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 under 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 near or at the end of its hydrogen fusing life. Also by odd coincidence, both were once pretty good southern pole stars (the result of 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). With a temperature of 5750 Kelvin, Beta Hydri is just 30 degrees cooler than the solar surface. The temperature and a precisely-known distance of 24.4 light years yield a luminosity of 3.7 times that of the Sun and a radius 1.9 times solar. Luminosity and temperature coupled with the theory of stellar structure and evolution give a mass of only about 1.2 times that of the Sun, while another more detailed study leads to 1.1 solar. A measured projected rotation speed of 3.3 kilometers per second gives a maximum rotation period of 29 days, quite similar to the 25-day period of our own star (though since the axial tilt is not known, the number could be much less). Whatever the details, we are looking at a star that is not much different from the one around which we revolve. Beta Hydri began its life some 5 to 6 billion years ago as a dwarf at the cooler end of class F, around class F8. 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 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 of only about half that which the star carries today. A high velocity relative to the Sun of 83 kilometers per second (five times greater than normal) suggests that the star is a visitor from a different part of the Galaxy.
Written by Jim Kaler. Return to STARS.