Magnitudes were first placed on stars by the Greek astronomer Hipparchus, more than two thousand years ago. He listed stars from first magnitude (the brightest) to sixth magnitude (the faintest) with a one magnitude increase corresponding to a star one-half as bright. In the mid-1800's astronomers made a more precise definition of magnitude, determining that the intensity difference between magnitudes was 2.512. This means that a second-mag. star appears 2.5 times as bright as a third-mag. star (close to Hipparchus' value!), and it works out perfectly so that a first-mag. star is 100 times as bright as a sixth-mag. star. With further measurements it was found that four stars were brighter than first-mag., but instead of changing the scale these stars were given negative magnitudes. The most important thing to remember is that as magnitude decreases a star's brightness increases.

Return to the Main Index