OMEGA LEO (Omega Leonis). Holding up the rest of Leo's Greek-lettered stars about 10 degrees west southwest of Regulus, find fifth magnitude (at 5.41 nearly sixth) Omega Leonis. Just 1.5 degrees east of Leo's border with Cancer, it's also Flamsteed's number 2, just beaten out for the first spot going east in Leo by better-known Kappa Leo (Al Minliar al Asad) well to the north. However, while Kappa is a rather ordinary class K giant, Omega (108 light years away, give or take 5) is intriguingly near-solar. Nominally a cool class F (F9) dwarf (perhaps even G1), it's also a double with the components at most a second or so of arc apart. Sixth magnitude (given as 5.69) Omega A, the F9 dwarf, is accompanied by seventh magnitude (7.28) Omega B, whose absolute visual brightness is consistent with it being a G2 dwarf like the Sun, or at least close (the two adding to magnitude 5.46, close enough). From the nineteenth century, Smythe and Chambers call them "An exquisite close double star... A pale yellow, B greenish." Herschel first measured them at a mere 0.4 seconds apart! At an average separation of 28.6 Astronomical Units, not quite Neptune's distance from the Sun, the two take 118.0 days to orbit each other, a hefty eccentricity bringing them as close as 12.6 AU out to 44.5 AU and back. They were last physically closest in 1959, and will be farthest apart in 2018. Kepler's Laws give a total system mass of 1.67 times that of the Sun, oddly low and probably showing that the orbit needs improvement (as also noted below).

Omega Leo Omega Leo B, a sunlike star, orbits somewhat more massive Omega A (at the cross) every 118 years at an average separation of 28.6 Astronomical Units, at least according to the orbital solution (the orbital size perhaps larger). In reality, the two orbit a common center of mass. The true major axis of the elliptical orbit, the dot-dash line, is offset from that of the observed ellipse because of the orbital tilt of 65 degrees to the plane of the sky combined with the orbit's orientation. From the Sixth Catalog of Orbits of Visual Binary Stars , W. I. Hartkopf and B. D. Mason, US Naval Observatory Double Star Catalog, 2006.

Omega A has a well-determined temperature of 5940 Kelvin, while we can adopt the Sun's 5780 Kelvin for "B." Minor adjustments for infrared radiation then yield a luminosity of 4.9 times that of the Sun for "A" (and suggest it to be a subgiant), 1.2 for B. Masses would then be approximately 1.3 and 1.0 Suns (the decimal uncertain), for a system mass of 2.3, well above that derived from the orbit. The two can be reconciled by expanding the orbital size by 11 percent to 31.7 AU. Once thought to be part of the widespread youngish (500 million year old) Ursa Major moving group (which is related to the Ursa Major cluster in the Big Dipper), Omega Leo seems actually to be the brightest member of a completely separate, much older (3 billion years) metal-rich moving group, Omega's metals relative to hydrogen up from solar by around 40 percent, the other half dozen or so members scattered around the sky. The age is roughly consistent with Omega A being a subgiant whose core hydrogen is close to running out, if it has not done so already. Omega Leo also displays no cycle in magnetic activity which, since stars rotate more slowly with time (rotation helping to produce magnetic fields), is consistent with higher age as well. While there is no evidence for any planets, how fascinating it would be for a resident to have another star to which one could easily travel.

Written by Jim Kaler 5/24/13. Return to STARS.