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5 UMI (5 Ursae Minoris). At fourth magnitude (4.25), number 5 in theFlamsteed list of numbered stars inUrsa Minor was obviously too faint for Bayer to make note of via Greek letter (though he and others indeed lettered fainter ones). Five Ursae Minoris may also be among the more prominent stars that everybody with a dark sky sees but pays no attention to. It's supremely easy to find and, given its position of only 14 degrees from theNorth Celestial Pole, it's visible from nearly everywhere north of 14 degrees north latitude any time of night. The front bowl stars of theBig Dipper,Merak andDubhe (Beta and Alpha UMa), are famed as the Pointers toPolaris, the North Star. Similarly, the front bowl stars of the Little Dipper,Pherkad andKochab (Beta and Gamma UMi), point right to 5 UMi. Only it's a lot closer, just 2 degrees north-northeast of second magnitude Kochab. It's so close that it might make a good addition to the Little Dipper as a drop of water flying off its lip. Beyond that, the star is among the most common types in the visual bestiary, a class K (but cooler than normal, K4)giant (oddly the same as Kochab)359 light years away (with a likely uncertainty of just 5 light years). The recorded temperatures have a large range, from 4070 to 4395 Kelvin, with an average of 4230, which somewhat compromises correction forinfrared radiation. Using the average temperature, 5 UMi shines with the light of 413Suns (with an uncertainty of perhaps 20 percent, really not all that bad). Temperature and luminosity then give a radius of 38 times that of the Sun, 0.18 Astronomical Units, roughly 60 percent the size of Mercury's orbit. Were it in our solar system, the orange star would glower nearly 20 degrees across, about the separation between Betelgeuse and Rigel. A low projected equatorial rotation speed of 1.9 kilometers per second indicates a rotation period that (were the rotation axis perpendicular to the line of sight) could be as long as 2.5 years. The theory of stellar structure and evolution give a mass perhaps as high as 4 times that of the Sun and show that the star is probably fusing its core helium to carbon and oxygen, though it could be just past that state as it makes ready to become an even bigger giant. The iron content is about three-fourths solar, nothing unusual, nor is the speed of motion relative to the Sun. At four solar masses, stars don't live very long; 5 UMi had a hydrogen-fusing lifetime of just 180 million years. After sloughing off its outer envelope, the core will turn into awhite dwarf of just under 0.8 of a solar mass. At 59 and 23 seconds of arc away are thirteenth and tenth magnitude "companions" whose motions clearly show both to be just line-of-sight "optical" coincidences.Five UMi is, however, listed as a mild barium star, which suggests the vaguepossiblility that there is a white dwarf companion that as a swollenenriced giant contaminated the star we see now.
Written byJim Kaler 4/11/14. Return toSTARS.
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