Why are some galaxies pinwheel-shaped and others not?
We do not fully understand why this is so, but it has to do with
the thinness of the disk of gas which is in orbit around the nucleus of the
galaxy where much of the luminous matter is located. If you look at
spiral galaxies with the very red light produced by the oldest populations of
stars, the spiral arm contrast almost vanishes completely and is replaces by a
smooth flat disk of old-population stars. The arms we see are regions where
star formation is occurring very rapidly, producing massive stars. Spiral
arms are not physical arms in the disk of a galaxy, but some type of
wave of compression which sweeps through the interstellar medium, compressing
it slightly, and triggering star formation which then 'illuminates' where
this disturbance is currently located in the otherwise smooth disk of stars
and gas.
Copyright 1997 Dr. Sten Odenwald
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