The Development of Photography
This nebula is designated M16 in Messier’s catalogue - in which he describes a cluster of stars “enmeshed in a faint glow“. With modern photographic techniques and large telescopes we can see the full extent of the the star cluster’s nebula. The reddish colour of this nebula is typical of most nebulae, as this is the colour of light emitted by the most common element in the universe – Hydrogen.
It has long been thought that a logical conclusion to these observations of nebulosity associated with star clusters was that stars must have been born in these nebulae, and theories of began to form of how a cloud of gas becomes a star. The thermal pressure of gas cloud will ordinarily be great enough to keep it from collapsing in upon itself under its own gravity. But if for some reason its density was increased in some region then this region could become overdense whereby the amount of mass in that region would cause it to gravitationally collapse. The question of what caused the density to increase in this otherwise stable gas cloud remained though, until the advent of propagating star formation theory.