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Published on Oct 28, 2014
The HR diagram gives us insight into what stars are made of. Our earliest understanding of stars were that they came in different colors. This was because the stars are different temperatures, ranging from red-hot to white-hot, to the still more unimaginably blue-hot. The great innovation came in the early 20th century, when Ejinar Hertzprung and Henry Norris Russell realized that when you had distances to stars and plotted how bright they REALLY are, most of them formed some kind of sequence. The blue stars are much bigger and brighter than the red ones, but at least there are tons more of the little red ones - but all of them obey this relationship because they're all fusing hydrogen into helium.
Adding to this are the giants, which are in the process of bloating out to huge size as they die, and the supergiants, which are the very final stages of stellar life. It's these red giants that leave behind hot and dim white dwarf cores, which was another surprising feature that the HR diagram seemed to separate out.
In recent years, we've pushed to objects redder and smaller than M dwarfs, to fill in the gap between stars and planets. These objects are brown dwarfs, and they really do extend down to sizes that are indistinguishable from planets. In many cases, it may be that the only difference between two objects is that one is orbiting a star.