ALCOR (80 Ursae Majoris). Alcor, forever tied to Mizar, is hardly ever spoken of unless as
"Mizar and Alcor," a naked eye
double in the tail of Ursa Major that are 11.8 minutes of arc apart and
that the Arabs referred to as the "horse and
rider." The two are usually taken as a test of minimal
vision. The name Alcor was stolen from that for Alioth. Both come from an Arabic word that
means the "black horse." The term was distorted in different ways
as it was applied to each of the two stars. Oddly, the "rider" of
the pair is the one with the name of the "horse," "Mizar" referring
not to a horse but to the "groin" of the Great Bear. A great many
stars with Bayer Greek letter names have no proper names. Alcor is
one of the very few in reverse, a star that has a proper names but
no Greek letter name. Instead, it is referred to as 80 Ursae
Majoris. In the early 1700s, the English astronomer John Flamsteed
organized a new catalogue of stars in which they were ordered from
west to east within the constellations, Alcor number 80 in Ursa
Major. "
Flamsteed numbers" are commonly used when the
Greek letter
names run out. Alcor is a fourth magnitude (4.01) white class A
(A5) hydrogen-fusing dwarf
star with a temperature of 8000 Kelvin and a luminosity 12
times that of the Sun. It appears coupled
with Mizar, but is it really a physical companion? We are still
not sure. Mizar itself is a quadruple star on the "double-double"
theme (two double stars in orbit about each other.) Precision
parallaxes with the Hipparcos satellite show Mizar to be 78.1 light
years away, but Alcor to be 81.1 light years distant. Mizar and
Alcor are part of the Ursa Major cluster, whose core consists of
the middle five stars of the Big Dipper.
A separation of over three light years, almost the distance between
here and Alpha Centauri, would make a gravitational pairing
unlikely as the neighboring stars would pull them apart. The
measured errors, however, allow a separation as close as 0.7 light
years. The errors in the distances are suspected of being greater
than listed, and the analysis of the orbit of Mizar A suggests that
Mizar might actually be FARTHER than Alcor! If they are actually
at the same distance, their minimum separation is only 0.27 light
years, making them close enough so that they could truly orbit,
though with a long period of three-quarters of a million years.
For a time Alcor was thought to be double, but it now appears that
early astronomers were fooled and that it is really single,
rendering Mizar and Alcor together a "quintuple star." While the
Mizar stars are slow rotators with peculiar chemical compositions
as a result of element separation, Alcor is a rapid spinner (218
kilometers per second, over 100 times solar). As a result, its
atmosphere is stirred and its composition normal. It is, however,
a slight pulsating variable. The inner five stars of the Big
Dipper are all at roughly the same distance and all are normal
hydrogen fusing main sequence dwarfs. Alcor's faintness next to
the them is a vivid reminder of the role that mass plays in the
stars. Alcor's mass is around 1.6 times that of the Sun. Alioth,
on the other hand, with twice Alcor's mass, is almost 10 times
brighter!