83 UMA (83 Ursae Majoris). In a constellation known for its white class A
stars and its orange class K giants, it's a surprise and a
treat to come across something quite different. Streaming just to
the east of Mizar in Ursa Major's Big
Dipper is a string of little stars
that starts with Alcor (80 UMa, wedded to
Mizar), then continues with Flamsteed's 81, 83, 84, and 86
Ursae Majoris. 82 UMa is a bit south of them. (The gap at 85 UMa
is filled by Alkaid at the end of the
Dipper's handle.) As a group, the string points southeasterly
toward the fingers of Bootes next door.
81, 86, and 82 are (surprise) rapidly rotating sixth magnitude white
class A stars, while 84 UMa is a more interesting sixth magnitude
magnetic star (like the variety's prototype, Cor Caroli in nearby Canes Venatici.) The prize in the set is 83
UMa, a red class M (M2) giant, which fits not at all with the rest
of the gang. Moreover, with a distance of 524 light years (give or
take 19) it's the most distant of them, though at 486 light years,
86 UMa comes close. Numerous temperature measures of 83 UMa average
out to a cool 3685 Kelvin, most of the star's radiation then falling
into the infrared. Using
distance and the infrared correction, 83 UMa is seen to shine with
the light of 3685 Suns, which with
temperature gives a radius of 59 times the solar value, or 0.27
Astronomical Units, 71 percent the size of Mercury's orbit. Theory
applied to luminosity and temperature strongly suggests that 83 UMa
is an ageing star that is brightening as a giant for the second time, now with a quiet
carbon-oxygen core. (In the first brightening as giants, stars have
dead helium cores that heat until they can fuse the helium into the
C-O mix. They then stabilize as the common orange giants of the sort
we see in Ursa Major, and when the helium runs out they brighten again
as 83 UMa seems to be doing now.) Such stars tend to be unstable.
As a "semiregular" variable it changes by just short of a tenth of
a magnitude over a poorly known "period" (if such exists) estimated
at 18 days or so. The best estimate of mass gives about twice that
of the Sun. It's hard to be exact as mature stars have a range of
masses with closely similar properties. Adopting two solar masses,
it began evolving as a giant around 300 million years ago, and is
now 1.4 billion years old. But don't take the numbers too seriously
as age is very dependent on mass. Flamsteed's 83 is listed as a weak
barium star, one that had a close companion that when evolving dumped
freshly made elements onto the star we now see. The classification
is probably spurious as real barium stars tend to be rather obvious.
83's most likely fate is to continue to brighten, and eject its outer
envelope. The revealed hot core will then light up the ejecta as
an expanding planetary nebula and die as a tiny
white dwarf.
Written byJim Kaler 7/18/14. Return to STARS.