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
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.