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The little-known cloud of cosmic gas and dust called Gum 15 is the birthplace and home of hot young stars. Beautiful and deadly, these stars mould the appearance of the nebula from which they formed and, as they progress into adulthood, will eventually also destroy it.
Hydrogen (H) is the most common element in the Universe, and can be found in virtually every environment investigated by astronomers. HII regions are different because they contain substantial amounts of ionised hydrogen — hydrogen atoms that have been stripped of their electrons through high energy interactions with ultraviolet photons — particles of light. As the ionised hydrogen nuclei recapture electrons they release light at different characteristic wavelengths. It is one of these that gives nebulae such as Gum 15 their reddish glow — a glow which astronomers call hydrogen alpha (Hα).
In HII regions the ionising photons come from the young hot stars within the region, and Gum 15 is no exception. At the centre of this image you can see one of the culprits: the star HD 74804, the brightest member of a cluster of stars known as Collinder 197. (Cited from ESO)
A full resolution version can be seen here
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