Nebula
From Aetilc
Nebula, 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. In 1864, a 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.
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[edit] Diffuse nebulae
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. 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. 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.
[edit] Planetary nebulae
Planetary nebulae appear through the telescope as small disks with well-defined boundaries. They are the last stage of evolution for most stars. 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).
[edit] 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.
[edit] 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.
