>> 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 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.
Last Modification: 25 Jan 1998, 16:05 MET