Apparent Magnitude (page 2)

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Author: Janice VanCleave

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The star Aldebaran, in the constellation Taurus, is about 50 times larger and about 100 times more luminous than the Sun. It appears far less bright, however, because it is so far away from Earth. Demonstrate how distance affects the apparent magnitude of stars with unequal luminosity. Repeat the experiment using two flashlights, fresh batteries in one and weak batteries in the other.

Design Your Own Experiment

  1. Design a model to illustrate how distance affects the apparent magnitude of stars with equal luminosity. One way is to make a drawing using lines to indicate luminosity. In Figure 8.2, the three stars emit the same amount of energy, as indicated by the number of lines (6) in the boxes. The numbers show, however, that their apparent magnitude varies, with the closest star appearing the brightest (having the lowest lowest magnitude number).
  2. Apparent Star Brightness

    1. Design an experiment to measure the apparent magnitude of stars. One way is to make a brightness viewer. The following steps provide a method for testing the brightness of a magnitude 1 star.
      • Punch two holes near the short edge of an index card. Label the card "Magnitude 1."
      • Cover one of the holes with a piece of transparent (not frosted) tape (see Figure 8.3). The covered hole is the testing hole. The uncovered hole is the viewing hole.
      • Apparent Star Brightness

      • To calibrate the viewer: (1) Through the viewing hole, look at a star known to be of magnitude 1. For information about star magnitude, see Terence Dickinson's Night Watch (Willowdale, Ontario: Firefly Books, 1998), pp. 100–119. (2) Look at the same star through the testing hole. The star should be barely visible. If it appears bright, add more layers of tape until the star is barely visible. Note: Be careful to keep the layers of tape clean and wrinkle free.
      • To use the viewer, find a star by looking through the viewing hole. Then move the viewer so that you look at the same star through the testing hole. If you can barely see the star, it has a magnitude of 1 or less.
    2. To identify stars with magnitudes greater than 1, repeat the previous experiment for stars of greater magnitude.
  4. The stars that make up the bowl of the constellation Ursa Minor, commonly called the Little Dipper, have magnitudes of 2, 3, 4, and 5 (see Figure 8.4). Use these stars as a "magnitude scale in the sky" against which the magnitudes of other stars can be compared and estimated.

Apparent Star Brightness

Get the Facts

Some stars give off much more light than the Sun, but Earth receives thousands of millions of times more light from the Sun than from any other star. Why? To compare the light outputs of the Sun and other stars, astronomers use an absolute magnitude scale. How was it developed? How does it work? For information, see the National Audubon Society Field Guide to the Night Sky (New York: Knopf, 1995), pp. 24–25.

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