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Apparent Magnitude

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Author: Janice VanCleave

Apparent Star Brightness

Stars vary widely in brightness. Some appear very bright, while others are barely visible to the naked eye. Around 150 B.C., long before the invention of telescopes, the Greek astronomer Hipparchus devised a scale to measure apparent magnitude, the brightness of stars as seen with the naked eye from Earth. He gave a value of 1 to the brightest star and a value of 6 to the dimmest. Today, we use a variation of his scale to measure the brightness of stars. Instead of observing and estimating magnitudes with the naked eye, we now use an instrument called a photometer, which produces more precise measurements. Also, the scale has been extended beyond 1 to 6 so astronomers can measure an even broader range of brightness.

In this project, you will demonstrate the effect of luminosity and distance on the apparent magnitude of a star. You will build an instrument to measure apparent magnitude. You will learn how apparent magnitude differs from intrinsic (natural) luminosity, which is the amount of light a star emits. You will also discover the difference between apparent and absolute magnitude.

Getting Started

Purpose: To demonstrate how distance affects the brightness of an object.

Materials

  • 3 pencils
  • yardstick (meterstick)
  • 2 identical incandescent flashlights with new batteries
  • 2 helpers

Procedure

  1. In an open area outdoors, stick a pencil in the ground to mark the starting point. Use the yardstick (meterstick) to measure two distances from the pencil, one at 10 feet (3 m) and the second at 30 feet (9 m). Mark these distances with pencils in the ground.
  2. At night, stand beside the first pencil.
  3. Ask your helpers to hold flashlights and to stand side by side at the second pencil, 10 feet (3 m) away.
  4. Instruct your helpers to turn on their flashlights and shine them toward you.
  5. Look at the lights just long enough to compare their brightness.
  6. Ask one of your helpers to move to the third pencil, 30 feet (9 m) away, while continuing to shine the light toward you (see Figure 8.1).
  7. Again, compare the brightness of the lights.
  8. Ask your other helper to move to the third pencil while continuing to shine the light toward you.
  9. As before, compare the brightness of the lights.

Apparent Star Brightness

Results

The lights appear equally bright at an equal distance from you. When they are at different distances, the closer light appears brighter.

Why?

Magnitude is a measure of how bright a celestial body appears to be. Apparent magnitude is a measure of how bright a celestial body appears as viewed with the naked eye from Earth. Apparent magnitudes are ranked on a magnitude scale, with an inverse relationship between brightness and magnitude numbers, expressed as magnitudes. For example, the magnitude 1 star in Figure 8.2 is brighter than the magnitude 3 star. Apparent magnitude is not a measure of luminosity. Luminosity is the amount of light energy a light source such as a star gives off in a given amount of time. When stars have the same luminosity, the closer star, like the closer flashlight, has a greater magnitude.

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