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Astrobiology Glossary C – Space

Term Definition
Carbohydrate Any of a group of organic compounds that includes sugars, starches, celluloses, and gums and serves as a major energy source in the diet of animals. These compounds are produced by photosynthetic plants and contain only carbon, hydrogen, and oxygen, usually in the ratio 1:2:1.
Carbonaceous Chondrite A rare type of meteorite which is rich in organic material. The Murchison meteorite is a famous example.
Carbonaceous Chondrite A meteorite with embedded pebble-sized granules that contain significant quantities of organic (complex carbon-rich) matter.
Celestial Sphere An imaginary sphere encompassing the Earth that represents the sky. Astronomers chart the sky using the celestial coordinates of the sphere to locate objects in the cosmos. This sphere is divided into 88 sections called constellations. Objects are sometimes named for the major constellation in which they appear.
Cell The smallest structural unit of an organism that is capable of independent functioning, consisting of one or more nuclei, cytoplasm, and various organelles, all surrounded by a semipermeable cell membrane.
Celsius (Centigrade) Temperature Scale A temperature scale on which the freezing point of water is 0° C and the boiling point is 100° C.
Cenancestor An alternative term for the Last Common Ancestor of all life on Earth.
Cepheid Variable A type of pulsating star whose light and energy output vary noticeably over a set period of time. The time period over which the star varies is directly related to its light output or luminosity, making these stars useful standard candles for measuring intergalactic distances.
Chandra X-Ray Observatory A space-based X-ray observatory; also known as the Advanced X-ray Astrophysics Facility (AXAF). Chandra is designed to observe X-rays from high-energy regions of the universe, such as hot gas in the remnants of exploded stars. The satellite was launched and deployed in July 1999.
Charge-Coupled Device (CCD) An electronic detector that records visible light from stars and galaxies to make photographs. These detectors are very sensitive to the extremely faint light of distant galaxies. They can see objects that are 1,000 million times fainter than the eye can see. CCDs are electronic circuits composed of light-sensitive picture elements (pixels), tiny cells that, placed together, resemble mesh on a screen door. The same CCD technology is used in digital cameras. The Hubble Space Telescope’s Wide Field and Planetary Camera 2 has four CCDs; each contains 640,000 pixels. The light collected by each pixel is translated into a number. These numbers (all 2,560,000 of them) are sent to ground-based computers, which convert them into an image.
Chemical Compound A pure substance consisting of atoms or ions of two or more different elements. The elements are in definite proportions. A chemical compound usually possesses properties unlike those of its constituent elements. For example, table salt (the common name for sodium chloride) is a chemical compound made up of the elements chlorine and sodium.
Chemical Evolution The chemical (i.e., pre-biological) changes that transformed simple atoms and molecules into the more complex chemicals needed for the origin of life. For example, hydrogen atoms in the cores of stars combine through nuclear fusion to form the heavier element helium.
Chemosynthesis The use of inorganic substances such as carbon dioxide to make carbohydrates from the energy released by chemical reactions.
Chirality A chiral molecule is a molecule with an asymmetric structure which can exist in two mirror image forms (or enantiomers). In living organisms such molecules are usually found in only one of the two possible enantiomers (homochirality). Thus amino acids are normally in the left handed or L enantiomer, while sugars are in the right handed or D enantiomer.
Chloroplast An organelle found in the cells of green plants in which photosynthesis occurs. According to the endosymbiosis theory, chloroplasts are descended from photosynthetic bacteria.
Chromosphere The middle layer of the solar atmosphere between the photosphere and the corona. The chromosphere is roughly 10,000 kilometers (6,200 miles) thick and is composed primarily of hydrogen. It varies in temperature from below 10,000 Kelvin (18,000° F) to over 100,000 Kelvin (180,000° F).
Closed Universe A geometric model of the universe in which the overall structure of the universe closes upon itself like the surface of a sphere. The rules of geometry in a closed universe are like those that would apply on the surface of a sphere.
Coacervate A primitive cell-like structure.
Collecting Area The area of a telescope’s primary light-collecting mirror. A telescope’s light-gathering power rises with an increase in its collecting area.
Colliding Galaxies A galactic ‘car wreck’ in which two galaxies pass close enough to gravitationally disrupt each other’s shape. The collision rips streamers of stars from the galaxies, fuels an explosion of star birth, and can ultimately result in both galaxies merging into one.
Collisional Process An event involving a collision of objects; for example, the excitation of a hydrogen atom when it is hit by an electron.
Color The visual perception of light that enables human eyes to differentiate between wavelengths of the visible spectrum, with the longest wavelengths appearing red and the shortest appearing blue or violet.
Comet A ball of rock and ice, often referred to as a ‘dirty snowball.’ Typically a few kilometers in diameter, comets orbit the Sun in paths that either allow them to pass by the Sun only once or that repeatedly bring them through the solar system (as in the 76-year orbit of Halley’s Comet). A comet’s ‘signature’ long, glowing tail is formed when the Sun’s heat warms the coma or nucleus, which releases vapors into space.
Comet Shoemaker-Levy 9 (SL-9) A comet that became gravitationally bound to Jupiter, colliding with the planet in July 1994. Prior to entering the planet’s atmosphere, the comet broke into several distinct pieces, each with a separate coma and tail.
Compton Gamma-Ray Observatory (CGRO) A space-based observatory that collected high-energy gamma-ray light from celestial objects. The Compton satellite consisted of the BATSE, COMPTEL, EGRET, and OSSE instruments. Astronauts aboard the space shuttle Atlantis deployed the CGRO into low-Earth orbit in April 1991. The satellite plunged into the Pacific Ocean in June 2000.
Conservation of Energy And Mass A fundamental law of physics, which states that the total amount of mass and energy in the universe remains unchanged. However, mass can be converted to energy, and vice versa.
Constellation A geometric pattern of bright stars that appears grouped in the sky. Ancient observers named many constellations after gods, heroes, animals, and mythological beings. Leo (the Lion) is one example of the 88 constellations.
Convection The transfer of heat through a liquid or gas caused by the physical upwelling of hot matter. The heat transfer results in the circulation of currents from lower, hotter regions to higher, cooler regions. An everyday example of this process is boiling water. Convection occurs in the Sun and other stars.
Convection Zone The region below a star’s surface where energy flows outward by the rising of hot gas known as convection.
Core The central region of a planet, star, or galaxy.
Corona The outermost layer of the atmosphere of a star, including the Sun. The corona is visible during a solar eclipse or when special adapters or filters are attached to a telescope to block the light from the star’s central region. The gaseous corona extends millions of kilometers from the star’s surface and has a temperature in the millions of degrees.
Coronal Hole Regions in the corona from which the high-speed solar wind is known to originate. Coronal holes, usually found near the Sun’s poles, are large regions in the corona that are less dense and cooler than the surrounding region.
Cosmic Abundances The relative proportions of chemical elements in the Sun, the solar system, and the local region of the Milky Way galaxy. These proportions are determined by studies of the spectral lines in astronomical objects and are averaged for many stars in our cosmic neighborhood. For example, for every million hydrogen atoms in an average star like our Sun, there are 98,000 helium atoms, 360 carbon atoms, 110 nitrogen atoms, 850 oxygen atoms, and so on.
Cosmic Microwave Background Radiative energy filling the universe that is believed to be the radiation remaining from the Big Bang. It is sometimes called the ‘primal glow.’ This radiation is strongest in the microwave part of the spectrum but has also been detected at radio and infrared wavelengths. The intensity of the cosmic microwave background from every part of the sky is almost exactly the same.
Cosmic Rays High-energy atomic particles that travel through space at speeds close to the speed of light; also known as cosmic-ray particles.
Cosmological Principle This principle states that the distribution of matter across very large distances is the same everywhere in the universe and that the universe looks the same in all directions. According to this principle, our view of the universe is like the view from a boat on an ocean, which is essentially the same for any other person on any other boat on any other ocean. Measurements of matter and energy in the universe on the largest observable scales support the cosmological principle.
Cosmology The investigation of the origin, structure, and development of the universe, including how energy, forces, and matter interact on a cosmic scale.
Crater A bowl-shaped depression caused by a comet or meteorite colliding with the surface of a planet, moon, or asteroid. On geologically active moons and planets (like Earth), craters can result from volcanic activity.
Critical Density The minimum average density that matter in the universe would need in order for its gravitational pull to slow the universe’s expansion to a halt.
Cyanobacteria A class of bacteria which make use of oxygen producing photosynthesis. Commonly referred to as blue green algae.