OMEGA GEM (Omega Geminorum). Bayer sometimes could not run through the entire Greek alphabet within each of the constellations. There had to be enough bright stars, which confined the "Omegas" to the larger figures and those nearer the Milky Way. Gemini has a good one, a fifth magnitude (5.18) yellow-white class G (G5) supergiant (though a lesser one, perhaps even a "bright giant"). Omega Gem's relative faintness is the result of its great distance, 1489 light years (with a significant uncertainty of 177). Even though the star is close to the path of the Milky Way, there is little dimming by interstellar dust. Distance and a temperature of 5064 Kelvin, from which we calculate a bit of infrared radiation, yield a luminosity of 1820 Suns and thus a radius 56 times solar. The theory of stellar structure and evolution gives a mass of 5.5 Suns and shows that the star is contentedly fusing helium to carbon and oxygen in its deep core. With an age of around 65 million years, it will before long lose its outer envelope to produce a planetary nebula, the carbon/oxygen core destined to become a white dwarf with a mass of around 90 percent solar. The star is similar to Psi Andromedae, though slightly more massive and without obvious companions.

All seems straightforward. Hardly. First, Omega Gem teaches a modest lesson about classification and thoroughly researching the literature. In 1977 the star was declared a Cepheid variable with a small variation of about 0.09 magnitudes and a period of 0.7282 days. How interesting, since Omega Gem is just 22 minutes of arc north of one of the great classical Cepheids of the sky, Mekbuda, Zeta Geminorum. Eleven years later, however, a dedicated study at three different observatories found no variation in Omega Gem whatsoever, the star steady as the proverbial rock. But the label stuck, and Omega Gem is listed as a Cepheid to this day. Second, a measurement of the angular diameter suggests a physical radius of 72 times that of the Sun, 29 percent higher than given by theory. If we push the uncertainties in distance and angular diameter to the limits, we close some, but not all, of the gap. However, the angular diameter was measured in the infrared part of the spectrum, and given the diffuse "surfaces" of supergiants, we can hardly expect the sizes in different wavelength bands to be the same. Third, Omega Gem is listed as a "barium star," one that's been contaminated with freshly-made elements during the evolution of a more-massive companion that is now a white dwarf. However there is no confirmation of the designation and neither is there evidence for a white dwarf in orbit about the bigger star. Not that one does not exist, as detection would be difficult. So we'll have to admire Gemini's star at the end of the alphabet for what it is, a distant supergiant, massive to be sure, but one well under the limit of 8 to 10 Suns above which stars explode as supernovae.
Written byJim Kaler 2/12/16. Return to STARS.