TANIA BOREALIS (Lambda Ursae Majoris). Three pairs of unrelated
stars mark the feet of Ursa Major, the
Great Bear. To the Arabs, they marked the "leaps of the gazelle,"
the first, second, and third leaps proceeding from east to west.
Tania Borealis (Lambda Ursae Majoris) is the more northerly of the
stars that make the "Second Leap," the name from a long phrase that
means just that (plus the Latin for "northern"). The southern star
of the pair is Tania Australis (Mu
UMa), while the first leap is made by Alula Borealis and Australis (Nu and Xi), the third from Talitha (Iota) and Kappa UMa. Positioned just north of Leo Minor, faint-third-magnitude (3.45) Tania
Bor is a class A2 subgiant that lies 134 light years away, 70
percent farther than the middle five Dipper stars that make the most notable portion of the
Ursa Major Cluster. With a surface
temperature of 8930 Kelvin, it radiates at a rate 59 times greater
than does the Sun, which leads to a radius
of 3.2 times solar, and from the theory of stellar structure and
evolution, a mass of 2.5 times solar. Though nearing the end of
its core hydrogen fusing lifetime, it seems not to be a subgiant (a
star that has just given up such fusion), but an older dwarf with
an age of 480 million years. Give it another 100 million and the
formal class will finally apply. As do many stars of its kind, it
appears to have an infrared enhancement (though quite mild) caused
by some sort of dusty circumstellar debris. More significantly,
Tania Bor is a "mild metallic line star" that (relative to hydrogen
and compared to the Sun) is enriched in zinc and the rare earths
(europium for example up from solar by a factor of 100) and
deficient in calcium. The variations from solar are caused by
gravity acting one way on some atoms, radiation acting the other,
in a relatively quite atmosphere. And sure enough, compared with
some, Tania Bor has a modest projected equatorial rotation speed of
50 kilometers per second, giving it a rotation period of under 3.2
days. The metallic differences are not enough, however, to give it
an "Am" class like more extreme Xi Cephei
or Mu Ori A. (Thanks to Jerry Diekmann,
who suggested this star.)
Written by Jim Kaler 1/04/08. Return to STARS.