PROCYON (Alpha Canis Minoris). The eastern anchor of the Winter Triangle, Procyon is the luminary
of Canis Minor, the smaller dog, and at
magnitude zero (0.34) is the sky's eighth brightest star. At a
distance of only 11.4 light years, it is the 14th closest star
system, which largely explains its brightness. The Greek name
means "before the dog," as in northern latitudes the star rises
before Sirius, the "Dog Star," and its
constellation Canis Major, announcing
their quick arrival. A white class F (F5) subgiant-dwarf, Procyon
radiates with the power of 7.0 Suns from a
somewhat warmer surface of 6530 Kelvin. From these parameters we
find a mass of 1.4 solar and see that indeed (true to the star's
subgiant status) that it is about to give up core hydrogen fusion
as it prepares to become a much cooler and brighter subgiant (the
star's age around three billion years). With a radius of 2.1 times
that of the Sun and an equatorial rotation speed of at least 3.2
kilometers per second, Procyon's rotational period could be as long
as 33 days. It is surrounded by a hot solar-type corona heated to
1.6 million Kelvin. The star's major claim to fame is a tiny
companion, Procyon B, one of three classic white dwarfs (the others Sirius B, and 40
Eridani-B). Though Procyon B was not actually seen until 1896,
its existence was known as early as 1844 from the wobbles it exerts
on the motion of much brighter Procyon A. With a visual magnitude
of only 10.82, 10.5 magnitude fainter than Procyon A (a factor of
over 15,000), and lying at most just 5 seconds of arc from Procyon
A, Procyon B is a real challenge to observe. Procyon A and B
circuit each other every 40.8 years at an average distance of 15.0
Astronomical Units, a rather high eccentricity taking them between
21.0 and 8.9 AU. From the orbit we find a precise mass for Procyon
A of 1.42 solar (satisfingly similar to that derived from the
theory of stellar structure), and 0.60 solar for Procyon B, notably
less than that of Sirius B. Procyon B's spectrum yields a temperature of
7740 Kelvin (a bit warmer than Procyon A), a radius of 1.35 times
that of Earth (typical of such stars), and a terribly low
luminosity of 0.00049 times that of the Sun. (From Procyon A,
Procyon B would shine with the light of only a couple of full
Moons.) White dwarfs are the end products of moderate-mass stars.
Since higher mass stars die before those of lower mass, the star
that created Procyon B must have had a mass a bit greater than
Procyon A's, perhaps around 2.1 solar. During the giant stages
that preceded the white dwarf, it lost roughly 3/4 of itself back
into space through winds. Given a huge mass stuffed into a small
volume, the average density of Procyon B is around a third of a
metric ton per cubic centimeter (but just
20 percent that of Sirius B). There are
two basic kinds of white dwarfs, those with thin, pure hydrogen
skins on them (heavier elements sinking under the action of
gravity), and those with helium atmospheres, the hydrogen having
most likely been lost through earlier winds. Sirius B of the first
kind, Procyon B of the second, but an odd example that has an
unusual collection of carbon and heavier elements. Someday,
Procyon A will join it as a white dwarf itself.