dark era: The final era of the Universe when only low energy photons, neutrinos, and some elementary particles (the ones that did no find a partner to annihilate) remain.


dark matter: Non-luminous matter. See hot dark matter and cold dark matter.


dark nebula: Dust and gas seen in silhouette.


daughter molecules: Relatively simple molecules in comets resulting from the breakup of  ore complex molecules.


declination: Celestial latitude, measured in degrees north or south of the celestial equator.


deferent: In the Ptolemaic system of the Universe, the larger circle, centered at the Earth, on which the centers of the epicycles revolve.


degenerate era: The future era when the Universe will be filled with cold brown dwarfs, white dwarfs, and neutron stars.


density: Mass divided by volume.


density wave: A circulating region of relatively high density, important, for example, in models of spiral arms.


density-wave theory: The explanation of spiral structure of galaxies as the effect of a wave of compression that rotates around the center of the galaxy and causes the formation of stars in the compressed region.


deuterium: An isotope of hydrogen that contains 1 proton and 1neutron.


deuteron: A deuterium nucleus, containing 1 proton and 1 neutron.


diamond-ring effect: The last Baily’s bead glowing brightly at the beginning of the total phase of a solar eclipse, or its counterpart at the end of totality.


differential forces: A net force resulting from the difference of two other forces; a tidal force.


differential rotation: Rotation of a body in which different parts have different angular velocities (and thus different periods of rotation).


differentiation: For a planet, the existence of layers of different structure or composition.


diffraction: A phenomenon affecting light as it passes any obstacle, spreading it out in a complicated fashion.


diffraction grating: A very closely ruled series of lines that, through diffraction of light, provides a spectrum of radiation that falls on it.


dirty snowball: A theory explaining comets as amalgams of ices, dust, and rocks.


discrete: Separated; isolated.


disk: (a) Of a galaxy, the disk-like flat portion, as opposed to the nucleus or the halo; (b) of a star or planet, the two-dimensional projection of its surface.


dispersion: (a) Of light, the effect that different colors are bent by different amounts when passing from one substance to another; (b) of the pulses of a pulsar, the effect that a given pulse, which leaves the pulsar at one instant, arrives at the Earth at different times depending on the different wavelength or frequency at which it is observed.  Both of these effects arise because light of different wavelengths travels at different speeds except in a vacuum.


D lines: A pair of lines from sodium that appear in the yellow part of the spectrum.


DNA: Deoxyribonucleic acid, a long chain of molecules that contains the genetic information of life.


Dobsonian: An inexpensive type of large-aperture amateur telescope characterized by a thin mirror, composition tube, and Teflon bearings on an alt-azimuth mount.


Doppler effect: A change in wavelength that arises from the Doppler effect, caused by relative motion toward or away from the observer.


Doppler shift: The change in wavelength that arises from the Doppler effect, caused by relative motion toward or away from the observer.


double star: A binary star; two or more stars orbiting each other.


double-lobed structure: An object in which radio emission comes from a pair of regions on opposite sides.


Drake Equation: An equation advanced by Frank Drake and popularized also by Carl Sagan that attempts to calculate the number of civilizations by breaking the calculation down into a series of steps that can be assessed individually, such as rate of star formation, the fraction of stars with planets, and the average lifetime of a civilization.


dust tail: The dust left behind a comet, reflecting sunlight.


dwarf ellipticals: Small, low-mass elliptical galaxies.


dwarf stars: Main-sequence stars.


dwarfs: Dwarf stars.


dynamo: A device that generates electricity through the effect of motion in the presence of a magnetic field.


dynamo theories: Explaining sunspots and the solar activity cycle through an interaction of motion and magnetic fields.