Al ZIRR (Xi Geminorum). Or Alzirr, as in "star name English," the Arabic article is often run together with the noun. The name means "the button," though the button of exactly what is not known. At the rather prominent foot of the eastern of Gemini's twins, the name is probably not part of the Greek tradition, but one of the ancient Arabic. But who knows, as the scholarship on it is lacking, as for that matter is the science, as little is actually known about this rather obvious third magnitude (3.36) star, which is far better known by its Greek letter name, Xi Geminorum. Fairly nearby, Al Zirr is a relatively lower-luminosity class F (F5) giant (really subgiant) only 57 light years away (though that is still 68 percent farther than the constellation's luminary, Pollux). From that distance, our Sun would still be visible to the naked eye, albeit at a mere sixth magnitude (6.05) and quite lost amidst the Milky Way's stars in what we here on Earth call Scutum (the Shield). Distance and a temperature of 6460 Kelvin lead to a luminosity 11.1 times that of the Sun and a radius of 2.7 solar. The theory of stellar structure and evolution then yield a mass between 1.5 and 1.6 times that of the Sun (depending on the exact state of evolution) and an age of around 2.5 billion years (just over half the solar age). While formally classed as a giant, Xi Gem is really a subgiant that has just given up (or is about to give up) its core hydrogen fusion, more-massive stars having shorter hydrogen-fusing lifetimes. The star is actually right at a critical point in the stellar spectral sequence where, as we go to warmer stars, fusion begins to run on the "carbon cycle" and where we hit the "rotation break," where stars begin to rotate much faster than does the Sun. (The Sun's fusion is by the "proton- proton chain," in which hydrogen builds to helium in a direct three-step process; in the more efficient carbon cycle, carbon is used as a nuclear catalyst.) Consistent with being at the rotation break, Al Zirr has a rather rapid lower limit to its equatorial rotation speed, 68 kilometers per second (34 times that of the Sun), which gives it a rotation period of less than two days. (We can know only lower limits because the tilt of the rotation axis is not known.) The rapid rotation, along with an outer layer in a state of convection, lead to a fair degree of magnetic activity, X- ray radiation, and a two-level outer corona, one with a temperature of two million Kelvin (much like that of the Sun), the other of eight million. Xi Gem is listed as a "Delta Scuti" star, one that subtly oscillates with multiple periods, but one whose character is also quite unexplored, consistent with the lack of other knowledge about the star. The star (which has no known companion) is now preparing to become a much more luminous red giant, and like all others of its modest mass, will eventually die as a modest white dwarf.
Written by Jim Kaler 2/23/07. Return to STARS.