The various colors in a galaxy (red bulge, blue disks) is due to the types of stars found in those
galaxy regions, called its stellar population. Big, massive stars burn their hydrogen fuel, by
thermonuclear fusion, extremely fast. Thus, they are bright and hot = blue. Low mass stars,
although more numerous, are cool in surface temperature (= red) and much fainter. All this is
displayed in a Hertzsprung-Russell Diagram of the young star cluster.
The hot blue stars use their core fuel much faster than the fainter, cooler red stars. Therefore, a
young stellar population has a mean color that is blue (the sum of the light from all the stars in the
stellar population) since most of the light is coming from the hot stars. An old stellar population is
red, since all the hot stars have died off (turned into red giant stars) leaving the faint cool stars.
The bottom line is that the red regions of a galaxy are old, with no hot stars. The blue portions of a
galaxy are young, meaning the stellar population that dominates this region is newly formed.
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