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ATLAS

(27 Tauri). Among the wonders of the sky is the grouping in Taurus that western astronomers have long called the Pleiades, or "Seven Sisters." Never mind that only six are usually visible with the naked eye, the seventh had to be there to make up the set. In Greek mythology, daughters of Atlas and Pleione, the sisters have lovely astronomical names: Alcyone, Celaeno, Electra, Taygeta, Maia, Sterope, and Merope (and wouldn't you love calling them all home from the back door). NINE of the stars, however, are named, as both our star Atlas (number 27 in the Flamsteed list) and Pleione are included as well. All are hot class B stars. B stars do not live very long, so the cluster must be young, only 100 or so million years old. Atlas, 425 light years away (as is the rest of the cluster), is (at class B8) a bit on the cooler side (for the class), with a temperature around 12,300 Kelvin. It makes up for it by being a giant, a star that is now evolving with a quiet helium core. Like the other naked-eye Pleiades (well, Dad actually), Atlas is brilliant, radiating 940 times as much light as the Sun, much of it in the invisible ultraviolet, the luminosity and temperature telling of a star with a mass about 5 times solar. Again like many other class B stars, Atlas is a rapid rotator, at least 212 kilometers per second, over 100 times faster than the Sun. Could we watch it spin, it would go around in under a day and a half. Atlas has a reputation as something of a mild "B-emission" star, implying a ring of radiating material formed as a result of its rapid rotation. Some 0.4 seconds of arc (at least 52 astronomical units, 30 percent farther then Pluto is from the Sun) away is a seventh magnitude class A companion that takes at least 150 years to circulate. The star also is variable, changing by a very subtle 0.2 percent over a few- day period. Some day all the bright Pleiades stars will be no more than small white dwarfs set within a merely modest cluster, one of thousands that dot the Milky Way.