5 UMI (5 Ursae Minoris). At fourth magnitude (4.25), number 5 in
the Flamsteed list of
numbered stars in Ursa Minor was
obviously too faint for Bayer to make note of via Greek letter
(though he and others indeed lettered fainter ones). Five Ursae
Minoris may also be among the more prominent stars that everybody
with a dark sky sees but pays no attention to. It's supremely easy
to find and, given its position of only 14 degrees from the North Celestial Pole, it's visible from
nearly everywhere north of 14 degrees north latitude any time of
night. The front bowl stars of the Big
Dipper, Merak and Dubhe (Beta and Alpha UMa), are famed as the
Pointers to Polaris, the North Star.
Similarly, the front bowl stars of the Little Dipper, Pherkad and Kochab (Beta and Gamma UMi), point right
to 5 UMi. Only it's a lot closer, just 2 degrees north-northeast
of second magnitude Kochab. It's so close that it might make a
good addition to the Little Dipper as a drop of water flying off
its lip. Beyond that, the star is among the most common types in
the visual bestiary, a class K (but cooler than normal, K4) giant (oddly the same as Kochab)
359 light years away
(with a likely uncertainty of just 5 light years). The recorded
temperatures have a large range, from 4070 to 4395 Kelvin, with
an average of 4230, which somewhat compromises correction for infrared radiation. Using
the average temperature, 5 UMi shines with the light of 413 Suns (with an uncertainty of perhaps 20
percent, really not all that bad). Temperature and luminosity
then give a radius of 38 times that of the Sun, 0.18 Astronomical
Units, roughly 60 percent the size of Mercury's orbit. Were it
in our solar system, the orange star would glower nearly 20 degrees
across, about the separation between Betelgeuse and Rigel. A low
projected equatorial rotation speed of 1.9 kilometers per second
indicates a rotation period that (were the rotation axis
perpendicular to the line of sight) could be as long as 2.5 years.
The theory of stellar structure and evolution give a mass perhaps
as high as 4 times that of the Sun and show that the star is probably
fusing its core helium to carbon and oxygen, though it could be
just past that state as it makes ready to become an even bigger
giant. The iron content is about three-fourths solar, nothing
unusual, nor is the speed of motion relative to the Sun. At four
solar masses, stars don't live very long; 5 UMi had a
hydrogen-fusing lifetime of just 180 million years. After
sloughing off its outer envelope, the core will turn into a white dwarf of just under
0.8 of a solar mass. At 59 and 23 seconds of arc away are
thirteenth and tenth magnitude "companions" whose motions
clearly show both to be just line-of-sight "optical" coincidences.
Five UMi is, however, listed as a mild barium star, which suggests the vague
possiblility that there is a white dwarf companion that as a swollen
enriced giant contaminated the star we see now.
Written byJim Kaler 4/11/14. Return to STARS.