Diffuse Nebulae

Click icon to view a diffuse nebula from Messier's catalog

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 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. 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 hydrogen, 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 hydrogen 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. From the physical viewpoint, the nebulae are an early stage of evolution of star clusters.

The first diffuse nebula discovered was the Orion Nebula, M42, observed telescopically in 1610 by N. Peiresc, followed by M43 (discovered in or before 1731 by De Mairan), the Omega or Swan Nebula, M17 (1745-46 by De Chéseaux), the Lagoon Nebula, M8 (1749 by Le Gentil), the Tarantula Nebula, NGC 2070 and the Carina Nebula, NGC 3372 (1751/52 by Lacaille), Messier's 1764 discoveries of the Eagle Nebula, M16 and the Trifid Nebula, M20, and Méchain's 1780 discovery of M78. Messier's Catalog contains 7 of the 9 diffuse nebulae known at the time of its publication, 1781.

Diffuse nebulae were longly be considered as distant, unresolved star clusters or star clouds, until in the 1860s spectroscopy revealed their gaseous nature by showing line spectra, in particular due to the pioneering research of William Huggins. Eventually, in 1912, Vesto 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 of these objects to be discovered; its nature as a reflection nebula was revealed in 1919, again by V.M. Slipher.

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, M16, M17, M20, M42, M43, M78.
Moreover, the Pleiades, M45, contain diffuse reflection nebulae; note that this is a rare and interesting case of a cluster passing through an independent reflection nebula (or dust cloud).
From their situation in the night sky, Messier's diffuse nebulae can be assembled into two groups (in the following, ordered by Right Ascension):