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ZOSMA

(Delta Leonis). A star with two names, one Greek, the other Arabic, Zosma rides the back of Leo the Lion. Its principal name, by which it is listed here, is from Greek and means "girdle." Unfortunately, Zosma is a transliteration of the wrong word, the correct one meaning "hip" or "back." The alternative name, Duhr, is more direct, and comes from an Arabic phrase that specifically means "the Lion's back." It is the fourth brightest star in the constellation, and Bayer got it right by calling it Delta. But it is fourth brightest only because just-brighter Algieba (Gamma Leonis) is a double whose components are too close together to be seen with the naked eye. Telescopically, each component is fainter than Zosma, which elevates Zosma to the rank of third. Zosma is one of the family of numerous white class A stars that flock the sky and make such a great part of the constellations. Lying nearby, only 58 light years away, Zosma shines with a luminosity 23 times that of the Sun, from a surface with a temperature of 8350 Kelvin, just slightly cooler than its neighbor to the southeast, Denebola (Beta Leonis). These data show the radius to be only about double that of the Sun. Though a seemingly ordinary main sequence (hydrogen fusing) dwarf like the Sun, Zosma has, like any other star, interesting things to recommend it. First, it is a fairly rapid rotator, spinning at a speed of at least 180 kilometers per second at its equator, 90 times solar, giving it a rotational period of less than half a day. Second, it is suspected to be a variable, of a kind called a "Delta Scuti star." These vibrate like subtle Cepheids (of which Mekbuda in Gemini is an example). Zosma's pulsations, however, are greatly suppressed by the natural stability of the main (hydrogen fusing) sequence. Third, it is part of a set of stars that surround us called the "Ursa Major Stream," all of which (including Sirius) share a common motion across the sky. Their physical relation is unclear. Fourth, unlike its neighbor Denebola, Zosma appears to have no surrounding dusty cloud that might be suspected of harboring planets. Finally, Zosma is so well studied that astronomers actually have an age for it. From its current luminosity and temperature (as well as other properties), it is between 600 and 750 million years old. Its mass of 2.2 solar masses allow a total hydrogen-fusing age of around a billion years (a tenth that of the Sun), so Zosma is well over half way toward beginning its death process, when its then-helium core will contract and its outer layers will expand, making it into an orange giant.