OMI-1 ORI (Omicron-1 Orionis). Omicron-1 Orionis lies at the northwestern fringe of Orion's formal pattern, it and Omicron-2 an extension of the Hunter's left arm (as he looks at you). The star really seems to belong more to Taurus than it does to Orion, as it falls less than five degrees to the southeast of first magnitude Aldebaran. A fifth magnitude (4.74) reddish cool giant 651 light years away (give or take 92), it has nothing to do with somewhat brighter Omicron-2 about a degree to the southeast, which is a stock class K2 giant 3.5 times closer. Omi-1 is a bit of an unusual star. First, its a semi-regular variable that flops between magnitudes 4.65 and 4.88 with a 31-day period, with another 71-day period found in the infrared. Second, along with Upsilon Geminorum and a gaggle of other stars that include R Leonis and R Coronae Borealis (see Ups Gem for a longer list), it is part of the Wolf 630 moving group, studies of which (moving groups in general) are significant for unravelling the history of our Galaxy. As such, the star is moving along relative to the Sun at 53 kilometers per second, nearly three times normal. Third and perhaps most important, it is listed as a rare star of class S (S3), the class characterized by enhanced zirconium oxide as opposed to the titanium oxide of normal class M giants. S stars are an intermediate stage on the road of an oxygen-rich class M giant to becoming a true carbon star (like 19 Piscium or R Leporis), in which the oxygen content is similar to that of carbon (in the Sun and normal stars, oxygen dominates the two). Chi Cygni is a prime example. Later studies have Omicron-1 as an "MS" star, one that is becoming an S star but has not quite made it. With a temperature of around 3570 Kelvin, and assuming no dimming by interstellar dust, Omi-1 shines with the light of 2130 Suns, which gives a radius of 120 times solar, or just over half an Astronomical Unit. The maximum dimming of 0.65 magnitudes could raise these to 3800 and 160 Suns. The mass is indeterminate, but may hover around double solar. The odd class is coming from chemical changes that take place on the stellar surface as a result of deep internal nuclear processes, wherein freshly-made chemical elements are brought upward by convection. Enhanced carbon in the final stage is the result of helium fusion, whereas the zirconium (which is up by a factor of three or so) and similar elements such as yttrium are created by neutron capture. Proof that such alchemy is taking place is the observation of the neutron-capture element technetium. Rare stuff, Tc has no stable isotopes and does not naturally exist on Earth or in the Sun. But there it is in Omi-1 Ori, technetium- 99 with a half-life of only two million years, far less than the age of the star. An alternative possibility is that Omi-1 is an "extrinsic" S star, one that was polluted by a one-time giant companion that is now a dead white dwarf, although such a neighbor has never been found. Omicron-1 is most likely in the first stages of brightening with a quiet carbon-oxygen core. It will eventually shed its outer layers perhaps to produce a "planetary nebula" (created by the ejected gas being illuminated by the exposed core), the current core to become a white dwarf in its own right.
Written by Jim Kaler 3/09/12. Return to STARS.