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.