Teacher Demo: Bright Star or Close Star?

When you gaze up in the night sky, some stars will be very bright while other stars are barely visible to the unaided eye. With the aid of binoculars, you may be able to observe different colours in the stars. The brightness and colour of a star depends on three factors: temperature, distance, and size.

There are two measures of how bright a star appears:

  • The “apparent magnitude” of a star describes how bright the star appears from Earth. This scale ranges from the brightest star in the sky, the Sun, which is set at –26.8. The dimmest stars, which are only visible with the largest telescopes, have an apparent magnitude of 25. The faintest stars visible by the naked eye have an apparent magnitude of 5.5. A decrease of 1 on the scale represents a 2.5 times increase in the apparent brightness.
  • The absolute magnitude of a star is the measure of brightness of a star if it were at a distance of 32.6 light years (ly) from an observer. By placing all stars at this distance, the true brightness of the stars can be compared.

We know that the apparent magnitude of the Sun is the brightest at –26.8. However, when using the absolute magnitude scale, the Sun would be barely visible to the naked eye, with a reading of 4.8.

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