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What evidence is there that The Orion region is actively forming stars?
1.) Distribution of star types (and therefore ages): Stellar type can be measured by examining the spectrum of a star, and size can be estimated by computing absolute magnitude (from distance and apparent magnitude). Orion has a number of large, young, hot stars that cannot be older than a few 10s of millions of years. Since we see lots of young stars there, it must be a star forming region.
2.) Existence and behavior of large clouds of dust: In Orion there are several large nebulae containing huge quantities of dust and gas. High-resolution photos in multiple parts of the spectrum from e.g. Hubble, IR satellite telescopes, and earth observatories show that there is a great deal of motion and heating in these clouds, and photographs also show the regions that have been cleared by young stars. This kind of motion, heat, and nebular clearing are indications of current turbulent activity, indicating condensation, star formation, and gas reacting to new stars.
3.) Spectra related directly to star formation: Protostars (stars that have heated due to gravitational collapse) and pre-main-sequence stars have unique spectra in infrared (for the protostars) and in visible light PMS stars. These spectra have been observed in many nebulae in the Orion region.