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Starry, Starry Night


Paul Kohlmiller


Procyon (PRO-see-on) is the brightest star in the constellation Canis Minor. Its name (originally from the Greek) means "before the dog". This is in relation to the fact that Procyon rises just a bit before Sirius, the dog star. Canis Minor is a relatively small constellation with only one other relatively bright star, Gomeisa, a variable star. Procyon has magnitude 0.38 while Gomiesa is around 2.8. I can't see what the ancients saw in constellations but I'm completely baffled as to how a small dog can be seen in just two stars.

Procyon is a double star, though it is difficult to resolve. However, as early as 1844 it was known to be a double star because of the wobbles caused by its companion. Procyon might be so large, about the twice the diameter of the sun, that it is currently shedding material that is captured by its smaller but hotter companion. The white dwarf companion is called Procyon B. This dwarf is roughly earth size but has more than half of a solar mass. Procyon A is one of the largest stars within 20 light-years of the sun.

Procyon is intrinsically brighter than the sun — most visible stars are. But the real reason that Procyon appears to be so bright, about as bright as Rigel, is that it is just over 11 light years away. Rigel, by comparison, is 775 light years away. In a list of the most visible stars, Procyon is the least intrinsically bright star visible from San Jose.

Procyon is a type F star — its surface temperature is 6500 kelvins.

Procyon is part of the winter circle of stars. It is closer to Pollux than to Castor in Gemini which is one way to remember which of the major Gemini stars is which. Pollux is closer to Procyon and Castor is closer to Capella — P near P; C near C. Others consider Procyon part of the Winter triangle along with Betelgeuse and Sirius.

Procyon is also known as Antecanis, another version of "Before the dog". Its Arabic name is Al Shira.

Canis Major and Canis Minor are separated by another constellation, Monoceros. How that constellation managed to horn in between the dogs, I don't know. But if you draw a line between Sirius and Procyon you will find the open cluster M50 along that line - closer to Sirius than to Procyon but inside of Monoceros.



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