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23.3: Star Classification

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    Why are the stars in the Orion constellation different colors?

    In this view of Orion you can see that the stars are different colors. The red star in the upper left is Betelgeuse (pronounced BET-ul-juice). The blue star in the lower right is Rigel. The fuzzy patch in the sword is the Orion nebula. The nebula will be discussed in the concept "Star Formation."

    Star Classification

    With a quick look, stars look the same. Look closer, though, and you can see differences. The most obvious differences are in size and color.

    Color and Temperature

    Think about the coil of an electric stove as it heats up. The coil changes in color as its temperature rises. When you first turn on the heat, the coil looks black. The air a few inches above the coil begins to feel warm. As the coil gets hotter, it starts to glow a dull red. As it gets even hotter, it becomes a brighter red. Next it turns orange. If it gets extremely hot, it might look yellow-white, or even blue-white. Like a coil on a stove, a star’s color is determined by the temperature of the star’s surface. Relatively cool stars are red. Warmer stars are orange or yellow. Extremely hot stars are blue or blue-white.

    Star temperatures are measured in degrees kelvin. The lowest temperature on the kelvin scale is absolute zero. That means molecules have no motion. Kelvin is related to Celsius and Fahrenheit in these ways:

    [°C] = [K] − 273.15

    [°F] = [K] × 9/5 − 459.67


    A graph of the brightness (absolute magnitude) of stars versus their color (temperature) is pictured below (Figure below). This is called Hertzsprung-Russell diagram.

    The Hertzsprung-Russell diagram plots luminosity against star color with the hottest stars on the left side

    The Hertzsprung-Russell diagram plots luminosity (absolute magnitude) against the color of the stars ranging from the high-temperature blue-white stars on the left side of the diagram to the low temperature red stars on the right side.

    Most stars fall along the main sequence curve. Stars in the main sequence fuse hydrogen into helium in the core. The horizontal branch also has many stars. These fuse helium in the core and burn hydrogen surrounding the core. Other stars are found in other regions.

    Object Size

    This diagram compares the relative sizes of stars of different masses

    Relative sizes of stars of different masses.

    This illustration (Figure above) shows the relative sizes of stars and their mass compared to the Sun. Red dwarfs are less massive and much smaller in size than the Sun. This means they have a very long lifetime. Our Sun is a fairly common type of star and has an average lifespan. Red giants are what some main sequence stars (like our Sun) become near the end of their lives. They are much larger than our Sun. Supergiants are very massive stars and are larger than our Sun, but have a smaller radius than red giants. Supergiants have very short lifetimes.


    • Stars are classified by color, which correlates with temperature. Red stars are the coolest and blue are the hottest.
    • Stars are plotted on a Hertzsprung-Russell diagram.
    • Star temperatures are found in a continuum ranging from 2000 K to more than 30,000 K.
    • Kelvin is a temperature measure in which the lowest temperature is absolute zero.


    1. Why are stars different colors?
    2. Where do most stars fall on the Hertzsprung-Russel diagram? Why?
    3. Why do stars that are different colors appear in the same constellation?
    4. If a cluster of stars is all the same color, what could that mean?

    Explore More

    Use the resource below to answer the questions that follow.

    1. What unique characteristics of a star can be used to classify it?
    2. What is the classification of our star?
    3. What was the problem with the original classification system set up for stars?
    4. What number was assigned to the brightest stars? What characterized stars later assigned the letter A? What did this characteristic mean?
    5. Why didn't this characteristic turn out to be a very good way to gauge temperature?
    6. What is the current letter classification, in order from hottest to coldest? What is the mnemonic device to remember that?
    7. Why is the temperature of a star a really important thing to know?
    8. What is identified on each axis of the Hertzsprung-Russell diagram?


    Image Reference Attributions

    [Figure 1]

    Credit: Courtesy of NASA
    License: Public Domain

    [Figure 2]

    Credit: Courtesy of NASA/GSFC;Courtesy of NASA
    Source: ;
    License: Public Domain

    [Figure 3]

    Credit: Courtesy of NASA, ESA and A. Feild (STScI);Courtesy of NASA
    Source: ;
    License: Public Domain

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