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 hydrogen) 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 reflection 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.

The term "Nebula" has varied in the history of astronomy. In pre-telescopic times it was used to distinguish objects which look non-stellar from the pointlike stars. Most "nebulae" known at that time have been shown to be open star clusters. The term "Nebula" was thus used for what we now call "Deepsky Object".

In early telescopic times, the nature of these objects was still widely unknown. With open clusters resolved, still all other deepsky objects were summarized as "Nebulae". Only the use of large telescopes, the discovery of spectroscopy and the invention of photography in the second half of the 19th century made it possible to distinguish "real" nebulae - i.e., gas and dust clouds - with certainty from objects made up of stars (globular clusters and galaxies).

Now that the nature of nebulae as interstellar masses of gas and dust is known, there are still several classification schemes. The first is based on spectroscopy and the light which is seen from the nebulae:

Emission Nebulae: Emit light because the atoms in their gases are excited by high energy radiation of stars involved. They show emission line spectra.
Reflection Nebulae: Reflect light of nearby stars by their dust particles. Therefore, their spectra are the same as those of the stars, typically continuous spectra.
Absorption Nebulae or Dark Nebulae: Absorb light: Their gas component can be seen as absorption spectra in the light of background stars, their dust component by absorbing and reddening background light.

This scheme tells nothing on the nature of the nebulae.

A more modern scheme distinguishes star-forming or pre-stellar nebulae (basically diffuse and dark nebulae) from post-stellar nebulae (basically planetary nebulae and supernova remnants). The first of these classes typically includes clouds of interstellar matter of a mass of several 100 or several 1,000 stars, while the latter is related to one specific star in advanced state of evolution, at or just beyond the end of its nuclear life.

There are a number of variations and special classes of nebulae such as the Herbig-Haro Nebulae (related to stars in the process of formation, and emit jets of gaseous material, thus often found near large diffuse nebulae with star formation) and Wolf-Rayet Nebulae (related to hot Wolf-Rayet stars, stars of some age that have ejected matter they now cause to shine).

On cosmic timescales, all these types of nebulae, in particular the bright nebulae, undergo rapid changes and have only comparatively short lifetimes, so that those we observe are all rather young objects. Planetary nebulae and supernova remnants usually have only a few thousands of years before they fade and spread their matter into the this interstellar matter of their environment, while star forming H II regions while shine bright for the few 100,000 or million years they are brightened by the very hot massive O stars that formed within them. The giant molecular clouds have a somewhat longer life of some 10s of millions of years, while they form new stars and star clusters.

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
Christine Kronberg
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