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The Question

(Submitted January 08, 1998)

First, why do stars clump together into arms in spiral galaxies and are there commonly a specific number of prominent arms in a typical spiral galaxy? Is there a correlation between star masses, rotational speeds, galactic size, etcetera and the spiral arms?

Second, and this will settle an argument with a coworker :). What is the typical evolution of a cluster of stars forming into a galaxy? My friend seems to think that we start with spirals then move to disks. He uses as an analogy, chocolate syrup mixing in a glass of milk (where the chocolate particles are stars and the milk is space). I think this is wrong. Is he correct? And if he is, why does this happen? Or if he is wrong then what is the right answer?

The Answer

The answers to your two questions are, not surprisingly, related. What causes spiral arms are density waves propagating through the stars and gas of the galaxy. This means that stars in an arm may not be in the arm after the density wave has moved on. What exactly causes these density waves is not known. Because of the increased density of gas in the arms, star formation in a spiral galaxy is concentrated in them, and so newer, bigger, and brighter stars tend to be in the arms.

As for the history of galaxies, the current thinking is that galaxies that have a relatively low total angular momentum form elliptical galaxies, and high angular momentum galaxies form spirals. Spirals probably start more like ellipticals, then collapse down to a disk (and a more spherical center bulge), and then the spiral arms form. Spirals all tend to be similar in mass, as opposed to the elliptical galaxies which vary in size from very small to the largest of galaxies. How these galaxies continue to develop is a topic for speculation, or you can ask me again in 5 or 10 billion years.

Thanks for your questions

Eric Christian
for Ask an Astrophysicist

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