The Hertzsprung-Russell Diagram
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Critical Thinking Questions
(Think about these carefully before you consult the answers below)
QUESTIONS
1) If White Dwarf stars are as hot as spectral type A and B stars, why aren't they as bright? <answer>
2) Why aren't there any green stars on the HR diagram? <answer>
3) If there are Red Dwarfs and White Dwarfs, why aren't there any blue dwarfs and yellow dwarfs? <answer>
4) When Ejnar Hertzsprung first made is diagram, he did not plot nearby stars, but rather stars from certain star clusters. Why did he do that? <answer>
5) Type O stars are much hotter than type M stars. What do you think this might say about the length of the star's lifetime. <answer>
ANSWERS
1) A white dwarf is the tiny corpse of a Main Sequence star. No longer producing nuclear fusion (except under certain circumstances on its surface), the White Dwarf star starts out very hot, with surface temperatures the same as some hot Main Sequence stars. Over a very long period of time, it loses its energy into space and the color changes, eventually into red and into infrared invisibility. (Do not confuse this with a Red Dwarf, which is completely different.) Although hot, and its surface bright, the White Dwarf is very small, roughly one-hundredth the radius of our Sun. So even though its surface is hot and bright, there just isn't enough surface area to radiate enough energy to make it highly luminous. Thus they are of low luminosity and absolute magnitude.
2) Actually, there are green stars. Type F stars reach their peak surface temperatures at green wavelengths. However, they do not appear green to the human eye, because the presence of other colors in the light (e.g., red, yellow and blue) mixes together to give the appearance of white. The Sun is yellow, but it appears white to most people, even though their are wavelengths of several colors present. Stars in the middle of the spectral types tend to get blended out to mostly whitish. So the reason we don't see green stars is due to the way our eyes work.
3) Or, you might add, "green dwarfs?" Aside from the problems our human eyes have in discerning green stars, the rest of the question is one of semantics. Technically, all stars on the Main Sequence are "dwarf" stars, a designation that traces back to the early years of the HR diagram when astronomers were attempting to contrast the truly huge giant and supergiant stars from more normal stars like the Sun. In that since, the Sun is a yellow dwarf, and their are other dwarfs with the colors of the other spectral types. An F5V star (a Main Sequence F5) would be a "white" dwarf. However, it should not be confused with a real White Dwarf, which, while it may have the same spectral type and temperature, is much smaller. So other than the Red Dwarfs, which are at the lower right end of the Main Sequence, astronomers tend not to use the term dwarf that much any more when referring to Main Sequence stars. Also, some real White Dwarf stars are so hot when formed that they are bluish in color. As they cool they become white, then yellow, orange, red and eventually fade from view as their temperature drops (although this takes a very long time).
4) Think about it. All the stars in a star cluster are at about the same distance to us, so even if he didn't know what that distance was, Hertzsprung knew that his chart would reflect a reasonably accurate representation of how the magnitudes and luminosities varied in the sample. Henry Norris Russell in the U.S. did essentially the same thing, except that he plotted stars of known distance (using their absolute magnitudes as we do today).
5) Since O stars are hotter, they are using up their nuclear fuel at a much greater rate than M stars of equivalent mass. If it uses up its fuel faster, it will run out sooner. Thus a O star typically has a very short lifetime on the Main Sequence, perhaps only a few dozen million years. A Main Sequence M type star, since it uses energy much more slowly, can last a hundred billion years or more, far longer than the current age of the Universe. (This doesn't apply to giant stars, which are a stage in the evolution of stars after they leave the Main Sequence, and which may exist as giants only a few million years.)
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