ZETA SER (Zeta Serpentis). Serpens is a strange constellation. It's divided into two quite separate parts by Ophiuchus, the Serpent Bearer, with the head of the giant snake (Serpens Caput) to the west, the tail (Serpens Cauda) to the east, the latter winding within the star clouds of the Milky Way. Yet it's still considered one constellation, Bayer's Greek letters jumping back and forth between the two. Alpha through Epsilon are in Serpens Caput, while the eastern side, Serpens Cauda, leads off with Zeta, then continues with Eta and Theta (one of just two Serpentine stars with proper names), while Iota and Kappa get kicked back to the other side. Oddly still, fifth magnitude (4.62) Zeta is not all that bright, and is overpowered by the star with the next letter, third magnitude Eta. Flamsteed numbers, which go west to east, divide better, the last one in Cauda being 50 Serpentis (Sigma), the first in Ser Caput 53, or Nu, Serpentis (who nu?). (Whatever happened to 51 and 52? From Morton Wagman in "Lost Stars," 51 Ser appears in Hercules as Omega Herculis and 52 was a mistake.) After all that, what about Zeta Ser itself, which seems to have been lost in the discussion. Fairly close to us, 77 light years away (give or take less than 1), it's listed as a class F (F2) subgiant (but as commonly noted, see below). Approaching solar proportions, it's not all that bright, shining at a temperature of 6730 Kelvin with the light of 6.1 Suns (almost all in the optical spectrum), which leads to a radius of 1.8 times solar. Theory then yields a mass of 1.5 Suns and shows it to be an ordinary dwarf (if any star can be thought "ordinary") about midway through its 1.5 billion year core hydrogen fusing lifetime. Zeta Ser, though, has one characteristic that places it more within the realm of higher mass stars. While the Sun rotates slowly, 2 kilometers per second at the equator, Zeta Ser whips around with a speed of at least 73 km/s, giving it a rotation period of under 1.3 days. The star highlights the "rotation break" at class F5, 1.25 or so solar masses. Below that mass, mature dwarfs rotate slowly, the result of drag produced by their magnetic fields, which are tied to their expanding winds. Among hotter and more massive stars (F5 into class A and beyond), the magnetic fields weaken then go away as a result of the disappearance of surface convection, which helps produce the fields. Stars like Zeta Ser have just never slowed down much from the time they were born, as has the Sun. And its rotation speed is small compared with those in class B, where stars like 48 Librae can hit 400 km/s and beyond.

Written by Jim Kaler 8/02/13. Return to STARS.