Originally a fixed, extended, and somewhat fuzzy white haze observed in the sky with a telescope. Many of these objects can now be resolved into clouds of individual stars and have been identified as galaxies. They are still sometimes referred to as extragalactic nebulae. The gaseous nebulae, however, cannot be resolved into individual stars and consist, for the most part, of interstellar dust and gas. In some of these gaseous nebulae the gas atoms have been ionized by ultraviolet radiation from nearby stars and light is emitted as these ions interact with the free electrons in the gas. These are called emission nebulae. In the dark nebulae, there are no nearby stars and these objects are consequently dark; they can only be detected by what they obscure.
A cloud of gas and dust in space. The term was originally applied to any object with a fuzzy telescopic appearance, but with the advent of larger instruments many ‘nebulae’ were found to consist of faint stars. In 1864 W. Huggins discovered that true nebulae could be distinguished from those composed of stars on the basis of their spectra. Nowadays the term ‘nebula’ means a gaseous nebula. The term extragalactic nebula, originally used for galaxies, is now obsolete. There are three broad types of gaseous nebula: emission nebulae, which shine by their own light; reflection nebulae, which reflect light from nearby bright sources such as stars; and dark nebulae (or absorption nebulae) which appear dark against a brighter background. This broad classification scheme has been carried over to other wavelengths, giving rise to terms such as infrared reflection nebula and infrared dark cloud. Emission nebulae include the diffuse nebulae or H II regions around young stars, planetary nebulae around old stars, and supernova remnants.
http://hubblesite.org/news_release/news/2015-01/3-nebulae/2015
A cloud of gas and dust in space. Nebulae are the birthplaces of stars, but some nebulae are produced by gas thrown off from dying stars (see planetary nebula; supernova). Nebulae are classified depending on whether they emit, reflect, or absorb light.
An emission nebula, such as the Orion nebula, glows brightly because its gas is energized by stars that have formed within it. In a reflection nebula, starlight reflects off grains of dust in the nebula, such as surround the stars of the Pleiades cluster. A dark nebula is a dense cloud, composed of molecular hydrogen, which partially or completely absorbs light behind it. Examples include the Coalsack nebula in Crux and the Horsehead nebula in Orion.
- negative binomial distribution
- negative booster
- negative charge
- negative correlation
- negative direction
- negative equity
- negative exponential distribution
- negative eyepiece
- negative feedback
- negative freedom
- negative income tax
- negative interest rate
- negative introspection
- negative inversion
- negative ion
- negative lens
- negative logic
- negative log-log link
- negatively skewed; negative skewness
- negative number
- negative paradox
- negative phase sequence
- negative photoresist
- negative pressure
- negative refractive index