AL SHAT (Nu Capricorni). Northwestern Capricornus is an interesting place. Here we find both the Alpha and Beta stars, Algedi and Dabih. Neither are the constellation's brightest, as that honor is reserved for Delta Cap (Deneb Algedi) over on the eastern side. Both Algedi and Dabih are wide "naked eye" doubles (though seeing both Beta 1 and sixth magnitude Beta 2 is a challenge). Algedi's apparent duplicity is just a line of sight coincidence, whereas Dabih's doubling is probably real. Less than a degree to the east southeast of Alpha (Algedi), actually pointed to by Algedi's two stars, lies fifth magnitude (4.76) Nu Capricorni, the proximity with Algedi making it ridiculously easy to find. At first, the star seems linked by Arabic mythology not to its close neighbor Algedi, but to Dabih, which lies two degrees to the south. "Dabih," however, originally referred to BOTH Beta and Alpha, resolving the matter. The name mysteriously alludes to the "lucky stars of the slaughterer," while little Nu is "Al Shat," the "sheep to be slaughtered" (making some sense of a fifth magnitude star having a proper name).
Alpha Cap Separated by 6.6 minutes of arc, the naked eye "double star" Algedi (toward the upper right) is actually an "optical double" that consists of two unrelated line-of-sight stars, Alpha-1 Capricorni at right, Alpha-2 at left. They point down and to the left toward Nu Capricorni. Dabih (Beta Cap), toward the bottom, is a real, though wide, binary, the pair far enough apart (3.4 minutes of arc) to produce an elongated image, with Beta-1 (the fainter of the two) a bit down and to the right.

The star itself, which clearly survived its name, is just barely a class B (B9.5) dwarf 253 light years away (give or take 5) that actually has nothing to do with either Alpha or Beta. And we'll stick to the Greek letter name. Its distance and well-determined temperature of 10,240 Kelvin (from which we can calculate the amount of invisible ultraviolet light) leads to a luminosity 79 times that of the Sun and a radius of 2.8 times solar (consistent with earlier estimates). For its class, Nu Cap is a slow spinner, its projected rotation period being only 21 kilometers per second, which gives a rotation period under 6.9 days. Slow rotation speeds in stars of this general class, however, allow separation of elements and odd chemical compositions. Nu Cap bears no evidence of anything of the sort (the iron content perhaps a bit lower than solar). The star is therefore probably spinning much faster and rotating more with its axis pointed toward us. The theory of stellar structure and evolution tells of a star with a mass of 2.7 times that of the Sun that is well past halfway through its hydrogen fusing lifetime of 500 million years. It will eventually slough off most of its outer parts and die as a white dwarf with a mass of about 70 percent that of the Sun. Nu Cap is listed as a "common proper motion" double, with a faint 12th magnitude companion travelling along with it at a separation of 53 seconds of arc. The change in separation though over 135 years of observation however, may suggest yet another line of sight coincidence. If the two represent a real pair, Nu Cap B has the brightness of a K5 dwarf that is at least 4000 Astronomical Units away from Nu Cap proper and takes over 140,000 years to orbit. At that distance, from Nu A, Nu B would have the brightness three times that of Venus, while from Nu B, Nu A would shine like a couple full Moons.

Written by Jim Kaler 10/11/13. Return to STARS.