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Diffuse Nebulae

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

The icon showsreflection nebulae in Scorpius around Antares. The globular cluster at the lower right isM4.

Diffuse nebulae, sometimes inacurately referred to as gaseous nebulae, areclouds of interstellar matter, namely thin but widespread agglomerations ofgas and dust. If they are large and massive enough they are frequently placesof star formation, thus generating big associations orclusters of stars. Some of the young stars areoften very massive and so hot that their high energy radiation can excite thegas of the nebula (mostly hydrogen) to shine; such nebula is calledemission nebula. If the stars are not hot enough, their light is reflected by the dust and can be seen as white or bluishreflection nebula.Note that many emission nebulae also have an additional reflection nebulacomponent (as they usually also contain dust); a most impressive example for this is theTrifid Nebula M20.

Diffuse emission nebulae are often called H II regions because they aremainly consisted of ionized hydrogen, H II - the roman number after theelement symbol (here H) designating the ionization level: 'I' would standfor neutral atoms, the 'II' here means first ionization, i.e. the hydrogenatoms 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 bornopen star cluster willremain. From the physical viewpoint, the nebulae are an early stage of evolution of star clusters.

The first diffuse nebula discovered was theOrion Nebula, M42, observed telescopically in 1610byN. Peiresc,followed byM43 (discovered in or before 1731 byDe Mairan),theOmega or Swan Nebula, M17 (1745-46 byDe Chéseaux),theLagoon Nebula, M8 (1749 byLe Gentil),theTarantula Nebula, NGC 2070 andtheCarina Nebula, NGC 3372 (1751/52 byLacaille),Messier's 1764 discoveries oftheEagle Nebula, M16 and theTrifid Nebula, M20,andMéchain's 1780discovery ofM78.Messier's Catalog contains 7 of the 9diffuse nebulae known at the time of its publication, 1781.

Diffuse nebulae were longly be considered as distant, unresolved starclusters or star clouds, until in the 1860s spectroscopy revealed theirgaseous nature by showing line spectra, in particular due to the pioneeringresearch ofWilliam Huggins.Eventually, in 1912,Vesto M. Slipher discovered that the nebulae in thePleiades, M45, had the same spectra as the starsilluminating them, thus proving their nature as reflection nebulae.Of Messier's nebulae,M78 is the only purereflection nebula, and the first of these objects to be discovered; itsnature as a reflection nebula was revealed in 1919, again by V.M. Slipher.

While all of Messier's diffuse nebulae belong to ourMilky Way galaxy, most othergalaxies (especially allspiral andirregular galaxies) also contain such objects.

Messier's diffuse nebulae:M8,M16,M17,M20,M42,M43,M78.
Moreover, thePleiades, M45, contain diffuse reflection nebulae; note that this is a rare and interesting case of a clusterpassing 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):