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Lynn Waddell, age 14, of San Diego, Calif., for her questions

What are Cepheid variable stars?

A variable star varies in brightness. There are several kinds of variable star, one of which is the Cepheid variable. The discovery of these stars is one of the most fascinating stories of astronomy. It dates back to 1596 when a Dutch astronomer named Fabricius spotted a rather temperamental star in the autumn constellation Cetus, the Sea Monster.

Cetus is a long, straggly constellation southwest oft he glimmering Pleiades. The brightness of a star is recorded on a scale of magnitude. A star of lst magnitude is 21 times as bright as a star of 2nd magnitude, and so on. Fabricius spotted. a star of 3rd magnitude in Cetus for which no previous record existed. It faded in a few weeks and was forgotten.

In 1638, Holwarda, another Dutch astronomer' discovered that the bright star found and lost by Fabricius had popped up again. Holwarda kept a check on this temperamental star and found that it blazed forth at 11 month intervals. The strange star was named Mira, the Wonderful.

The Arabs may have noticed a variable star in the fall constellation Perseus. They loved the stars and usually gave them flattering names. This fellow, however, they named Algol, the Demon. About 1782, the English astronomer Goodricke got around to studying Algol, the Demon, in detail.

Goodricke found Algol to be a variable star with a period of 2 days and 21 hours. He then went on to study a variable star in the constellation Cepheid. The special variables in this constellation provided the name for the class of Cepheid variable stars.

A Cepheid variable star is a giant, often several hundred times bigger and brighter thin our sun. Its short blazing and fading period is very punctual. However, it does not vary so much as a Miry type star, which may vary from 10th to 2nd magnitudes

The Cepheid. varies from a half to one and a half magnitudes. We are not sure what causes all the different variable stars, but some think that the Cepheid. explodes and subsides, pulsing like a beating heart.

In 1908, Miss Leavitt of the Harvard Observatory found a fact which made the Cepheids useful. to astronomers. She was studying these stars in the small Magellanic Cloud, which is outside our Galaxy. She proved that the longer the period, the greater the brightness of a Cepheid.

Now a star has two magnitudes. The apparent magnitude is how bright it appears to be as seen from the earth. Its absolute magnitude is how bright it actually is, regardless of distance. By comparing the absolute and the apparent magnitude of a star, we can estimate its distance. Since 1917, the Cepheid variable stars have been used to estimate the distances of star clusters and. of faraway galaxies. They are perhaps the most valuable yardsticks in the heavens.

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