Astrobiology Glossary A – Space

Term Definition
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.
Absorption 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 covers twice the area, has twice the sharpness, and is up to ten times more efficient than the telescope’s Wide Field and Planetary Camera 2. The camera’s wavelength range spans from ultraviolet to near-infrared light. The camera’s sharp eye and broader viewing area enable astronomers to study the life cycles of galaxies in the remotest regions of the cosmos. Astronauts installed the camera aboard the telescope in March 2002.
Afterglow 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.
ALH84001 Martian meteorite discovered in the Allan Hills ice field in Antarctica in 1984. It has been suggested that this meteorite contains evidence of martian microbial life.
Alkalinity pH values above 7
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.
Amino Acid Simple organic molecules containing an amino group (NH2) and a carboxyl group (COOH). Amino acids link together in chains to form proteins.
Amplitude 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.
Antimatter 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.
Archaea One of three domains of life, along with the Bacteria and the Eucarya. The archaea are prokaryotic organisms which were shown to be a distinct group from the bacteria by molecular phylogeny using 16S ribosomal RNA. The archaeal domain includes many extremophilic organisms.
Archaebacteria An alternative name for the domain archaea.
Archean The period in the history of the Earth from 4000 million years ago to 2500 million years ago, part of the Precambrian. The Archean is preceeded by the Hadean and followed by the Proterozoic. The earliest fossil evidence for life on Earth appears during the Archean period.
Asexual Involving reproduction that occurs without the union of male and female gametes, as in binary fission or budding.
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.
Asteroid A small, rocky object revolving around the Sun, sometimes called a minor planet or planetoid. The vast majority of asteroids is found in the asteroid belt, a region between the orbits of Mars and Jupiter. The largest known asteroid, Ceres, has a diameter of 926 kilometers (579 miles).
Asteroid Belt A region of space between Mars and Jupiter where the great majority of asteroids is found.
Astronomer A scientist who studies the universe and the celestial bodies that reside 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 The scientific study of matter in outer space.
Atmosphere The layer of gases surrounding the surface of a planet, moon, or star.
Atom 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.
ATP Adenosine triphosphate The molecule which is the source of energy for most metabolic processe in living organisms.
Aurora 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.
Autotroph A self-nourishing organism.

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