Seen through the telescope, the silvery stars turn into jewels of every color known. Since the glow of a star is from heat, te color depends on its temperature. Red stas are relatively cool, with surface temperatures of about 6,000 °F. Yellow stars, like our Sun, are hotter by thousands of degrees, and blue stars are still hotter. The hottest ultraviolet star may be more that 100.000 °F.
Relationships exist between tbe color and size of stars and their age and Iocation in tbe galaxies. Stars tend to fall into two great categories called Population I and Population II. Population I consists of stars in the arms of spiral galaxies and in irregular galaxies like the Malleganic Clouds. Population II consists of stars in the nuclei of spiral galaxies, in elliptical galaxies, and in globular clusters.
The biggest, brightest stars in Population I are blue giants, which spread a blue radiance around them. The biggest, brightest stars of Population II are red giants, which give their surroundings a reddish-orange tint.
In Population I, the smaller stars are red and relatively cool — that is, cool for stars. The larger ones are blue and hot. Until a few decades ago, astronomers believed that the bigger stars are, the hotter they are. Then, as telescopes probed deeper into space, new populations of stars were found. Out in remote globular clusters and still more distant galaxies, the giant stars were red and cool. And even in our own galaxv some stars have been discovered that change periodically in size and brightness. These are the so-called pulsating, or variable, stars. Some change regularly; some are irregular.
As nuclear physics developed, astronomers learned more about processes that go on in the stars. Then it became clear that different types of stars represent different stages in star evolution. Apparently Population I stars are in earlier stages of their evolution, and Population II stars are probably in later stages.
