Variable Stars
Variable stars are stars that have regular or semi-regular, periodically varying brightnesses, and they have been known since medieval times.
One of the most famous is known as Algol, which is Arabic for "Demon Star." Its brightness varies by more than one full magnitude every 2.86739 days. This star is not a true variable; the change in brightness is due to its being an eclipsing binary star. During each orbit, the two stars eclipse one another, causing the visual brightness to drop. Variable stars of this type only occur when the binary star system is properly aligned for the observer to see one member being eclipsed by the other.
The Danish astronomer David Fabricius (1564-1617) is famous for his naked-eye discovery of the first long-term variable star in 1596. Named Omicron Ceti, this star varies from third magnitude to ninth, during an average period of 331 days. (Any star below sixth magnitude is beyond the limit of the naked eye to see.) Fabricius thought he had discovered a nova, so when it faded from view he did not go back to look for it. This star so amazed astronomers, it was dubbed "Mira," which is Latin for "Wonderful." Long-term variables, like Mira, form the most numerous type in the universe. They have a very great range of intensity, have periods of 60 to 700 days, have cycles that do not repeat with complete regularity, and they are all red giant stars. Because red giant stars are nearing the end of their life cycle, the variation could be caused by hydrogen depletion, and the onset of helium fusion.
Another type of variable are the Cepheids. Cepheid variable stars are pulsating variables that possess a very significant characteristic; in the early 1900s Henrietta Leavitt and Harlow Shapley found that the speed at which they pulsate is directly related to the their brightness. By finding the distance to a few Cepheids, the distances to other objects containing Cepheids could be determined. They became one of the yardsticks of astronomy.
The first star of this type, Delta Cephei, was discovered by John Goodricke in 1784. Cepheids are short-term variables, having periods ranging from 3 to 50 days. Most are massive, young yellow stars. Their pulsation is apparently caused by the expansion and contraction of their surface layers.
RR Lyrae stars, also named after the first of its class to be discovered in Lyra, were discovered by Williamina Fleming (1857-1911) in 1901. They are quite old, have very short periods which range from 0.3 to 0.7 day, and their brightness never varies more than two magnitudes. They are found primarily in globular star clusters. While they are similar to Cepheid variables, they do not have enough brightness to be used for measuring distances.
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