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 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.
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
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.
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
- 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)
Last Modification: 25 Jan 1998, 16:23 MET