ESO Astronomical Glossary - D
Damped Lyman Alpha Absorber
Damped Lyman Alpha Absorbers (also: Damped Lyman alpha systems or Damped Lyman alpha absorption systems) are gaseous objects detected in the spectra of quasars, with a column density larger than 1020 atoms/cm2. Their spectra consist of neutral hydrogen Lyman alpha absorption lines, broadened by radiation damping.
Dark energy is a type of "negative gravity" that seems to play a role in the acceleration of the expansion of the Universe. Current models predict that dark energy makes up 70% of the Universe. Its nature is still unknown.
Dark matter is material that exerts gravitational force but does not emit any detectable light or radiation. Dark matter is thought to make up around 25% of the content of the Universe but its exact nature remains unknown.
Disc is a general term that is used to describe flattened circular objects. Other uses of the term disc include disc-shaped outer regions of a spiral galaxy surrounding the central bulge (see galactic disc). See also accretion disc and (proto)planetary disc.
The Doppler effect (or Doppler shift) is an increase or decrease in the wavelength of the radiation emitted by an object, as observed from Earth, as the object moves relative to the observer. The wavelength is shortened, or the radiation appaears 'bluer', when the source is moving towards the observer (blueshift) and lengthened, appearing 'reddened' when moving away from them (redshift, often denoted by the symbol 'z'). Most objects in the Universe are seen to recede from each other, and thus appear 'redshifted'. From the shift, the object's speed and distance can be calculated. Studying objects with high redshift can give us information on conditions in the early Universe.
A double star is a pair of stars that appear close to one another in the sky. Some are true binaries (two stars that revolve around one another); others just appear close together from the Earth because they are both in the same line-of-sight (optical doubles).
Very different from our common household dust, 'cosmic' dust is found in many parts of the Universe: around stars, between galaxies, and throughout the solar system. It is composed of microscopic particles (in the range of a micron to a millimetre in size) of carbon and/or silicates. Once thought to be an annoyance, as it obscures our view of interesting objects, dust is now known to play an important role in many astrophysical processes, and is now a field of study in astronomy in its own right.
Dwarf galaxies are small galaxies containing up to a few billion stars (compared with several hundred billion in the Milky Way galaxy). They are further classified according to their shape, as with regular-sized galaxies, and are often seen to orbit larger galaxies.
Dwarf planets are a category of solar system bodies created by the International Astronomical Union in 2006 to describe objects orbiting the Sun that are big and heavy enough to resemble a planet, but not big enough to 'clear' a free path on its orbit. Since 2006, Pluto has been reclassified as a dwarf planet. The dwarf planet class also contains the largest asteroid Ceres and the trans-Neptunian object Eris.