(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
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
Thanks for your questions
for Ask an Astrophysicist