IOTA GEM (Iota Geminorum). While not a part of the classic figure of Gemini, Iota Gem holds a prominent place in the northern part of the constellation almost directly on a line passed southwesterly between the Twin's luminaries, Castor and Pollux. If you create your own constellation (rather fun to do), and think of Castor and Pollux as two eyes in a face, then fourth magnitude (3.79) Iota Geminorum would be the nose. Though occasionally called "Propus," the name is a mis-assignment, and properly belongs to Eta Geminorum, near the southwestern end of the constellation. Like Pollux, the star is a helium-fusing giant, though at class G9 just a bit warmer than its K0 neighbor (though not by much, Iota's temperature of 4788 Kelvin only a hair above that of Pollux). From a distance of 126 light years, the star (decidedly single, with no known companion) shines with the light of 52 Suns, again a bit more than Pollux's radiance. (The star appears fainter than Pollux only because it is nearly four times farther away.) Luminosity and temperature then give us a radius of 11 times solar, befitting a quiet giant with a mass of about double that of the Sun. A very slow projected rotation speed (again befitting a giant star) of just 1.5 kilometers per second (just a bit less than that of the Sun) gives a rotation period of under 200 days, the limit imposed by our not knowing the axial tilt. Now 1.4 billion years old, the star began life as much hotter class A2 dwarf, then gave up its core hydrogen fusion some 250 million years ago when it swelled to become a giant and fired up its internal helium to fuse to carbon and oxygen. When after a similar time span the core helium is gone, Iota Gem will swell further as it brightens to nearly 1000 solar luminosities, begins to pulsate like Mira, brightens even more into the thousands of Suns, then sloughs off its outer envelope to become a common white dwarf with a mass of about a third of that with which it began. In the transition to white-dwarf-hood, it may give a fine but brief show as the central star of a luminous "planetary nebula" (in which the old core lights up its fleeing outer shell, the term having nothing to do with planets). To give a sense of the majesty of giant stars such as this one, were you on an orbiting planet (for which there is no evidence), the Sun would be lost as an eighth magnitude dot amidst the Milky Way to the northeast of the Little Milk Dipper of Sagittarius.
Written by Jim Kaler 2/27/09. Return to STARS.