UPS UMA (Upsilon Ursae Majoris). Placed in front of the Big Dipper, in the neck Bayer's depiction of Ursa Major (the Greater Bear), lies Upsilon UMa. Together with nearby 23 UMa (just 4.5 degrees to the north), it provides a fine example of the coincidences that abound in astronomy (akin to our Sun and similar Alpha Centauri A being so close to each other). Both are fourth magnitude, rapidly rotating class F (respectively F2 and F0) subgiants that are Delta Scuti (multiple-period) variables with low mass companions. That said, concentrate here on Upsilon, the fainter (3.80) and farther (115 light years). With a surface temperature of 7170 Kelvin, the star radiates at a modest rate (for naked-eye stars) of 29 times that of the Sun, which yields a radius of 3.5 times solar, a mass 2.0 solar, and the understanding that the star is at the end of its core-hydrogen-fusing life (consistent with its subgiant spectrum). However, the temperature is anomalously high for the spectral class, which should be closer to F0 (like 23 UMa) or even A9. Rotating with a high equatorial speed of at least 124 kilometers per second (implying that the axis is fairly vertical to the line of sight), the star completes a rotation in just 1.4 days, or even somewhat less. The subtle "Delta Scuti" variation, between 0.05 and 0.10 magnitudes, is not visible to the naked eye. The principle period is just 3.19 hours, but others (different parts of the star pulsating inward while others pulsate outward) have been found between 1.6 and 2.1 hours. Separated by 11.6 seconds of arc (at least 410 Astronomical Units) lies a dim companion of magnitude 11.5, which implies that it is a class M0 red dwarf. With a mass of about half solar, the companion orbits Ups UMa A with a period of at least 5200 years. From Ups proper, the companion, dim as it is, would still shine with the light of 20 percent that of the full Moon, while from the companion, Ups A would radiate 200 full Moonlights. Off in the distance, a planetary rider (none is known) would see the sister star, 23 UMa, 40 light years away from Ups, shining at second magnitude (2.30). Among the biggest differences between pair is that unlike 23 UMa, Upsilon is a high-velocity star, sailing past us from a different part of the Galaxy with a speed of 62 kilometers per second, about four times normal.
Written by Jim Kaler. Return to STARS.