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15.3: Star Classification

Difficulty Level: At Grade Created by: CK-12
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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, 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).

A Hertzsprung-Russell diagram shows the brightness and color of main sequence stars. The brightness is indicated by luminosity and is higher up the y-axis. The temperature is given in degrees Kelvin and is higher on the left side of the x-axis. How does our Sun fare in terms of brightness and color compared with other stars?

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.

Classification of Stars By Color and Temperature
Class Color Temperature Range Sample Star
O Blue 30,000 K or more Zeta Ophiuchi
B Blue-white 10,000–30,000 K Rigel
A White 7,500–10,000 K Altair
F Yellowish-white 6,000–7,500 K Procyon A
G Yellow 5,500–6,000 K Sun
K Orange 3,500–5,000 K Epsilon Indi
M Red 2,000–3,500 K Betelgeuse, Proxima Centauri

(Sources: http://en.wikipedia.org/wiki/Stellar_classification; http://en.wikipedia.org/wiki/Star, License: GNU-FDL)

For most stars, surface temperature is also related to size. Bigger stars produce more energy, so their surfaces are hotter. Figure below shows a typical star of each class, with the colors about the same as you would see in the sky.

Typical stars by class, color, and size. For most stars, size is related to class and to color. The colors are approximately as they appear in the sky.


  • 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.


Use this resource to answer the questions that follow.


1. How are stars classified?

2. What is the Hertzsprung - Russell diagram?

3. What are most stars?

4. What is our star? Be specific.

5. Define luminosity.

6. What is the Yerkes classification scheme?

7. Describe main sequence stars.

8. Explain the difference between a red and blue giant star.

9. What is a a white dwarf?

10. What is a brown dwarf?


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?

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Difficulty Level:
At Grade
Date Created:
Feb 24, 2012
Last Modified:
Aug 15, 2016
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