon, as viewed from the Earth, has an angular diameter of one-half a degree.


An electrical device used to send or receive electromagnetic waves. The aerial (a long piece of metal attached to the front or rear fender) on a car is the antenna for the radio.


Matter made up of elementary particles whose masses are identical to their normal-matter counterparts but whose other properties, such as electric charge, are reversed. The positron is the antimatter counterpart of an electron, with a positive charge instead of a negative charge. When an antimatter particle collides with its normal-matter counterpart, both particles are annihilated and energy is released.

Apparent Brightness (Apparent Magnitude)

A measure of the brightness of a celestial object as it appears from Earth. The Sun is the brightest object in Earth's sky and has the greatest apparent magnitude, with the moon second. Apparent brightness does not take into account how far away the object is from Earth.

Arc Minute

One arc minute is 1/60 of a degree of arc. The angular diameter of the full moon or the Sun as seen from Earth is about 30 arc minutes.

Arc Second

One arc second is 1/60 of an arc minute and 1/3600 of an arc degree. The apparent size of a dime about 3.7 kilometers (2.3 miles) away would be an arc second. The angular diameter of Jupiter varies from about 30 to 50 arc seconds, depending on its distance from Earth.


An orderly arrangement or impressive display. For radio telescopes, an array is a group of individual radio dishes that work together. The VLA (Very Large Array) has 27 telescope dishes arranged in a “Y” pattern.

Association of Universities for Research in Astronomy, Inc. (AURA)

A consortium of educational and other non-profit institutions that operates world-class astronomical observatories. Members include five international affiliates and 29 U.S. institutions, including the Space Telescope Science Institute in Baltimore, Maryland, the science operations center for NASA’s Hubble Space Telescope.


A small solar system object composed mostly of rock. Many of these objects orbit the Sun between Mars and Jupiter. Their sizes range anywhere from 33 feet (10 meters) in diameter to less than 620 miles (1,000 kilometers). The largest known asteroid, Ceres, has a diameter of 579 miles (926 kilometers).

Asteroid Belt

A region of space between Mars and Jupiter where the great majority of asteroids is found.


A scientist who studies the universe and the celestial bodies residing in it, including their composition, history, location, and motion. Many of the scientists at the Space Telescope Science Institute are astronomers. Astronomers from all over the world use the Hubble Space Telescope.

Astronomical Unit (AU)

The average distance between the Earth and the Sun, which is about 150 million kilometers (93 million miles). This unit of length is commonly used for measuring the distances between objects within the solar system.


Astronomy is the study of the universe and the celestial bodies that reside in it, including their composition, history, location, and motion.


The layer of gases surrounding the surface of a planet, moon, or star.

Atmospheric distortion

The blurring of an image due to the layer of gases surrounding the surface of Earth. As starlight travels through the atmosphere, pockets of air act like little lenses and bend the light in unpredictable ways. This distortion causes stars to appear to twinkle.


The smallest unit of matter that possesses chemical properties. All atoms have the same basic structure: a nucleus containing positively charged protons with an equal number of negatively charged electrons orbiting around it. In addition to protons, most nuclei contain neutral neutrons whose mass is similar to that of protons. Each atom corresponds to a unique chemical element determined by the number of protons in its nucleus.

Atomic Nucleus

The positively charged core of an atom consisting of protons and (except for hydrogen) neutrons, and around which electrons orbit.


A phenomenon produced when the solar wind (made up of energized electrons and protons) disturbs the atoms and molecules in a planet’s upper atmosphere. Some of the energy produced by these disturbances is converted into colorful visible light, which shimmers and dances. Auroras have been seen on several planets in our solar system. On Earth, auroras are also known as the “Northern Lights” (aurora borealis) or “Southern Lights” (aurora australis), depending on in which polar region they appear.


An imaginary line through the center of an object. The object rotates around this line.

BATSE (Burst and Transient Source Experiment)

A high-energy astrophysics “experiment” used to investigate gamma-ray bursts (GRBs). BATSE consisted of eight detectors that were mounted on the corners of NASA’s Earth-orbiting Compton Gamma-Ray Observatory, whose mission ended in 2000.

Barred Spiral Galaxy

A galaxy with a “bar” of stars and interstellar matter, such as dust and gas, slicing across its center. The Milky Way is thought to be a barred spiral galaxy.


The distance between two or more telescopes that are working together as a single instrument to observe celestial objects. The wider the baseline, the greater the resolving power.


Batteries provide all the electrical power to support Hubble operations during the night portion of its orbit, when the telescope is in Earth’s shadow. The telescope’s orbit is approximately 97 minutes long. Roughly 61 minutes of Hubble’s orbit are in sunlight and 36 minutes are in Earth’s shadow. During Hubble’s sunlight or daytime period, the solar arrays provide power to the onboard electrical equipment. The solar arrays also charge the spacecraft’s batteries so they can power the spacecraft during the night portion of Hubble’s orbit. Hubble has six nickel-hydrogen batteries. These batteries, which had been onboard Hubble since the telescope was launched in 1990, were replaced during Servicing Mission 4.


A space-based X-ray observatory built and operated by the Italian Space Agency and the Netherlands Agency for Aerospace Programs. BeppoSAX has been instrumental in identifying and locating gamma-ray bursts.

Big Bang

A broadly accepted theory for the origin and evolution of our universe. The theory says that the observable universe started roughly 13.7 billion years ago from an extremely dense and incredibly hot initial state.

Binary Star System

A system of two stars orbiting around a common center of mass that are bound together by their mutual gravitational attraction.

Black Hole

A region of space containing a huge amount of mass compacted into an extremely small volume. A black hole’s gravitational influence is so strong that nothing, not even light, can escape its grasp. Swirling disks of material — called accretion disks — may surround black holes, and jets of matter may arise from their vicinity.

Blue Star

A massive, hot star that appears blue in color. Spica in the constellation Virgo is an example of a blue star.


The shortening of a light wave from an object moving toward an observer. For example, when a star is traveling toward Earth, its light appears bluer.


Large, brilliant meteors that enter the Earth’s atmosphere. Friction between a fast-moving meteor and Earth’s air molecules generates tremendous heat, which causes the meteor to heat up, glow, and perhaps disintegrate. In some cases, the meteor literally explodes, leaving a visible cloud that dissipates slowly.

Brown Dwarf

An object too small to be an ordinary star because it cannot produce enough energy by fusion in its core to compensate for the radiative energy it loses from its surface. A brown dwarf has a mass less than 0.08 times that of the Sun.


The spherical structure at the center of a spiral galaxy that is made up primarily of old stars, gas, and dust. The Milky Way’s bulge is roughly 15,000 light-years across.

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