Astrobiology Glossary D – Space

Term Definition
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.
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.
Density 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.
Deuterium A special form of hydrogen (an isotope called ‘heavy hydrogen’) that has a neutron as well as a proton in its nucleus.
Differentiation 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.
DNA Deoxyribonucleic Acid The molecule which constitutes the genome of living cells. DNA molecules have a double stranded helical structure built from a sugar phosphate backbone and a set of four bases (adenine, guanine, cytosine and thymine). The sequence of bases specifies the genetic information.
Domain The highest taxonomic division in the classification of living organisms. The three domains are the Archaea, the Bacteria and the Eucarya. Domains are subdivided into kingdoms. While the three domain model is widely used in astrobiology, some biologists prefer other schemes such as the Five Kingdom system.
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.
Doppler Shift The shift in wavelength of light or other electromagnetic radiation due to the motion of the source relative to the observer. Such observations are used to detect extrasolar planets.
Drake Equation An equation proposed by SETI pioneer Frank Drake to calculate the number of civilizations in the galaxy.
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.
Dyson Sphere A type of orbiting habitat that gathers energy from the sun.