Tuesday, August 30, 2005
Looking East-Northeast 1-2 Hours Before Dawn
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Today's chart is aimed east-northeast about an hour before sunrise.
That's where you'll find two bright stars flanking the waning
crescent moon. Even when the moon has moved on, these stars are
noticeable for being bright, yet close together on the sky's dome.
They are Castor and Pollux, and they're the brightest lights in the
constellation Gemini the Twins. The stars themselves are often
referred to as "twins," but they're not really connected in any
way.
Castor's estimated distance is 52 light-years, while Pollux is
estimated to be 34 light-years away. In other words, these two
stars are about 18 light-years apart . . . more than four times
farther apart than our sun and its nearest neighbor, Alpha
Centauri. To the eye, Pollux shines with a golden color, while
Castor is blue-white. There's a marvelous description of Castor in
the great book Turn Left at Orion by Guy Consolmagno and Dan M.
Davis.
A small telescope would show the single point of light we see as
Castor to be two stars of similar brightness, with a third fainter
star some distance away. The two primary stars are known as "A" and
"B." Each one is also a double star. All four stars in the A and B
pairs are hot and young, roughly twice the size of our sun and 10
times as bright.
What's more, the stars in each pair orbit very closely, separated
by just two or three star-diameters. Because they're so close, each
pair of stars spins around its common center of gravity very
quickly! The A pair completes its mutual orbit in only nine days.
The B pair orbits in only three days. That's in contrast to our
Earth's orbit around the sun of 365 days.
But don't forget the third fainter star, orbiting some distance
from the other two. Star "C" is also really two stars, which orbit
each other in less than a day. By the way, the C pair is pursuing a
long orbit indeed around the A and B pairs. The C stars need 10,000
years to go around their brother stars. And remember, although
we're talking about a total of six stars in the A,B and C pairs . .
. all six stars reside in the single starry point we know as
Castor.
By the way, there's another starry point below the moon and twins
that you won't want to overlook. It looks like a star . . . but
it's really the planet Saturn, which now lies low on the
horizon.
