Nebula (Latin: "mist") is a term formerly used in astronomy to denote any object situated beyond the solar system that is seen as a bright or dark area, in contrast to the pointlike images of stars. This definition, adopted at a time when very distant objects could not be resolved into great detail, unfortunately includes two unrelated classes of objects: the extragalactic nebulae, now called galaxies, which are enormous collections of stars and gas; and the galactic nebulae, which are composed of the interstellar medium (the gas between the stars, with its accompanying small solid particles) within a single galaxy. Today the term nebula generally refers exclusively to the interstellar medium.

In a spiral galaxy the interstellar medium makes up 3 to 5 percent of the galaxy's mass, but within a spiral arm its mass fraction increases to about 20 percent. About 1 percent of the mass of the interstellar medium is in the form of "dust"--small, solid particles that are efficient in absorbing and scattering radiation. Much of the rest of the mass within a galaxy is concentrated in visible stars, but there are strong indications from the rotation of galaxies about their centres that there is also some form of dark matter accounting for a substantial fraction of the mass in the outer regions. This material might consist of dim stars or compact objects such as neutron stars or black holes.

The most conspicuous property of interstellar gas is its clumpy distribution on all size scales observed, from the size of the entire Milky Way Galaxy (about 10{sup 20} metres, or hundreds of thousands of light-years) down to the distance from the Earth to the Sun (less than 10{sup 11} metres, or a few light-minutes). The large-scale variations are seen by direct observation; the smallest are observed by fluctuations in the intensity of radio waves, similar to the "twinkling" of starlight caused by unsteadiness in the Earth's atmosphere. Various regions exhibit an enormous range of densities and temperatures. Within the Galaxy's spiral arms about half of the mass of the interstellar medium is concentrated in molecular clouds, in which hydrogen occurs in molecular form (H) and temperatures are as low as 10 kelvins. These clouds are inconspicuous optically and are detected principally by their carbon monoxide (CO) emissions in the millimetre wavelength range. Their densities in the regions studied by carbon monoxide emissions are typically 1,000 H molecules per cubic centimetre. At the other extreme is the gas between the clouds with a temperature of 10 million kelvins and a density of only 0.001 H{sup +} ion per cubic centimetre. Such gas is produced by supernovas, the violent explosions of unstable stars.

Excerpt from the Encyclopedia Britannica without permission.