Framed by brilliant Mars and Sirius, Procyon and the constellaton Canis Minor are easy to see across the east-southeast around 8 o’clock. The entire groups is visible throughout the winter. Created with Stellarium
Mars and the brightest star Sirius are now well placed in the eastern sky around 8 o’clock. This coming week Mars will be the closest it’s been to Earth in over two years and almost match Sirius in brightness. Can you tell the difference between them? On the magnitude scale used by astronomers, Mars is -1.2 magnitude and Sirius -1.4, just a tad brighter.
Framed by these two stellar luminaries, Procyon (PRO-see-on) in Canis Minor the Little Dog is easily passed over. Its name means "before the dog" which refers to its rising before the brilliant dog star Sirius — 42 minutes earlier if you’re watching from Duluth, Minn. Sirius is the alpha male in the much larger constellation Canis Major while Procyon belongs to Canis Minor, a stick-shaped pattern of just two stars. Appropriate I suppose for a small dog.
Procyon and Canis Minor get a leg up on the big dog Sirius, who resides at Orion’s feet. The faint constellation of Monoceros the Unicorn lies between the two canines. Created with Stellarium
Both Sirius and Procyon have small, compact companion stars called white dwarfs in orbit about them. We looked at these exotic, superdense stars when we got acquainted with the star Omicron in Eridanus the River two weeks ago. A white dwarf is the end of the road for stars like the sun, Sirius and Procyon. Five billion years from now, the sun will have blown away its outer atmosphere to reveal what it’s been cooking up all those years: a tiny Earth-sized star so dense a thimbleful of it weighs more than a ton.
Compare the sizes of the sun, Procyon and Sirius. Procyon is four times the diameter of the sun, Sirius is 3.5. The little dog star is not so little after all! (sizes shown are approximate) Illustration: Bob King
Procyon is a white star about 3 1/2 times larger than the sun and radiates seven times more brightly. At 11.5 light years away, it’s the 14th closest star system to Earth. Back in 1844 German astronomer Friedrich Bessel measured a slight wobble or shift in Procyon’s and Sirius’ positions he believed were caused by the gravitational tug of unseen companions. 18 years later, Sirius’ companion — Sirius B — was discovered by the keen-eyed telescope maker Alvan Clark. Procyon’s companion, Procyon B, is both closer to its parent star and fainter than Sirius B and had to wait until 1896 for discovery. Bessel’s method of measuring stellar wobbles is still used to this day to detect companion stars and extrasolar planets in orbit around their parent stars.
Procyon’s companion star Procyon B is half again as small as shown in this diagram. Despite the enormous difference in size, the dwarf contains more than one third of the mass of its parent star Procyon. Illustration: Bob King
While Procyon is some three million miles in diameter, its companion star is only about 10,700 miles across. I’ve tried to show the white dwarf’s size in relation to Procyon but at the scale that fits on this page, I couldn’t draw it any smaller. Picture a pinpoint next to Procyon and you’ll have an even better idea of its size.
Procyon and its little buddy circle around each other once every 40.8 years at a average distance similar to that separating the sun and Uranus (1.4 billion miles). The little dwarf is much, much fainter than its parent and would shine only a couple times more brightly than the full moon as seen from an imaginary planet closely orbiting Procyon. From Earth, 11th magnitude Procyon B is lost in Procyon’s brilliant glare and inaccessible to most amateur astronomers’ telescopes but we can picture there in our imagination the next clear night. In the far future, Procyon will shed it stellar cloak just as its companion once did and become a white dwarf, too.
Two peas in the pod if there ever could be.