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.

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. These nebulae

quickly expand and fade while their matter is spread in the interstellar

surroundings.

Stars which are considerably more massive than our Sun, and have at least about

3 solar masses, 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, and ejecting at

least the very greatest part of the stellar matter in a violently expanding shell,

called supernova remnant.

The only supernova remnant in Messier’s catalog is the first object, the

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.

Last Modification: 25 Jan 1998, 16:23 MET