Emission nebula

As an emission nebula (Latin emittere " send ", " let out " ) are used in astronomy clouds of interstellar gas (fog ) indicates that themselves emit light in different colors. They therefore differ from reflection nebulae, which merely reflect incident light.

The energy source that excites the nebula to shine usually are high-energy photons of one or more neighboring hot stars. This bright in the UV stars are not always visible to the eye. To emit light through the atoms of the mist gas, it comes in two ways:

The effect of these mechanisms that the spectral analysis of emission nebulae not a pure continuum emission shows, but discrete, stronger emission lines occur.

The stars, who are responsible for the lighting of the emission nebulae are mostly hot, young stars of spectral types O, B, or A, since only they can radiate the necessary energy. Mostly it is in the mists around the remains of the gas clouds from which these stars were created. This type of emission nebulae are often H II regions, ie regions in which hydrogen occurs ionized.

Another of the emission nebulae are inherently the planetary nebula in which, however, a hot white dwarf, so the remnant of a star, for the enlightenment provides. Here, the mist from the cast-off outer gaseous envelopes of the former star.

The color of the fog depends on its chemical composition and of the energy of the incident light. Because of the abundance of hydrogen in the interstellar gas and its relatively low ionization energy light many fog with his characteristic red at a wavelength of 656.3 nm If more energy is available, it is also possible that other elements are ionized, and mist with green and blue color originate. Astronomers can determine the elements contained in the spectrum of a nebula. Most emission nebulae consist of 90 % hydrogen, further of helium, oxygen, nitrogen and other elements.

Beautiful Examples of emission nebulae are the Lagoon Nebula M8 and the Orion Nebula M 42

Emission nebula often contain darker regions where dense dust clouds, so-called dark clouds, no light let through. Such combination of emission nebulae and dark clouds arise interesting looking objects whose shape often influenced the naming, such as the Cone Nebula NGC 2264th

Emission and reflection nebulae are often observed together and are sometimes collectively referred to as diffuse nebulae. Examples of these are the Omega Nebula M 17 and M 20, the Trifid Nebula