21.3 Star Classification

Why are the stars in Orion's Belt different colors?

The ancient Greeks thought this group of stars looked like a hunter, so they named it Orion after their mythical hunter. The line of three stars at the center is "Orion's Belt." The many different colors of stars reflect the star’s temperature. The bright, red star in the upper left (with an arrow pointing to it), named Betelgeuse (pronounced BET-ul-juice), is not as hot than the blue star in the lower right, named Rigel.

Color and Temperature

Think about how the color of a piece of metal changes with temperature. A coil of an electric stove will start out black, but with added heat will start to glow a dull red. With more heat, the coil turns a brighter red, then orange. At extremely high temperatures the coil will turn yellow-white, or even blue-white (it’s hard to imagine a stove coil getting that hot). A star’s color is also determined by the temperature of the star’s surface. Relatively cool stars are red, warmer stars are orange or yellow, and extremely hot stars are blue or blue-white (Figure below).

Classifying Stars by Color

Color is the most common way to classify stars. Table below shows the classification system. The class of a star is given by a letter. Each letter corresponds to a color, and also to a range of temperatures. Note that these letters don’t match the color names; they are left over from an older system that is no longer used.

For most stars, surface temperature is also related to size. Bigger stars produce more energy, so their surfaces are hotter.

Summary

• Stars are classified by color, which correlates with temperature.
• A Hertzsprung-Russell diagram is used to learn about the characteristics of a star.
• Red stars are the coolest and blue are the hottest in a continuum ranging from 2000 K to more than 30,000 K.

Review

1. What information is contained in a Hertzsprung-Russell diagram?
2. What is the order of star colors from coolest to hottest? How is that related to size?
3. Why do stars that are different colors appear in the same constellation?