Nebulae are classified in three basic types: emission nebulae, dark nebulae, and reflection nebulae. Ultraviolet radiation from nearby bright blue stars excites hydrogen atoms in the gas of an emission nebula (A), and these atoms give off a fluorescent light. An absorption, or dark, nebula (B) is seen as a dark patch, sometimes surrounded by a halo of light; the light from stars behind the nebula is either absorbed or scattered by the nebular material. In a reflection nebula (C), the dust particles in the cloud reflect and scatter the light from stars that are not hot enough to make the nebula itself emit light.

Even with the unaided eye, faint hazy patches of light can be seen dotted about the night sky. Through a telescope these patches are resolved into various kinds of diffuse nebulae, which are clouds of dust and gas that can be seen in the visible part of the spectrum. In one type, the gas glows. This is an emission nebula. In a dark, or absorption, nebula, the dust appears silhouetted against other glowing material; and when the dust reflects light emitted by other objects, it comprises a reflection nebula.
In the early days of optical astronomy, there was also a category of spiral nebulae, so-called because they appeared to have gaseous “arms” spiraling out from their centers. These “nebulae” are now known to be other galaxies outside our own. A lot of work in classifying nebular objects was carried out by the French astronomer Charles Messier. He was more interested in studying comets but drew up a catalog and assigned numbers (called Messier or M numbers) to nebular objects so that he would not confuse them with new comets. A century after this catalog was published (in 1784), a more extensive list (the New General Catalog) was compiled, based on the observations of William Herschel.

Classification of nebulae

All nebulae are termed gaseous, or galactic (to distinguish them from extragalactic nebulae, the old misleading name for galaxies). There are two types: bright nebulae can be fairly compact in form or can be composed of extremely delicate filaments, or wisps, that are relatively extensive; dark nebulae can be determined against a lighter background of stars, and some have edges that are illuminated like a halo. Whatever its form, the nature of a nebula is determined by the density of the gas and dust from which it is composed, the chemical composition of these materials, and the absence or presence of nearby stars.
An emission nebula is associated with the presence of a hot star or stars (of spectral type B or O, for example). Ultraviolet radiation from the star ionizes hydrogen atoms in the cloud so that it emits light. One of the most famous emission nebulae is the Great Nebula of Orion, in the Sword of Orion; it is so bright that it can be seen with the naked eye. Its luminosity results from the presence of the O-type multiple star Theta Orionis. Another example of an emission nebula is the Rosette Nebula in the Milky Way constellation of Monoceros (the Unicorn).
A dark nebula may be dark when there is no star near it. Also, interstellar material often absorbs light, so that stars behind the nebula appear less bright. But if the nebula itself absorbs sufficient light then the stars are completely obscured. A striking example of a dark nebula is the Horsehead Nebula in Orion, in which a dark dustcloud protrudes into a bright emission nebula. This phenomenon proves that dark nebulae do not simply represent an absence of stars.
Reflection nebulae occur when the stars adjacent to a cloud of dust and gas are not hot enough to cause the cloud itself to emit light, but are bright enough to make the dust particles reflect their light. The Pleiades cluster in Taurus is a good example of bright stars among reflection nebulosity.

Planetary nebulae

The name of this group of nebulae is misleading because they have nothing to do with planets or a star system at all. Many are associated with a central star surrounded by a dislike shell of nebulosity. These central stars are usually very hot and often their emission is greater in the ultraviolet part of the spectrum. Optically, therefore, the nebula is seen only as a faint ring. Measurements of the Doppler shift in the spectra of planetary nebulae indicate turbulence and motion within the cloud. These nebulae are also expanding with a velocity of a few miles per second.

Supernovae remnants

The nature of certain nebulae suggests that they are the remnants of huge stellar explosions. The Crab Nebula in Taurus is perhaps the most famous example and is believed to consist of material ejected by a star. The supernova that created it was witnessed by Chinese astronomers in a.d. 1054. Its nebulosity is luminous and spreading.
A supernova represents a late evolutionary stage of a massive star. Many nebulae are, however, stellar birthplaces. The formation of a new star is a very slow process because the clouds of gas and dust are extremely rarefied. All stars begin in this way, and the eventual mass of the star depends upon its initial mass when it forms from nebular matter.

Interstellar matter

Space is pervaded by clouds of cold hydrogen gas that are permeated by cosmic dust. Such clouds do not emit radiation in the visible part of the spectrum, but they do emit radio waves at a wavelength of 21 centimeters. Positive proof of interstellar matter was not obtained until 1904, when stationary lines in the spectrum of a star in Orion (studied by the astronomer Hartmann) indicated that there was material between the star and the observer. Interstellar molecules were not identified until recently; cold hydrogen was detected in 1951, and more molecules are being identified each year.
Interstellar gas has a very low density. There are about 500,000 hydrogen atoms per cubic yard—a lower density than that of the gas in a laboratory vacuum. Grains of cosmic dust, formed by the accretion of matter around molecules, are about 10-7 millimeters (0.1 micron) in diameter and are about 1,000 million times less common than are hydrogen atoms.
The study of nebulae and interstellar matter brings together optical, radio, and infrared astronomy.

The Trifid Nebula is split into three by dark lanes of dust. This emission nebula consists of glowing hydrogen gas atoms that have been “excited” by the presence of a very hot, bright star. The hot gas is expanding into regions of cold gas and dust that surround the nebula.