Diffuse Nebulae

[M Diffuse Nebula] Click icon to view a diffuse nebula from Messier's catalog

>> Messier's diffuse nebulae; Links

The icon shows reflection nebulae in Scorpius around Antares. The globular cluster at the lower right is M4.

Diffuse nebulae, sometimes inacurately referred to as gaseous 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. Note that many emission nebulae also have an additional reflection nebula component (as they usually also contain dust); a most impressive example for this is the Trifid Nebula M20.

Diffuse emission nebulae are often called H II regions because they are mainly consisted of ionized hydrogene, H II - the roman number after the element symbol (here H) designating the ionization level: `I' would stand for neutral atoms, the `II' here means first ionization, i.e. the hydrogene atoms have lost their single electron, and for other elements higher numbers (ionization levels, or numbers of lost electrons) would be possible (e.g., He III, O III or Fe V).

After some million years, the gas and dust of the nebula will have been used up for forming stars (and planets), or blown away by the stellar winds of the young hot stars. A newly born open star cluster will remain.

The first diffuse nebula discovered was the Orion Nebula, M42, observed telescopically in 1610 by N. Pereisc. The diffuse nebulae were longly be considered as distant, unresolved star clusters, or star clouds, until in the 1860s spectroscopy revealed their gaseous nature. Eventually, in 1912, V.M. Slipher discovered that the nebulae in the Pleiades, M45, had the same spectra as the stars illuminating them, thus proving their nature as reflection nebulae. Of Messier's nebulae, M78 is the only pure reflection nebula, and the first one to be discovered.

While all of Messier's diffuse nebulae belong to our Milky Way galaxy, most other galaxies (especially all spiral and irregular galaxies) also contain such objects.

Messier's diffuse nebulae: M8, M17, M20, M42, M43, M78.
Moreover, open star cluster M16 (NGC 6611) is physically connected with the Eagle Nebula IC 4703, and the Pleiades, M45, contain diffuse reflection nebulae.
Other early known diffuse nebulae: NGC 2070, NGC 3372.

Links

References

Planetary Nebulae

Supernova Remnants

Dark Nebulae

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

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Last Modification: 25 Jan 1998, 16:05 MET