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23.1: Star Constellations

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    Why do the constellations appear in the same patterns all the time?

    This is a constellation, a pattern of stars in the night sky. This constellation is called Orion. The features you can see best are his belt and sword. You can see Orion's belt in the sky from many locations. These stars are very bright. For many constellations, the stars are not near each other. They just happen to appear near each other in our sky.


    When you look at the sky on a clear night, you can see hundreds of stars. A star is a giant ball of glowing gas that is very, very hot. A star generates energy through nuclear fusion reactions. Most of these stars are like our Sun. However, some stars are smaller than our Sun, and some are larger. Except for our own Sun, all stars are so far away that they only look like single points—even through a telescope.


    The stars that make up a constellation appear close to each other from Earth. In reality, they may be very distant from one another. Constellations were important to people, like the Ancient Greeks. People who spent a lot of time outdoors at night, like shepherds, named the constellations. They told stories about them. Pictured below is one of the most easily recognized constellations (Figure below). The ancient Greeks thought this group of stars looked like a hunter. They named it Orion, after a great hunter in Greek mythology.

    Star map of the stars in the constellation orion

    Orion has three stars that make up his belt. Orion's belt is fairly easy to see in the night sky.

    The constellations stay the same night after night. The patterns of the stars never change. However, each night the constellations move across the sky. They move because Earth is spinning on its axis. The constellations also move with the seasons. This is because Earth revolves around the Sun. The constellations in the winter are different from those in the summer. For example, Orion is high up in the winter sky. In the summer, it's up only in the early morning.

    Constellations are useful. They help astronomers and other observers orient in the night sky. The star Betelgeuse, for example, is Orion's right shoulder. Betelgeuse is the eighth brightest star in the sky. The star is an excellent example of a red supergiant.

    Apparent vs. Real Distances

    The stars in a constellation appear close together in our night sky. But they are not close together at all out in space. In the constellation Orion, the stars visible to the naked eye are at distances ranging from just 26 light-years (which is relatively close to Earth) to several thousand light-years away.

    What Is Astrology?

    Ancient Babylonian astronomers created the Zodiac. The Zodiac is a circle that divides the ecliptic into twelve 30-degree zones. Each zone contains a constellation, many of them animals. Horoscopes based on these astrological signs first appeared in Ptolemaic Egypt in around 50 BC. These early people used astrology to explain things that are now much better explained by science. You can see an example of a Zodiac below (Figure below).

    An ancient zodiac

    How did astrology come to be?

    There is no reason to think that the alignment of the stars has anything to do with events that happen on Earth. The constellations are patterns made from stars in the sky. The patterns do not reflect any characteristics of the stars themselves. When scientific tests are done to provide evidence in support of astrological ideas, the tests fail. When a scientific idea fails, it is abandoned or modified. Astrologers do not change or abandon their ideas when they fail. So astrology is not science.


    • The points of light in the night sky are stars that are balls of gas. They are lit by nuclear fusion.
    • Constellations are patterns of stars seen from Earth. These stars are usually not near each other.
    • Stars in a constellation may be fairly close together. More likely, though, they are extremely far apart.


    1. Why are constellations so important to people when they think about stars?
    2. Are constellations useful?
    3. Are stars in a constellation close to each other?
    4. What is astrology? How is it different from astronomy?

    Explore More

    Use the resource below to answer the questions that follow.

    1. What does the word constellation mean?
    2. What are the names of the constellations based on?
    3. What was Orion? Why did the Greeks say the constellation disappeared for part of the year?
    4. Why do constellations change position in the sky?
    5. How many constellations are there?
    6. How do zodiac constellations differ from other constellations?


    Image Reference Attributions

    [Figure 1]

    Credit: Courtesy of NASA
    License: Public Domain

    [Figure 2]

    Credit: IAU and Sky & Telescope magazine (Roger Sinnott & Rick Fienberg)
    License: CC BY 3.0

    [Figure 3]

    Credit: Courtesy of NASA
    License: Public Domain

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