>> Messier's diffuse nebulae; Links
The icon shows reflection nebulae in Scorpius
The globular cluster at the lower right is M4.
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.
Other early known diffuse nebulae: NGC 2070, NGC 3372.
Last Modification: August 27, 2007