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.