Star Formation Along Spiral Arms

Text of HST press release.

Why do we believe (based on data) that stars form near or in spiral arms?

• H II regions are signatures of star formation and they are found only near spiral arms. H II regions are signatures of star formation because they are produced by O and B stars which have very short lifetimes.

• However, low mass stars 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 billions of year lifetimes.

  We know that 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 1,000,000 years, a spiral arm moves around 20 parsecs with respect to the disk material. So, objects with short lifetimes will be confined to the region around the spiral arm. In 1,000,000,000 years, a spiral arm moves around 2,000 parsecs with respect to the disk material. Long-lived stars can move away from spiral arms.