The Dog Nights of Winter

© 2001 by Robert C. Moler

Everyone's heard of the dog days of summer, those hot days of July and August. It was thought by the ancients that the Dog Star, Sirius, supposedly augmented the sun's heat to produce those hot days. Sirius is not so hot by itself as it shines like a cold diamond in the bone chilling cold night sky of winter.

Sirius is located in the heart of the constellation of Canis Major, Orion's large hunting dog. It is the brightest night time star, though it is not as bright as the planets Jupiter and Venus which also decorate our winter evening sky this year. Sirius sparkles with a clear blue-white light the planets can't match. Sirius is called the Dog Star due to its position in the constellation of the Great Dog. The name Sirius actually means dazzling one, due to the brightness and its twinkling as its rides low in the sky.

Though it stands out above all other stars in brightness, another way to find it is to follow the line of Orion's belt stars to the lower left.

To the Egyptians this bright star was Sothis, whose heliacal rising in June announced the flooding of the Nile and the beginning of their agricultural year. A heliacal rising is a star or planet's first appearance in the dawn sky after leaving the evening sky.

Sirius' rank as the brightest night time star is due mainly to its closeness to the earth. It is 8.6 light years away. While it is 23 times brighter than the sun, many stars are much brighter than that. On the scale of surface temperatures from hot to cool, that is called spectral class denoted by the letters OBAFGKM ("Oh Be A Fine Girl/Guy, Kiss Me"), Sirius is class A (white) to the sun's G (yellow). To my eyes Sirius as a decidedly arc light bluish cast.

Because hotter stars shine brighter per square inch, or centimeter than cooler stars, the 23 times brighter star has only 3.24 times greater surface area. Sirius is 1.8 times as large as the sun. We can measure Sirius' mass because it is a binary star. This is done by measuring the gravitational effect of the companion star on Sirius and Sirius on the companion, along with the size and period of the orbit. It turns out that Sirius is some 2.35 times the mass of the sun.

A little mass difference leads to a great difference in brightness. This makes stars more massive than the sun use up their available hydrogen fuel faster than the sun does, shortening their lives.

The companion to Sirius was discovered indirectly in the first half of the 19th century by F. W. Bessel. Sirius and its companion orbit about their mutual center of gravity causes Sirius to move across the sky in a wavy path. All stars, including the sun move in the sky. The motion seen against the sky is called proper motion. Sirius moves about the width of the moon every 1,300 years. Superimposed on that is a 50 year period wiggle. The Stradivarius of telescope makers Alvan Clark was the first to spot the companion, Officially named Sirius B, but nicknamed "The Pup".

Sirius B was the first of a new class of stars discovered, white dwarfs. Here is a star with the mass of the sun, only twice the diameter of the earth. It is a star which has run out of hydrogen in its core, succumbing to the force of gravity which has squeezed it down to this small size.

The other dog star in the sky is Procyon, in the constellation of Canis Minor, the lesser dog . With but two stars, Canis Minor could represent a hot dog. Procyon has the cryptic meaning, "Before the Dog". This alludes to the fact that though east of Sirius, Procyon rises before the Dog Star. This is because Procyon is also north of Sirius. This only works for observers north of 30 degrees north latitude.

Procyon is like Sirius in many ways. It is not as hot as Sirius, being spectral type F (white). It is 11.4 light years away. It is also 5.8 times the brightness of the sun, and about the same size as Sirius. Procyon also has a white dwarf, about the size of the Pup, but only 65% the mass. Its orbit of Procyon is a bit less than 41 years.

So remember the cool dogs of winter while you're sweltering during the dog days of next summer.

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Uploaded: 01/31/01