BETA HYI (Beta Hydri). Toward the bright end of third magnitude (2.80) and the closest bright star to the South Celestial Pole, the Beta star in modern (and deep southern hemisphere) Hydrus (the Water Snake) just barely beats out Alpha (magnitude 2.86) as the constellation's luminary. By rather odd coincidence, both are relatively lower mass stars. Alpha is a class F (F0) dwarf of about two solar masses, while class G (G2) Beta is close to being a clone of the Sun, differing only slightly in mass, though as a subgiant apparently near or at the end of its hydrogen fusing life (as actually is Alpha). Also by odd coincidence, both were once pretty good southern pole stars (the result of the 26,000-year precession of the Earth's rotation axis), Beta around 150 BC (when it was just under two degrees off the South Celestial Pole), Alpha around 2900 BC (when it was a bit over two degrees away). Beta Hyi is also the closest bright star to the Small Magellanic Cloud, a small (as expected) nearby naked-eye irregular galaxy 200,000 light years away. With a temperature of 5840 Kelvin, Beta Hydri is just 60 Kelvin warmer than the solar surface. The temperature and a precisely-known distance of 24.3 light years (with an uncertainty of under 0.1) yield a luminosity of 3.56 times that of the Sun and a radius 1.85 times solar. Luminosity and temperature coupled with the theory of stellar structure and evolution give a mass somewhat over 1.1 times that of the Sun, while another more detailed study leads to more exactly to 1.1. "Asteroseismology," the study of minute stellar oscillations (with which we can probe the interior), gives a lower value of 1.02 Suns. The difference remains unexplained, at least for now. A measured projected rotation speed of 1.7 kilometers per second gives a maximum rotation period under 54 days. Since the axial tilt is not known, the number could be much lower. Whatever the details, we are looking at a star that is not much different from the one around which we revolve. At 1.1 solar masses, Beta Hydri began its life some 7 billion years ago as a dwarf at the cooler end of class F, around class F9. As it used up its internal hydrogen, it brightened (in part as a result of core contraction and heating) and expanded to the star we see today. At the end of its core-hydrogen-fusing life, Beta is about to make its grand transition to becoming a red giant. By the time the core is hot enough to re-stabilize the star as a helium-burner (helium fusing to carbon and oxygen), it will have brightened to perhaps 1000 solar luminosities, cooled to under 3400 Kelvin, and will have swollen to a radius half the size of the Earth's orbit. Beta Hydri will then lose its outer envelope, the nuclear-burning core dying as a white dwarf with a mass just over half that which the star now carries. Beta Hyi is zipping along with a proper motion against the stellar background of 2.2 seconds of arc per year, which when coupled to the distance and line of sight velocity of 22 kilometers per second gives a total velocity relative to the Sun of 82 km/s, four to five times greater than normal, which suggests that the star is a visitor from a different part of the Galaxy.

Written byJim Kaler 10/30/05; revised 5/05/07, 6/19/15. Return to STARS.