Hertzsprung-Russell Diagram

Main sequence stars are, by definition, normal. The other luminosity classes, of which the main ones are III and I, contain stars that are very different.

Consider class III stars. They are fairly cool since they lie near the right side of the H-R diagram. But they are also much brighter than any normal K and M star should be—perhaps 100 times as luminous as the Sun. We know that luminosity depends on temperature. Normally cool stars would not be as bright as hot stars, just as a glowing ember in a campfire gradually gets dimmer as it cools off. However, luminosity also depends on the size of an object. Imagine a glowing ember the size of a marble and another one, equally hot, the size of a beach ball. Clearly the larger one will be brighter, simply because there is more of it. Therefore, class III stars must be huge to be so bright and yet so cool.

For this reason, stars in luminosity class III are called giant stars. For example, Aldebaran, a bright K5 III star in the constellation Taurus (the Bull), has a diameter roughly 100 times greater than the Sun's. Aldebaran and many of the other bright but reddish stars you can see with the unaided eye are giants. If they were small main-sequence stars, they would be too faint to see.

Now consider luminosity class I, lying at the very top of the H-R diagram. If red stars 100 times brighter than the Sun are large, red stars 10,000 times brighter must be monstrous indeed. And they are: Antares, the M1 I star in the constellation Scorpio (the Scorpion), is so large that astronomers have been able to measure its diameter directly. Antares, it turns out, is about 400 times larger than the Sun. If placed at the center of the solar system, Antares would extend past the orbit of Mars. All four inner planets, including Earth, would be swallowed in a 4,000K (6,741°F [3,727°C]) inferno. Stars like this are called supergiants, and Antares as well as hotter supergiants like Rigel (the foot of Orion, spectral type B8 I) are among the largest, most luminous, and most massive stars in the galaxy.