Sites of Star Formation
H II regions (Trifid Nebula (M20)), regions of ionized hydrogen gas are signatures of star formation. They are only found near spiral arms. H II regions are created by the ultraviolet radiation from hot O and B stars (which have very short lifetimes). H II regions typically glow with a reddish hue because of emission from Hydrogen Balmer lines. See the figure to the right of the spiral galaxy M33.Low mass stars, however, are found spread throughout the disk of our Galaxy, they are not confined to the spiral arms. Is this a problem? No. Low mass stars have lifetimes many billions of years long and can wander large distances from their birth sites. Spiral arms are like compression waves which move through the disk of our Galaxy. That is, they move with respect to material in the disk (say, like a sound wave moves through this room). Spiral arms move through the disk at around 20 kilometers per second. So, in 1 million years, a spiral arm moves around 65 light years with respect to the disk material. Objects with short lifetimes will thus be found nearby spiral arms (their birth sites). In 1 billion years, a spiral arm moves around 65,000 light years with respect to the disk material. Long-lived stars may thus easiliy wander off from spiral arms (their birth sites). |
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B: Stars form in large ISM clouds known as
Giant Molecular Clouds (GMC).
