As darkness descends this
week, cast a glance over toward the east-southeast and you will see a brilliant,
bluish-white star scintillating just above the horizon. You'll be looking at
the brightest star in the sky, the Dog Star, Sirius.
Sirius is the brightest
star of the constellation Canis Major, which is Latin for the "Greater
Dog." Among all the visible stars, Sirius is one of only four stars of
negative magnitude (-1.44). The lower the figure of magnitude, the brighter
the star.
Of the 21 brightest stars,
twelve are considered to be of first magnitude, and five are of zero magnitude.
But only Arcturus, Alpha Centauri, Canopus and Sirius are bright enough to be
assigned a magnitude value lower than zero, or of a negative magnitude.
According to Burnham's Celestial
Handbook other names for Sirius include "The Sparkling One" or "The
Scorching One." The star normally appears a brilliant white with a tinge
of blue, but when the air is unsteady, or when it is low to the horizon it seems
to flicker and splinter with all the colors of the rainbow. At a distance of
just 8.7 light years, Sirius is the fifth-nearest known star. Among the naked-eye
stars, it is the nearest of all, with the sole exception of Alpha Centauri.
Many astronomy books suggest
you can locate Sirius by using the belt of Orion; that the belt points southeast
directly toward Sirius. Of course, that is absolutely true (in fact, following
the belt in the other direction takes you to another first magnitude star, Aldebaran,
in Taurus, the Bull). Of course, being the brightest star in the sky has its
advantages. It's almost impossible to overlook as it gleams in the southeastern
evening sky these frosty evening.
If you're still uncertain
about what you are seeing and want to make a positive identification, then by
all means, use Orion's belt to verify that you're indeed looking at Sirius.
Sirius will be due south
between 9 and 9:30 p.m. local time all of this week, and doesn't set below the
west-southwest horizon until after around 2:15 a.m.
Over thousands of years,
Sirius appears to move in a wavy line across the sky. In 1862, Alvan G. Clark
first saw Sirius B, also known as "the Pup," the companion star responsible
for the wiggle. Sirius B is only one ten-thousandth as bright as Sirius A, but
by 1914, spectroscopic observations had demonstrated that its temperature was
about the same.
From physical laws it follows
that B emits the same amount of light per unit surface area as A, therefore
to be so dim it must be very small. Later calculations have shown that A has
just over twice the mass as our Sun, but B has nearly one solar mass. Since
it is so small, B must be exceedingly dense.
In fact, it packs 98 percent
of one solar mass into a body just 2 percent of the Sun's diameter. To
do that, Sirius B must have a density 90,000 times that of the Sun.
Unbelievably, just a teaspoon
of this star's material would weigh about 2 tons!
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Joe Rao
serves as an instructor and guest lecturer at New York's Hayden Planetarium.
He writes about astronomy for The New York Times and other publications, and
he is also an on-camera meteorologist for News
12 Westchester, New York.
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