Movatterモバイル変換
[0]ホーム
16 and 17 DRA (16 and 17 Draconis), two for one of an unusualrelated duo. Naked eyedoublestars, likeMizar andAlcor inUrsa Major,are rather rare. Most that look like pairings are just chancecoincidences on the sky, one star much farther than the other. Buthere is a close-to real one, an oddity whose components (like theone in the Bear's Tail) carry different common names, from west toeastFlamsteed's 16 and 17Draconis, which lie withinDraco about10 degrees due west of the Dragon's head. Separated by 90 secondsof arc, they are not quite a naked-eye pair, and require at leastbinoculars to split them. Close to the same apparent brightness,fifth magnitude (5.03) 17 Dra slightly outranks just-barely sixthmagnitude (5.53) 16 Dra, a class B (B9.5) hydrogen-fusing dwarf. But that is a bit of a cheat, because 17 is ALSO double, made of afifth magnitude (5.38) class B9 dwarf that closely matches 16 Draand a sixth magnitude (6.42) class A (A1) dwarf just 3.3 seconds ofarc apart (Smythe and Chambers in the nineteenth century oddly calling them "pale yellow and faint lilac").Parallaxmeasures put 16 Dra 427 light years away, 17 Dra 412 light years,which, given the uncertainties, effectively place the stars at thesame distance. Moreover, the motions are closely similar. Sincethey are a true double, we adopt an average distance of 420 lightyears. As a true trio, the 17 Dra pair is called "A" and "B,"while 16 is referred to as the "C" component.
With respective temperatures of 10,500 and (estimated) 9400 Kelvin,17 Dra A and B shine with the light of 132 and 40Suns, which lead to radii of 3.4 and 2.4 timesthe solar value. Both are fast spinners, A and B with projectedequatorial values of 217 and 231 kilometers per second, which leadto rotation periods under 0.8 and 0.5 days, the masses (fromstructure and evolutionary theory) coming in at 3.1 and 2.4 Suns. Separated by at least 425 Astronomical Units, fromKepler's Laws, they must take at least 3800 years to orbiteach other. Now to 16 Dra, the outlying "C" member. Pretty mucha clone of 17 Dra A, with slightly less radiance, it shines at 112times the solar rate, the radius 3.2 times solar. At 77 km/srotating somewhat slower, 16 Dra takes under 2.1 days to make afull turn. A mass of 3.0 Suns leads to a system age of around 250million years, well short of the 350 million hydrogen-fusinglifetime.
Next for the "kicker," as 16 Draconis is ALSO a double star, onequite different from 17. The companion has never actually beenseen. But we know it's there from satellite observations thatreveal a powerful energetic ultraviolet and X-ray signature, onethat cannot be produced by an ordinary class B9-A0 dwarf. Theconclusion is that the companion must be a hotwhite dwarf, which thoughdim to the eye (probably around magnitude 15-16), is radiant athigh energies. It most likely has a temperature in theneighborhood of 30,000 Kelvin. To have died first, it must haveevolved from a mass greater than 3.1 solar, and to match the age ofthe system, more like 3.7, which would have made it a B6 dwarf atbirth. The white dwarf mass (stars vigorously losing mass as theyage) would now be in the neighborhood of 0.7-0.75 times that of theSun. All that makes 16-17 Dra a quadruple star, one that at onetime must have appeared like a higher-temperature version of thefamed double-double,Epsilon Lyrae. About two minutes of arc away is another "companion," 11thmagnitude "D," which, from its motion, is clearly just in the lineof sight. From their separations, 16 and 17 Draconis are at least11,500 AU apart, which, with total mass now known, gives an orbitalperiod of at least 38,000 years. From 16 Dra, the 17 Dra pairwould appear a couple degrees apart, the brighter one shining withthe light of a gibbous Moon. (Inspired by "Binocular Showpiecesfor Light-Polluted Skies," by Hugh Bartlett, which appeared in theSeptember 2010 edition ofSky and Telescope.) Written byJim Kaler 8/20/10. Return toSTARS.
[8]ページ先頭