The popular multiple star
Castor (the Horseman) is one of the bright stars which make up the
heads of the twins of the prominent winter constellation Gemini.
Castor is the northwestern most star of the two, and slightly
fainter. Even a small telescope will split Castor into two
separate stars. These stars are physically bound by gravity in
an orbital dance similar to the earth and moon. According to
Burnham's, this pair was the first widely recognized gravitationally
bound objects outside of our solar system.
Above is the appearance
of the bright pair in a 6-inch at 270x under excellent seeing
conditions. North is down and east is to the right.
The diagram on the right
illustrates the apparent motion of the two stars relative to the
brighter star, as if it were held motionless. North is up and
east is to the left in the diagram. The fainter component
appears to orbit the brighter in a clockwise direction. The
stars were closest together in the late 1960's and will continue to
widen until the end of the 21st century. The average physical
separation of these two stars is 118 Astronomical Units (1.0 AU is
the mean distance from the sun to the earth).
A third component (C)
is an 8.8 magnitude star which lies 73" distant. This
star is also physically bound, very slowly orbiting the main pair.
Each of the three
visible stars in this system is itself a double; there are at least
6 stars total here. Each of these close pairs orbit much too
closely to separate in even the largest of telescopes. We know
of the existence of these stars from spectroscopic measurements.
Rather than a single set of absorption lines from a single star, two
sets of lines are superimposed on one another. The relative
motions of the orbiting stars results in a periodic shift of each
set of absorption lines. Such measurements can tell us a great
deal about stars we can't even see as separate entities.
The fainter
"C" component is the variable star YY Gem, an eclipsing
binary. The two stars orbit about one another every 19 hours,
32 minutes. The orbit is seen edge-on, so twice each orbit one
of the stars passes in front of the other. When this happens
the single star we see at this location can fade from 8.9 magnitude
to 9.6. One of the component stars is also a flare star, which
can brighten dramatically from time to time.
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