Diffuse nebulae appear as light or dark clouds (called bright and dark nebulae), are irregular in shape, and range up to 100 light-years in diameter. Some bright nebulae, composed primarily of hydrogen gas ionized by nearby hot blue-white stars, radiate their own light; they are called emission nebulae and are characterized by narrow spectral emission lines. Other bright nebulae, existing near cooler stars and not receiving the radiation necessary to make them self-luminous, reflect the starlight and are called reflection nebulae. Over 300 bright nebulae have been cataloged; prime examples are the Orion Nebula, visible to the unaided eye, the Eta Carinae Nebula, and the smaller North America Nebula. Dark nebulae are detected as empty patches in a field of stars or as dark clouds obscuring part of a bright nebula in the background, as in the case of the Horsehead Nebula. Smaller bodies of dark nebulous matter having unusually high densities have been observed in some bright nebulous regions. Many astronomers believe that these bodies, called globules, are in the process of condensation and are the initial stages in the birth of stars.
Planetary nebulae appear through the telescope as small disks with well-defined boundaries. They are the last stage of evolution for most stars, including the sun. Each consists of a shell of gaseous material surrounding a central hot star that emits radiation causing this material to glow. These shells measure about 20,000 AU in diameter (1 AU is the mean distance between the earth and the sun) and are slowly expanding, which suggests that they were expelled by the stars in nova eruptions (see variable star).
See L. Allen, Atoms, Stars, and Nebulae, (3d ed. 1991).
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