Diffuse Nebulae

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; 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.

References

James Kaler, Cosmic Clouds (German edition: Kosmische Wolken)

Last Modification: 25 Jan 1998, 16:05 MET