The dark cloud Barnard 68 absorbs all background starlight because of the amount of interstellar dust it contains. Credit: ESO.
Interplanetary dust grain collected from high in the atmosphere. Credit: NASA Johnson Space Center.
Cosmic dust is microscopic grains of matter that occur in space. These grains play an important role in the origin of planetary systems (see planetary systems, formation) and possibly even that of life itself. The composition, size, and other properties of cosmic dust particles vary from one location to another. Grains in dense interstellar clouds, for example, are larger than those in the general interstellar medium, while larger particles still are found in circumstellar dust disks.
Most of the dust in interstellar space comes from stars that have moved off the main sequence and entered the red giant phase of their evolution (see stars, evolution). These stars have extended atmospheres rich in silicon, oxygen, and carbon – elements that were manufactured in the stellar core but that have been dredged to the surface by convection currents. Depending on its life history, a red giant may have surface layers that are rich in either carbon or oxygen. A carbon star gives rise to a dense pall of carbon particles, in the form of graphite flakes or amorphous lumps, each measuring about 0.01 microns across. In the case of an oxygen-rich star, the oxygen atoms react with silicon and any metal atoms in the star's atmosphere to form silicate grains, roughly 1 micron across. As the grains are blown away from the star by radiation pressure and their temperature falls, they begin acquiring additional atoms of hydrogen, oxygen, carbon, nitrogen, and sulfur, which have also escaped from the parent star. These accreted materials build up into icy mantles of water ice and solid ammonia, methane, and carbon dioxide. Through a variety of chemical "sticking" processes, other substances may then be added to the mantle ices, including small molecules such as carbon monoxide (CO) and hydrogen sulfide (H2S). Bombardment by ultraviolet radiation from local hot stars, or more remote stars, triggers reactions between the different chemical species on a grain's surface and leads to the formation of simple organic substances interstellar chemistry.
Dust grains that have drifted into the general interstellar medium find their way into denser clouds and, eventually, into molecular clouds where the density is sufficiently high for more complex organic synthesis to take place (see interstellar molecules). How far up the scale of prebiotic synthesis such interstellar cookery can lead has yet to be determined, but it certainly extends as far as the simplest of amino acids. According to Fred Hoyle and Chandra Wickramasinghe, it extends as far as life itself, with the largest of dust grains being none other than living bacteria. Although this extreme theory has few supporters, mainstream science has gradually come around to the view that organically-laced debris from space may have been an important means by which life was able to originate so early on Earth (see Earth, early conditions).
Interstellar dust particles strongly absorb, scatter, and polarize visible light at wavelengths comparable to their size, reemitting the light in the far-infrared region of the spectrum. The amount of visual interstellar extinction is wavelength-dependent and leads to both a dimming and a reddening of starlight, as blue wavelengths tend to be scattered the most. Views along the plane of the Milky Way are severely limited by the dust that congregates there. Elliptical galaxies have less dust than our galaxy (but are not dust-free), while some galaxies are experiencing such tremendous episodes of star formation that the dust in them converts nearly all the visible light into infrared, resulting in an ultra-luminous infrared galaxy.