10 UMA (10 Ursae Majoris) = HR 3579 Lyncis. Something is odd here. Seeming to belong to two constellations at the same time, poor 10 UMa is the victim of shifting constellation boundaries. At the time of the assignment of the Flamsteed numbers (around the turn of the 18th century), the broad area considered to be Ursa Major extended southward to cover the star, which appears as a rather natural southern extension of the western-most of the three "leaps" (the ancient Arabs "leaps of the gazelle"), the one made of Iota and Kappa UMa (falling between them in west- east progression). When the final rectangular constellation boundaries were drawn in the 1920s, however, the star quite naturally fell among those of Lynx (the eponymous Lynx). (Numerous stars have suffered similar "fates.") Today, 10 UMa actually makes part of the outline of the dim constellation, and indeed is Lynx's third brightest star (after Alpha and 38 Lyn). Because of the confusion, it goes more by its number in the Bright Star Catalogue, HR 3579. Shining from a distance of 54 light years, the apparent mid-fourth- magnitude (3.97) brightness is in part the product of duplicity. HR 3579 is a tight double that consists of a fourth magnitude (4.11) class F (F5) dwarf in mutual orbit with near-solar clone, a sixth magnitude (6.18) class G (G5) dwarf. The average separation of 10.6 Astronomical Units (about half a second of arc as seen on the sky) coupled with an orbital period of 21.78 years leads to a total system mass of 2.54 solar masses, and an estimate of the mass ratio tells that the primary class F component carries 1.44 solar, while the lesser class G star carries 1.1 solar.
The G5 dwarf component of 10 UMa orbits the more massive F5 dwarf with a period of 21.8 years at an average distance of 10.6 Astronomical Units. The orbit seen here is tilted by 55 degrees to the line of sight. The dashed line is the original orbit, the solid a new one, showing that the data are all remarkably accurate. In reality the two stars orbit a common center of mass, whereas here, we place the coordinates such that the lesser seems to orbit the more massive of the two. The dot-dash line shows the "line of nodes," where the apparent orbit seems to slice across the "plane of the sky." The scale is in seconds of arc. Names from double- star catalogues are in the upper left hand corner. From a paper by W. I. Hartkopf, B. D. Mason, and H. A. McAlister in the Astronomical Journal, vol. 111, p. 370, 1996.
A modest eccentricity varies the separation between 12.2 and 9.0 AU. The distance, brightnesses, and temperatures (respectively 6500 and 5600 Kelvin) lead to respective luminosities of 4.8 and 0.8 solar and (with application of stellar structure theory) masses of 1.4 and 1.0 solar, the sum of which is very close to that inferred from the binary orbit. A rotation velocity for the primary (10 UMa A) of at least 34 kilometers per second yields a rotation period less than 2.6 days. The class G star (10 UMa B) closely resembles what the Sun looked like in its younger days, when it was but 2 billion years old (it is now 4.6 billion) and slightly dimmer, something to contemplate as we gaze into the otherwise obscure stars of the Lynx.
Written by Jim Kaler. Return to STARS.