The Hertzsprung-Russell diagram is used to classify stars based on their color/heat and absolute magnitude/luminosity. Stars group into main sequence (dwarfs), white dwarfs, subgiants, giants, and supergiants. The diagram also shows the standard names for stars.
The Hertzsprung-Russell diagram is a tool used by astronomers and astrophysicists to classify different types of stars. It is sometimes called a color-magnitude diagram (CMD).
The Hertzsprung-Russell diagram primarily plots a star’s color/heat versus its absolute magnitude/luminosity. Color and warmth are the x-axis. They share an axis because color and warmth correlate very well with each other. Up to about 3500 K surface temperatures stars are red, up to about 4500 K they are orange, up to about 6000 K they are yellow, up to about 9000 K they are white, and beyond that they are blue. Some stars, such as Wolf-Rayet stars, have surface temperatures of up to 25,000 K.
Absolute magnitude/luminosity is the y-axis of the Hertzsprung-Russell diagram. Again, as with the x-axis, the y-axis can share two variables because the two are related. Absolute magnitude is measured using a standard luminosity scale, while the luminosity side is measured in terms of solar units. The solar units side is a logarithmic scale, because stars vary in their luminosity from 1/100,000 of the Sun to ~100,000 times the Sun. During a supernova, a star shines up to five billion times the Sun, although this is only temporary.
The Hertzsprung-Russell diagram shows that stars group into several natural categories: the main sequence (dwarfs), of which our Sun is a member, consists of stars that fuse hydrogen; white dwarfs, which have exhausted their nuclear fuel and are cooling down; and subgiants, giants, and supergiants, which fuse elements heavier than hydrogen, such as helium, together.
White dwarfs are much smaller than the Sun: they have a diameter similar only to that of the Earth! This is because a white dwarf is a remnant of the core of a star that previously generated its own energy. Dwarf stars are much larger, about the size of our Sun. Giants are much larger still, as large as the orbit of Mars. That’s because they melt their nuclear fuel more quickly, heating them up and expanding their envelope.
The standard name for stars comes from the Hertzsprung-Russell diagram. For example, if a star is red and belongs to the class of supergiants, it is called a red supergiant. Our Sun is a white dwarf. There are yellow giants, blue giants, orange dwarfs, red dwarfs, and many other categories of known stars.
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