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Nebula, a localized mass of the gases and finely divided dust particles that are spread throughout interstellar space. Before the invention of the telescope, the term nebula (Latin, “cloud”) was applied to all celestial objects of a diffuse appearance. As a result, many objects now known to be star clusters or galaxies were called nebulae.
The Ant Nebula
Nebulae exist within other galaxies as well as in our own, the Milky Way. They are classified as planetary nebulae, supernova remnants, and diffuse nebulae, including reflecting, emission, and dark nebulae. Small, very bright nebulae known as Herbig-Haro objects are found in dense interstellar clouds, and are probably the products of gas jets expelled by new stars in the process of formation.

Planetary nebulae, or planetaries, are so called because many of them superficially resemble planets through telescopes. They are actually shells of material that an old star of average mass sheds during a late, red giant stage in its evolution, before becoming a white dwarf. The Ring Nebula in the constellation Lyra, a typical planetary, has a rotational period of 132,900 years and a mass calculated to be about 14 times that of the Earth's sun. Several thousand planetaries have been discovered in the Milky Way. More spectacular but fewer in number are nebulae that are the fragments of supernova explosions, perhaps the most famous of which is the Crab Nebula in Taurus, now fading at the rate of about 0.4 per cent per year. Nebulae of this kind are strong emitters of radio waves, as a result of the explosions that formed them and the probable pulsar remnants of the original star.

Diffuse nebulae are extremely large, often many light years wide, with no definite outline and a tenuous, cloud-like appearance. They are either luminous or dark. The former shine as a result of the light of neighbouring stars. They include some of the most striking objects in the sky, such as the Great Nebula in Orion (the middle “star” in the Sword). The tremendous streams of matter in the diffuse nebulae are intermingled in violent, chaotic currents. Many thousands of luminous nebulae are known. Spectral studies show that light emanating from them consists of reflected starlight and also, in so-called emission nebulae, of stimulated radiation coming from ionized gases and dust in the nebulae.

Dark, diffuse nebulae are observed as nonluminous or faintly luminous clouds, obscuring portions of the Milky Way. They are too distant from the stimulation of neighbouring stars to reflect or emit much light of their own. One of the most famous dark nebulae is the Horsehead Nebula in Orion, so named after the shape of the dark mass silhouetted in front of a more luminous nebular region. The longest dark rift observed on photographic plates of the star clouds of the Milky Way is a succession of dark nebulae. Both dark and luminous nebulae are considered likely sites for the processes of dust-cloud condensation that lead to the formation of new stars.

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