Emission nebula
In astronomy, clouds of interstellar gas ( nebula ) that emit light in a wide variety of colors are called emission nebulae ( Latin: emittere, "send out", "let out") . This is where they differ from reflection nebulae , which only reflect radiated light.
The energy source that excites the nebula to glow are mostly high-energy photons from one or more neighboring hot stars . These stars, which mainly shine in the UV, are not always visible to the eye. The atoms of the fog gas emit light in two ways:
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Ionization and recombination : atoms are initially by photons whose energy is higher than the necessary ionization energy is, electrons torn; there arise ions and free electrons. This process is also called photoionization . The free electrons can then be captured by ionized atoms, with the electrons releasing their kinetic energy in the form of a photon ( recombination lights ).
A captured electron will initially be located on a higher than the energetically lowest, not fully occupied electron shell , whereby the atom is in an excited state . After a short time, however, it will jump onto a lower shelland emitthe energy difference between the two shells as a photon with a characteristic wavelength and energy. This continues until the basic state is reached, in which it is no longer possible to jump to a lower shell. - Excitation states : An electron bound to an atom isonly lifted to a higher electron shell, into an energetically higher state, by a photon of a certain energy. The photon energy must 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 with recombination.
These mechanisms have the effect that the spectral analysis of emission nebulae does not show pure continuum radiation, but rather discrete, stronger emission lines appear.
The stars that are responsible for the glow of the emission nebula are mostly hot, young stars of the spectral classes O, B or A, since only they can emit the necessary high-energy radiation. Most of the time, the nebulae are the remains of the gas clouds from which these stars arose. These types of emission nebulae are often H-II areas ; H. Areas in which hydrogen occurs ionized.
The planetary nebulae also belong to the emission nebulae , in which, however, a hot white dwarf , i.e. the remnant of a star, shines with its intense UV radiation. Here the nebula consists of the shed outer gas envelopes of the former star.
The color of the emission nebula depends on its chemical composition and the energy of the incident 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 a wavelength of 656.3 nm. If more energy is available, other elements can also be ionized, so that parts of the nebula glow in green and blue tones. From the spectrum of a nebula, astronomers can determine the elements it contains . Most emission nebulae consist of 90% hydrogen, as well as helium , oxygen , nitrogen and other elements.
Nice examples of emission nebulae are the Lagoon Nebula M 8 and the Orion Nebula M 42.
Emission nebulae often contain darker regions where dense clouds of dust, so-called dark clouds , do not let light through. Such a combination of emission nebulae and dark clouds result in interesting-looking objects, the shape of which often influenced the naming, e.g. B. with the cone nebula NGC 2264.
Emission and reflection nebulae can often be observed together and are sometimes collectively referred to as diffuse nebulae . Examples are the Omega Nebula M 17 and the Trifid Nebula M 20.
literature
- Susanne Friedrich et al .: Handbook Astronomy , Chapter "Gas" and "Galactic Nebula". 560 p., Oculum-Verlag, Erlangen 2015
- Arnold Hanslmeier : Introduction to Astronomy and Astrophysics . 3rd edition, 624 p., Verlag Springer / Spektrum, Berlin-Heidelberg 2014
