Diffuse nebulae and planetary nebulae are two major classifications of these objects. 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. 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. The Columbia Encyclopedia, Sixth Edition. Click here to shop the Bartleby Bookstore .