Why can astronomers conclude that star formation is occurring in regions like Orion Nebula?
Astronomers can see the various stages of star formation within the Orion Nebula.
The Orion nebula is one of the most identifiable features in the night sky, sitting in the middle of the sword in the constellation Orion. It is also relatively close to Earth, making it highly photogenic and therefore a popular choice for study.
Deeper observations reveal darker clouds of collapsing dust that block visible light behind them. These dark clouds, called Bok globules are the first stage of star formation.
Bok globules form as supernova shockwaves and stellar winds from nearby stars push nebular gas and dust together. Eventually gravity takes over and continues to pull the particles together. As these Bok globules collapse, the more dense parts will heat up, eventually forming protoplanetary disks.
Inside these protoplanetary disks, or proplyds, the bulk of the mass is collecting in the center, heating up. Once the density increases enough, fusion will begin and a new star will have been born.
Stellar winds from these newborn stars will blast the remaining loose dust and gas back into the greater nebula, leaving behind any planets, asteroids, and other bodies that might have formed.
This page has some great pictures along with a continued explanation of star formation.