Nebulae

Click the icon to view Nebulae of the Messier Catalog

The icon shows the Horsehead Nebula (Barnard 33), a dark nebula superimposed on an emission nebula (IC 434).

Diffuse Nebulae

Diffuse nebulae are clouds of interstellar matter, namely thin but widespread agglomerations of gas and dust. If they are large and massive enough they are frequently places of star formation, thus generating big associations or clusters of stars. Some of the young stars are often very massive and so hot that their high energy radiation can excite the gas of the nebula (mostly hydrogene) to shine; such nebula is called emission nebula. If the stars are not hot enough, their light is reflected by the dust and can be seen as white or bluish reflection nebula. As most diffuse emission nebulae also contain dust, they typically have a reflectin nebula component also.

Planetary Nebulae

When a star like our sun has used up all its central nuclear fuel, it finally ejects a significant portion of its mass in a gaseous shell which is then visible in the light emitted due to high-energy excitation by its extremely hot central star, which previously was the core of the stellar progenitor (thus, planetary nebulae are a special kind of emission nebulae). These nebulae quickly expand and fade while their matter is spread in the interstellar surroundings.

Supernova Remnants

Stars which are considerably more massive than our Sun, and have at least about 3 solar masses left after their giant state, can most probably not evolve quitely into an end state as a white dwarf, but when coming to age, explode in a most violent detonation which flashes up at a luminosity of up to 10 billion times that of the sun, called supernova (of type II) and ejecting the very greatest part of the stellar matter in a violently expanding shell. Alternatively, infalling matter on a white dwarf star can cause it to explode as a supernova of type I. The nebulous ejecta of supernovae of either type are called supernova remnants.

The only supernova remnant in Messier's catalog is the first object, the Crab Nebula M1, the remnant of a type II supernova.

Dark Nebulae

Although none of them is in Messier's catalog, some of these objects are conspicuous. Unlike the others, the bright nebulae, these dust clouds are only visible by the absorption of light from objects behind them. They are distinguished from diffuse nebula mainly because they happen to be not illuminated by embedded or nearby stars.

One should keep in mind that all Messier nebulae are members of our Milky Way Galaxy (together with many others). Other galaxies contain nebulae, too, which can be detected with considerably sensitive instruments within the images of these galaxies.

References

Webb Society Deep-Sky Observer's Handbook Vol. 2: Planetary and Gaseous Nebulae, by Kenneth Glyn Jones (ed.). Enslow Publishers 1978/1979.
James B. Kaler, Cosmic Clouds, Scientific American Library, W.H. Freeman, 1997 (German edition: Kosmische Wolken, 1998)

Hartmut Frommert (spider@seds.org)
Christine Kronberg (smil@lrz.uni-muenchen.de)