Probably the most spectacular feature of our Milky Way galaxy is its spiral arms.
We can’t get a probe far enough out yet to take a galactic selfie, but astronomers arereasonably sure that we live in a spiral galaxy. Observations of other spiral galaxies offer clues to what kind of objects can help us trace out the shapes of spiral arms, called spiral tracers. Using those spiral tracers, we’ve been able to map out patterns within our own galaxy that appear to be spiral arms.
Over the years, astronomers have tested the spiral arm hypothesis against the evidence again and again, and there is now a great deal of confidence that the Milky Way is a spiral galaxy.
More than that–star formation, which we know is limited to the disk of the galaxy (rather than its central bulge or halo), appears to be specifically found in the spiral arms.
But why? And for that matter…what even are spiral arms?
Astronomers have a pretty solid idea of how stars are born. They begin within the dense, cold dust of an interstellar cloud such as this one. They heat up and get more luminous as they contract, and then drop in luminosity as they continue to contract steadily toward the main sequence.
I’m going to spend at least the next ten or so posts talking about the main-sequence portion of a star’s life cycle. Basically, we’re talking about a star’s adulthood.
You know what, while we’re at it, why don’t I draw up an analogy between a star’s life cycle and that of a human:
When a human is a mere fetus developing within its mother, a star is a protostar.
We say a star has been “born” when it crosses the birth line—basically, satisfies certain expectations for its temperature and luminosity for its specific mass—and becomes visible.
After that, a star steadily approaches adulthood. A “child” star is referred to as a Young Stellar Object (YSO) or a pre-main-sequence star.