They were dying stars about the same size as our own sun. A nebula, we know, is an immense cloud of gas and dust that is dense enough to be seen. They become visible either by blocking the light of distant stars, by reflecting the light of a nearby star, or by being lit from within. Near the Southern Cross in the Southern Hemisphere is the Coal Sack Nebula, a classic example of a dark nebula. This patch of dust is dense enough to block the light from stars beyond and has no stars within or nearby to illuminate it.
Reflection nebulae are dust clouds that reflect the light of stars nearby.
The Witch Head Nebula near Rigel in the constellation of Orion and other reflection nebulae normally appear bluish through long exposure photographs because the reflected light is scattered by small dust and gas particles that match the shorter wavelengths in the blue end of the spectrum of visible light. Although all nebulae are beautiful, emission nebulae are among the best known and most sought-after deep sky objects for astrophotographers and other telescopic viewers. These areas of active star birth can sometimes be seen with the unaided eye as faint fuzzy patches. Nebulae were spotted by the early telescopic watchers of the sky, but they didn't know what they were. Charles Messier, the legendary comet hunter, even cataloged a number of nebulae in the 16th century that are still known today by their Messier number.