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Bright or dark clouds hovering in the interstellar medium (the space between the stars) are called nebulae. Nebula, Latin for "cloud," is a visual classification rather than a scientific one. Objects called nebulae vary greatly in composition. Some are really galaxies, but to early astronomers they all appeared to be clouds.

Bright nebulae

Some categories of bright nebulae include spiral, planetary, emission, and reflection. Others are remnants of supernova explosions.

Words to Know

Cepheid variable: Pulsating yellow supergiant star that can be used to measure distance in space.

Infrared radiation: Electromagnetic radiation of a wavelength shorter than radio waves but longer than visible light that takes the form of heat.

Interstellar medium: Space between the stars, consisting mainly of empty space with a very small concentration of gas atoms and tiny solid particles.

Light-year: Distance light travels in one solar year, roughly 5.9 trillion miles (9.5 trillion kilometers).

Red giant: Stage in which an average-sized star (like our sun) spends the final 10 percent of its lifetime; its surface temperature drops and its diameter expands to 10 to 1,000 times that of the Sun.

Stellar nursery: Area within glowing clouds of dust and gas where new stars are being formed.

Supernova: Explosion of a massive star at the end of its lifetime, causing it to shine more brightly than the rest of the stars in the galaxy put together.

Ultraviolet radiation: Electromagnetic radiation of a wavelength just shorter than the violet (shortest wavelength) end of the visible light spectrum.

In 1923, American astronomer Edwin Hubble made a remarkable discovery about a spiral-shaped nebula: it was actually a gigantic spiral galaxy. Previously, astronomers had considered the Great Nebula in the constellation Andromeda to be a cloud of gas within our galaxy, the Milky Way. Hubble identified a variable star known as a Cepheid (pronounced SEF-ee-id; a blinking star used to measure distance in space) in the Andromeda nebula, estimating its distance to be about one million light-years away. This was far beyond the bounds of the Milky Way, proving the existence of galaxies outside of our own. Since then, many other spiral nebulae have been defined as galaxies.

Planetary nebulae truly are clouds of gas. They are called planetary because when viewed through a telescope, they appear greenish and round, like planets. Astronomers believe a planetary nebula is a star's detached outer atmosphere of hydrogen gas. This is a by-product of a star going through the later stages of its life cycle. As it evolves past the red giant stage, a star sheds its atmosphere, much like a snake sheds its skin. One of the most famous of these is the Ring Nebula in the constellation Lyra.

An emission nebula is a glowing gas cloud with a hot bright star within or behind it. The star gives off high-energy ultraviolet radiation, which ionizes (electrically charges) the gas. As the electrons recombine with the atoms of gas, the gas fluoresces, or gives off light. A well-known example is the Orion Nebula, a greenish, hydrogen-rich, star-filled cloud

that is 20 light-years across. Astronomers believe it to be a stellar nursery, a place where new stars are formed.

Reflection nebulae are also bright gas clouds, but not as common as emission nebulae. A reflection nebula is a bluish cloud containing dust that reflects the light of a neighboring bright star. It is blue for a similar reason that Earth's sky is blue. In the case of our sky, the blue wavelength of sunlight is scattered by gas molecules in our atmosphere. In the same way, the nebula's dust scatters starlight only in the wavelengths of blue light.

The final type of bright nebula is that produced by a supernova explosion. The most famous nebula of this type is the Crab Nebula, an enormous patch of light in the constellation Taurus. At its center lies a pulsar, a rapidly spinning, incredibly dense star made of neutrons that remains after a supernova explosion.

Dark nebulae

Dark nebulae are also scattered throughout the interstellar medium. They appear dark because they contain dust (composed of carbon, silicon, magnesium, aluminum, and other elements) that does not emit light and that is dense enough to block the light of stars beyond. These nonglowing clouds are not visible through an optical telescope, but do give off infrared radiation. They can thus be identified either as dark patches on a background of starlight or through an infrared telescope. One example of a dark nebula is the cloud that blots out part of the Cygnus constellation in our galaxy.

[See also Infrared astronomy; Interstellar matter ]

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nebula (nĕb´yŏŏlə) [Lat.,=mist], in astronomy, observed manifestation of a collection of highly rarefied gas and dust in interstellar space. Prior to the 1960s this term was also applied to bodies later discovered to be galaxies, e.g., the so-called Great Nebula in the constellation Andromeda. In 1864, William Huggins confirmed William Herschel's conclusion that nebulae are not swarms of stars by determining that the spectra of nebulae are made of bright lines characteristic of radiating gases. Diffuse nebulae and planetary nebulae are two major classifications of these objects.

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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neb·u·la / ˈnebyələ/ • n. (pl. -lae / -lē/ or -las ) 1. Astron. a cloud of gas and dust in outer space, visible in the night sky either as an indistinct bright patch or as a dark silhouette against other luminous matter. ∎  (in general use) any indistinct bright area in the night sky, for example, a distant galaxy. 2. Med. a clouded spot on the cornea causing defective vision.

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nebula film over the eye XVII; cloud-like cluster of stars XVIII. — L. nebula, rel. to OE. nifol dark, OS. neƀal (Du. nevel), OHG. nebul (G. nebel) cloud, OIr. nēl, Gr. nephélē cloud, the simple IE. base being repr. by OSl. nebo, Gr. nēphos cloud, Skr. nábha- cloud, mist.
So nebuly (-Y5) (her.) wavy like the edges of clouds. XVI. — F. nébulé, medL. nebulātus. nebulous XVI. — F. nébuleux or L. nebulōsus.

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nebula (Lat. ‘cloud’) Region of interstellar gas and dust. There are three main types of nebula. Emission nebulae are bright diffuse nebulae that emit light and other radiation as a result of ionization and excitation of the gas atoms by ultraviolet radiation. In contrast, the brightness of reflection nebulae results from the scattering by dust particles of light from nearby stars. Dark nebulae are not luminous: interstellar gas and dust absorb light from background stars, producing apparently dark patches in the sky.

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nebula (neb-yoo-lă) n. a faint opacity of the cornea that remains after an ulcer has healed.