Emission nebula

Emission nebulae (lat. emittere "to send out", "to let out") are clouds of interstellar gas (nebulae) in astronomy that emit light in a wide variety of colours. This distinguishes them from reflection nebulae, which merely reflect incident light.

The energy source that causes the nebula to glow is usually high-energy photons from one or more neighbouring hot stars. These stars, which radiate mainly in the UV, are not always visible to the eye. Light is emitted by the atoms of the nebular gas in two ways:

1. Ionization and recombination: Atoms are first deprived of electrons by photons whose energy is higher than the necessary ionization energy; ions and free electrons are produced. This process is also called photoionization. The free electrons can then be captured by ionized atoms, whereby the electrons release their kinetic energy again in the form of a photon (recombination glow).
   In this process, a captured electron will usually initially reside on a higher than the lowest energy electron shell that is not fully occupied, leaving the atom in an excited state. After a short time, however, it will jump to a lower shell and emit the energy difference between the two shells as a photon with characteristic wavelength and energy. This continues until the ground state is reached, in which it can no longer jump to a lower shell.
2. Excited states: An electron bound to an atom is only raised to a higher electron shell, to an energetically higher state, by a photon of certain energy. The photon energy must thereby correspond exactly to the energy difference between the original and higher shell and can also be applied by the interaction of several photons. The jumping back, possibly in several steps, takes place as in recombination.

These mechanisms mean that the spectral analysis of emission nebulae does not show pure continuum radiation, but discrete, stronger emission lines appear.

The stars responsible for the glow of emission nebulae are usually hot, young stars of spectral classes O, B or A, since only they can emit the necessary high-energy radiation. In most cases, the nebulae are the remnants of the gas clouds from which these stars formed. These types of emission nebulae are often H-II regions, i.e. regions in which hydrogen is ionized.

Planetary nebulae also belong to the emission nebulae in principle, but here a hot white dwarf, i.e. the remnant of a star, is responsible for the glow with its intense UV radiation. Here the nebula consists of the ejected outer gas shells of the former star.

The color of the emission nebula depends on its chemical composition and on the energy of the irradiated light. Because of the abundance of hydrogen in the interstellar gas and its relatively low ionization energy, many nebulae glow in the characteristic red of the H-alpha line at 656.3 nm wavelength. If even more energy is available, other elements can also be ionized, so that parts of the nebula glow in green and blue hues. From the spectrum of a nebula, astronomers can determine the elements it contains. Most emission nebulae consist of 90% hydrogen, helium, oxygen, nitrogen and other elements.

Beautiful examples of emission nebulae are the Lagoon Nebula M 8 and the Orion Nebula M 42.

Emission nebulae often contain darker regions where dense dust clouds, so-called dark clouds, do not allow light to pass through. Such combinations of emission nebulae and dark clouds result in interesting looking objects, whose shape often influenced the naming, e.g. the cone nebula NGC 2264.

Emission and reflection nebulae can often be observed together and are sometimes collectively called diffuse nebulae. Examples are the Omega Nebula M 17 and the Trifid Nebula M 20.