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Cepheid variables.

There is a group of variable stars in which the rise and fall of brightness follows a distinctive pattern, although the period--the time taken by one rise and fall--varies from star to star. These stars are called Cepheid variables because the first one discovered was in the constellation Cepheus.

The American astronomer Henrietta Swan Leavitt (1868-1921) was interested in Cepheid variables and studied a number of them in the Magellanic Clouds, two large groups of stars that lie beyond the Milky Way (see 1678).

Her observations, beginning in 1904, showed that the brighter the Cepheid, the longer the period. This fact was obscured among the stars nearer to us, because some Cepheids seem dim only because they are quite far away, while others seem bright only because they are near. In the Magellanic Clouds, however, all the Cepheids are about the same distance from us, so that their apparent brightness reflects their real brightness, or luminosity.

By 1912 Leavitt had worked out a method for determining the luminosity of a Cepheid variable from its period. Once its luminosity was known, its distance could be calculated from its apparent brightness. What was needed to make this method work was a reliable estimate, by some other method, of the absolute distance of at least one Cepheid variable. This was a knotty problem, for even the nearest Cepheid is too far away for its absolute distance to be determined easily.

Once this was accomplished, however, the Cepheid variables could be used as a yardstick to determine distances for greater than those that could be determined by the method of parallax (see 150 B.C.).

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Author:Asimov, Isaac
Publication:Asimov's Chronology of Science & Discovery, Updated ed.
Article Type:Reference Source
Date:Jan 1, 1994
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