10 Ursae Majoris

10 Ursae Majoris (10 UMa) is a star in the constellation Lynx. Its apparent magnitude is 3.96.

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.

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.^[3]

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