23 UMA (23 Ursae Majoris). Among the best informal asterisms of the sky is Ursa Major's Big Dipper (the "plough" to some, the "wagon" to others), whose seven stars dominate northern spring skies. Immediately to the west of the Dipper's front bowl stars lies another pair, 23 and Upsilon Ursae Majoris, which bear a striking similarity to each other. Both are fourth magnitude, rapidly rotating class F (respectively F0 and F2) subgiants that are Delta Scuti (multiple-period) variables with low mass companions. Curiously, the fainter (magnitude 3.80) of the two carries the Greek letter and is the more studied, leaving Flamsteed's 23 UMa (at 3.67) somewhat neglected. The relative ascendancy of 23 UMa is caused not by greater luminosity, but by proximity, the star lying 76 light years away as opposed to Upsilon's 115. With a surface temperature of 7080 Kelvin (almost exactly that of Upsilon's), 23 UMa shines with the light of 14 Suns, only half Upsilon's radiance, from which we find a radius of 2.5 solar. The temperature and luminosity imply a mass of 1.75 Suns (appropriately less than Upsilon's mass of 2 solar), and that the star is actually a hydrogen fusing dwarf with an age of 1.2 billion years rather than a subgiant, in which core hydrogen fusion has ceased. Spinning with an equatorial speed of at least 147 kilometers per second, the star makes a full rotation in under 0.85 days. (The high rotation speed suggests that the rotation axis is tilted pretty much perpendicular to the line of sight, so that the rotation period is close to reality.) As do many of the stars in its realm of temperature and luminosity, 23 UMa is a subtly varying star of the "Delta Scuti" class, but one grossly understudied. An observation nearly a century old has the star varying between magnitudes 3.3 and 3.8, which seems quite impossible, as more recent data suggest a mere 0.03 - 0.07 magnitude variation with a period of the order of two hours. A comprehensive study has never been done. Some 23 seconds of arc away lies a dim ninth magnitude (9.19) companion, its absolute brightness implying a class K (K7) dwarf with a mass of 0.63 solar. The true separation of at least 530 Astronomical Units (the foreshortening is not known) implies an orbital period of at least 7900 years. If that distance is correct, from 23 UMa proper, the companion would shine at 35 percent that of the full Moon, while from the companion, 23 UMa A would seem to radiate 60 full Moons. If there were a planet (and none is known), its residents would see Upsilon UMa 40 light years away glowing at nearly first magnitude (1.51), much as Canis Major's Adhara appears in our earthly sky.
Written by Jim Kaler. Return to STARS.