CHARA (Beta Canum Venaticorum). A lovely name for a northern star
that vaults the heavens in northern spring, "Chara" from Greek
meaning "joy." Not terribly bright, just fourth magnitude (4.26),
Chara (Beta Canum Venaticorum) is the fainter of the pair of stars
that dominate the modern constellation of
Canes Venatici, the Hunting Dogs,
invented by Hevelius in the mid-1600s. The dogs are held by Bootes as he pursues Ursa Major around the pole, Chara and Cor Caroli (the Alpha star) helping make
the "southern dog," the "northern dog" represented by a small group
of stars to the northeast of Cor Caroli. Chara was originally the
name for the southern dog itself, the northern called "Asterion"
for "Little Star." But with the brighter of those that make the
southern dog called "Cor Caroli," little Chara got the name to
itself. Chara's most interesting aspect is its similarity to the
Sun. The majority of bright naked eye stars
are considerably more luminous than our own star, some vastly so,
a natural result of their intrinsic brilliance, such stars visible
over great distances. However, our Sun, a modest star in the
middle of the full range of stellar brightness, would be invisible
to the eye if only 70 light years away. At a distance of 27.3
light years (give or take just 0.1; second Hipparcos reduction), Chara provides
a modestly good chance to see what the Sun would look like at
stellar distances. It's a warm class G (G0) hydrogen-fusing dwarf
with a temperature of 5880 Kelvin (only 100 degrees hotter than the
Sun), a luminosity just 18 percent greater than solar, and a radius
four percent larger. Direct measure of angular diameter gives very
satisfying agreement of a radius three percent larger. Theory
tells that the mass is very close to that of the Sun, the larger
luminosity coming from a more advanced age (hydrogen-fusing stars
slowly brightening and swelling as they age), Chara more like 7
billion years old, as opposed to the Sun's 4.6 billion. (The range
is large, though, as other age estimates range from 4.1 billion
years through 8.6 billion, even unto 12 billion.) Even though
stars rotate more slowly with age, Chara (with a rotation speed of
at least 3 kilometers per second) is spinning at least 50 percent
faster than the Sun, giving a rotation period under 17 days. (Here
we encounter even more uncertainty, as another estimate of rotation
speed gives 11.5 km/s and a rotation period under 4 days, which is
more in line with stellar youth.) Consistently, it's also detected
in the X-ray part of the spectrum, implying that it too has a
surrounding hot corona (rotation helping induce the necessary
magnetic field). The biggest difference, other than luminosity,
seems to be a metal content less than solar, the star having only
about 60 percent as much iron. So (assuming the older age values)
if you want to see the Sun from afar, plus what it will look like
in a couple billion years, take a look at Canes Venatici's
"southern dog." While there is no evidence for an orbiting planet, Chara may have a spectroscopically
detected companion that orbits
every 6.65 years at a probable distance of around 3 Astronomical
Units. The reality of it, however, has been severely challenged,
and it may not exist. If there IS a planet, its residents (given
any) would see the Sun looking almost the same from there as Chara
does to us.
Written by Jim Kaler 4/16/98; revised
10/13/09. Return to STARS.