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KAPPA UMA (Kappa Ursae Majoris). WithTalitha (Iota Ursae Majoris), Kappa makesthe eastern paw ofUrsa Major, the GreaterBear, and from ancient Arabic times the "third leap" of the Gazelle(two other similar pairslying to the east). Such pairs can bedeceiving. While seemingly close to Talitha, at a distance of 425light years Kappa is nearly nine times farther away. A white classA (A1) star of magnitude four (3.60), it seemingly shines with thelight of 540 Suns from a surface heated to a temperature of 9600Kelvin. Kappa, however, is deceptive, as it is not one star buttwo nearly identical stars lying very close together, theseparation typically a mere couple tenths of a second of arc, thebrighter only 0.2 magnitudes above the fainter. Upon separation wesee that Kappa-A shines with the light of 290Suns, Kappa-B with 250. Though the stars areclose, sophisticated optical systems have allowed the calculationof an orbit. The two waltz around each other with a period of 35.6years at an average separation of 24 Astronomical Units. A fairlyhigh eccentricity brings then as close as 11 AU, and as far apartas 37 AU. From these parameters, we find a total mass to thesystem of 11 times that of the Sun. However, temperature,luminosity, and theory suggest masses of 3.4 solar for each of them(for a total of 7 solar), showing that at least some of themeasures (distance, orbital period, and separation) are a bit off. Both stars are close to giving up on core hydrogen fusion and areturning into "subgiants," both having begun life as class B7 starssome 250 million years ago. What makes the system more intriguingis that one, or both, are rare class A "emission" (Ae) stars thatradiate energy from hydrogen, which is characteristic of acircumstellar disk. Such stars are cooler counterparts of the muchbetter known B-emission (Be) stars, epitomized byGamma Cas andZetaTauri (which they might well have been). Consistently, one orboth rotate at a projected equatorial velocity of a quite-high 201kilometers per second, which (allowing for axial tilt) gives arotation period of only 3.5 days (as opposed to 25 days for theSun), the rapid spin somehow connected to the orbitingcircumstellar disk.
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