Glossary items by letter: c — d

Carbonaceous Chondrite

A meteorite with embedded pebble-sized granules that contain significant quantities of organic (complex carbon-rich) matter.

Cassegrain Telescope

A type of reflecting telescope whose eyepiece is located behind the primary mirror. The primary mirror is cast with a hole in the center. When light enters the telescope, it reflects from the primary mirror to the secondary mirror. The secondary mirror reflects the light back through the hole in the primary mirror to the eyepiece.


Of or relating to the sky or visible objects in the sky, like the Moon, Sun, planets, comets, asteroids, stars, and galaxies.

Celestial Object

An object in the sky – examples include the Moon, the Sun, planets, comets, asteroids, stars, and galaxies.

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.

Celsius (Centigrade) Temperature Scale

A temperature scale on which the freezing point of water is 0° C and the boiling point is 100° C.

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.

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.

Chromatic aberration

Visible light is made of different colors. When visible light passes through a glass lens or a prism, it gets dispersed, or split, into its many colors. A lens focuses each color at a different point, causing a fringe of color to appear around bright objects.

Looking at only red and blue light:


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.


A system of two moveable mirrors used in solar telescopes. The mirrors follow the Sun and keep its image in the same location as Earth rotates.

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.


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.


The cloud of gas and dust that forms around a comet's nucleus. This cloud is created when the solar wind strikes the surface of the nucleus.


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 Nucleus

The core of a comet, made up of ice, dirt, and rock.

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.

Comet Tail

A tail is made up of dust and gas from a comet’s coma. A tail forms when the solar wind separates dust and gas from the coma, pushing it outward and away from the Sun in either a slightly curved path (for dust) or a straight path (for gas).

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.

Concave vs. convex

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.


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.


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.


The central region of a planet, star, or galaxy.


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.

Corrective Optics Space Telescope Axial Replacement (COSTAR)

An apparatus installed during the 1993 First Servicing Mission. By placing small and carefully designed mirrors in the telescope, COSTAR successfully improved restored Hubble’s vision to its original design goals. All the new instruments installed during the servicing missions have internal corrections for spherical aberration and do not require the services of COSTAR. Hubble’s last original instrument, the Faint Object Camera, was replaced by the Advanced Camera for Surveys during SM3B. COSTAR was replaced by the Cosmic Origins Spectrograph during Servicing Mission 4 and returned to Earth in the space shuttle.

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 Origins Spectrograph (COS)

A spectrograph that detects ultraviolet light. A spectrograph works by breaking up light from an object into its individual wavelengths so that its composition, temperature, motion, and other chemical and physical properties can be analyzed. COS will study the structure of the universe and how galaxies, stars and planets formed and evolved. Astronauts installed COS during SM4.

Cosmic Rays

High-energy atomic particles that travel through space at speeds close to the speed of light; also known as cosmic-ray particles.

Cosmic background radiation

Electromagnetic 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.

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.


The investigation of the origin, structure, and development of the universe, including how energy, forces, and matter interact on a cosmic scale.


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.

Crown glass

Originally the main material used to make flat planes of glass for windows, it is composed of soda-lime glass. It can be used to make lenses and prisms. Crown glass bends and disperses, or spreads out, light less than flint glass.

Dark Dust Cloud

A region of interstellar space that contains a rich concentration of gas and dust. Such a cloud is often irregular in shape but sometimes has a well-defined edge. Visible light cannot pass through these clouds, so they obscure the light from stars beyond them.

Dark Matter

Matter that is too dim to be detected by telescopes. Astronomers infer its existence by measuring its gravitational influence. Dark matter makes up most of the total mass of the universe.

Dark energy

A mysterious force that seems to work opposite to that of gravity and makes the universe expand at a faster pace.

Declination (DEC)

One of two celestial coordinates required to locate an astronomical object, such as a star, on the celestial sphere. Declination is the measure of angular distance of a celestial object above or below the celestial equator and is comparable to latitude. To familiarize yourself with declination, hold out your arm in the direction of the North Star (Polaris). You are now pointing at plus 90 degrees declination. Move your arm downward by 90 degrees. You are now pointing at 0 degrees declination.

Degree of Arc

One degree of arc is 1/360 of a full circle. The apparent sizes of objects as seen from Earth can be measured in degrees of arc. The angular diameter of the full moon or the Sun as seen from Earth is one-half of a degree.


The ratio of the mass of an object to its volume. For example, water has a density of one gram of mass for every milliliter of volume.


A device used to measure the amount of electromagnetic radiation emitted by celestial objects. Frequently, detectors are used to sense light that is not visible.


A special form of hydrogen (an isotope called “heavy hydrogen”) that has a neutron as well as a proton in its nucleus.


The distance from one side of a circle to the other measured through the center. For telescopes, the diameter of a lens or mirror is measured from one side to the opposite side, passing through the center.


The separation of heavy matter from light matter, thus causing a variation in density and composition. Differentiation occurs in an object like a planet as gravity draws heavier material toward the planet’s center and lighter material rises to the surface.

Diffraction Grating

A device that splits light into its component parts or spectrum. A diffraction grating often consists of a mirror with thousands of closely spaced parallel lines, which spread out the light into parallel bands of colors or distinct fine lines or bars.

Digital Image

A visible image that is recorded by an electronic detector and subdivided into small picture elements (pixels). Each element is assigned a number that corresponds to the brightness recorded at its physical location on the detector. Computer software converts the numerical information into a visual image. The Hubble Space Telescope records digital images.


Visible light is actually made up of different colors. Each color bends by a different amount when refracted by glass. That’s why visible light is split, or dispersed, into different colors when it passes through a lens or prism. Shorter wavelengths, like purple and blue light, bend the most. Longer wavelengths, like red and orange light, bend the least.

Doppler Effect

The change in the wavelength of sound or light waves caused when the object emitting the waves moves toward or away from the observer; also called Doppler Shift. In sound, the Doppler Effect causes a shift in sound frequency or pitch (for example, the change in pitch noted as an ambulance passes). In light, an object’s visible color is altered and its spectrum is shifted toward the blue region of the spectrum for objects moving toward the observer and toward the red for objects moving away.

Double stars

A system of two stars that are gravitationally bound to each other. They orbit each other around a common center. They can also be called binary stars.

Dwarf Galaxy

A relatively small galaxy. The Large and Small Magellanic Clouds, visible in the Southern Hemisphere, are two dwarf irregular galaxies that are neighbors of the Milky Way.

Dwarf Planet

A celestial body within the solar system that shares the characteristics of planets. It orbits the Sun, is not a moon, and has a spherical or nearly spherical shape. Unlike a planet, however, a dwarf planet has not cleared away any loose cosmic rubble from its orbit. Dwarf planets include Ceres, Pluto, and Eris.

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