BETA SER (Beta Serpentis). Unique among constellations, Serpens (the eponymous Serpent) is divided in two parts, Serpens Caput (the Head) on the west side of snake- enwrapped Ophiuchus, Serpens Cauda (the Tail) on the east side. While Bayer gave the luminary, Unukalhai (the "serpent's neck") the Alpha designation, he seems to have lettered most of the rest of the plethora of rather similarly-dim stars by position. Which leaves fourth magnitude (3.67) Beta Ser (falling in brightness behind Eta, Mu, and Xi Ser) within the northern extreme of Serpens' X-shaped head. The star's relative obscurity, however, belies its significance as a curious multiple with as many as five components. In the nineteenth century, Smythe and Chambers saw it as "A delicate double star on the Serpent's under-jaw...both of a pale blue tint, and there is a distant telescopic star also blue..." The early observers, however, were fooled again by dimness and by contrast effects. Beta Ser A, the main member, is a common class A (A2) subgiant, which would make it as close to pure white as possible. From a well-determined distance of 155 light years (plus or minus just 2) and a temperature of 8549 Kelvin (which places nearly all the radiation in the visual spectrum), we find a luminosity of 61 times that of the Sun and a radius of 3.6 solar. Madly spinning with an equatorial velocity of at least 189 kilometers per second, Beta Ser rotates in under 23 hours. Theory shows that the star is indeed a subgiant, one that has given up (or is about to give up) core hydrogen fusion, the mass falling between 2.4 and 2.5 Suns depending on the exact state of evolution. Separated from Beta Ser by 31 seconds of arc lies 10th magnitude (9.7) Beta Ser B, which is keeping pace with much brighter Beta A as it moves through space and clearly belongs to it. Hardly blue, the small companion is an orange class K3 hydrogen-fusing dwarf (showing how easily the eye can be deceived). With a true separation of at least 1500 Astronomical Units and an estimated mass of roughly three-fourths that of the Sun, Beta-B takes at least 31,000 years to make an orbit around Beta-A. From its brightness, Smythe and Chambers' "distant telescopic star" (200 seconds of arc from A), 11th magnitude Beta Ser C, which also keeps pace and seems to belong, must be another orange mid-class K dwarf. With an orbital radius of at least 9500 AU, it would take more than half a million years to make a full circuit around the inner AB pair. Going vastly farther afield, nearly half a degree away, we find eighth magnitude Beta-D, which is ALSO keeping good pace. If a real companion, it would orbit the inner trio in a minimum-million-year path that is at least 80,000 AU wide. Bright enough to have a name of its own, Beta Ser D is better known as HD 140665 (from the Henry Draper spectroscopic catalogue, Beta Ser A also HD 141003). Beta-D's distance of 151 light years, which given an error of 10 l-y puts it at the same distance as Beta-A, confirms the connection. A close look reveals that it TOO is double! and that it consists of a sunlike star in mutual orbit with yet another class K dwarf, the two separated by at least 285 AU, giving a period of at least 3700 years. If all this is correct, we thus have a highly unusual quintet with A and B in orbit, then C going around AB, with fragilly-linked double D going around ABC. It's almost hard to see how such an arrangement could last over the system's 600 million year lifetime (but then look at much older Alpha and Proxima Centauri). From Beta Ser B, A might shine with the light of up to 15 full Moons, while from double D it would appear more like a few times Venus's brightness. From A, double D would look like a Jupiter orbiting a star like Vega separated by perhaps up to several minutes of arc, presenting a fine sight were there anyone there to see it, the existence of any "debris disk" not clear.
Written by Jim Kaler 5/13/11. Return to STARS.