Procyon

Procyon

Also known as Alpha Canis Minoris, 10 Canis Minoris, GCTP 1805.00, BD+05º1739, HD 61421, LHS 233, GJ 280, HIP 37279

     Poor Procyon. Seventh brightest star in the heavens, yet gets no respect. Anywhere else in the sky, Procyon (pronounced PRO-see-yon), alpha star of the constellation Canis Minor, would be one of the night’s more prominent luminaries. But hemmed in as it is on every side by glories such as Betelgeuse, Rigel, Sirius, Castor, Pollux, Capella, and Aldebaran, Procyon is just one more light in the dazzling display of the winter Milky Way. Even its name is an indication of secondary status. The Greek root of Procyon translates roughly as “Before the Dog”, a reference to its rising prior to the Dog Star, or Sirius. And it doesn’t help that its immediate setting, Canis Minor, is a rather undistinguished constellation containing but a single naked eye companion of just over 3rd magnitude, the amazingly young (160 million year old) Beta Canis Minoris, a.k.a. Gomeisa (Arabic for “the bleary-eyed woman”).

     But at magnitude 0.34, there is no chance of not finding this close neighbor. At eleven and one half light years distance, it stands near the edge of the stellar neighborhood as we have defined it. But at twice the size of our own Sun and nearly seven times its luminosity, Procyon certainly makes its presence known! (We ourselves would be seen as a magnitude 2.5 star from the vicinity of Procyon – still respectable, but nothing like this blazing luminary.)

     The three billion year old Procyon belongs to the curiously-named class of stars known as subgiant-dwarfs. This means that it is a main sequence stars on the verge of running through the last of its store of core hydrogen, prior to moving on to the next stage of stellar evolution (within the next 10 to 100 million years) by simultaneously expanding and cooling off, eventually becoming a red giant with perhaps 150 times its current diameter. Its greater mass (1.4 times that of the Sun) means that Procyon is racing through its evolutionary development at a faster pace than our own star. So long before the Sun will have left the main sequence, Procyon will have blown off its outer layers to briefly form a glorious planetary nebula, prior to collapsing upon itself into a white dwarf scarcely larger than the earth.

                                                   Friedrich Wilhelm Bessel
 

     Of perhaps even more interest, Procyon is not a solitary star. It has a companion (Procyon B), which, having long since run through every phase of stellar evolution, is already a white dwarf. Decades before it was finally confirmed observationally, astronomers suspected an invisible companion to Procyon A due to wobbles detected in the star’s motion. Friedrich Wilhelm Bessel (of 61 Cygni fame – see the posting concerning that star for details) first deduced B’s existence in 1844 after extensive astrometric measurements using a device known as a filar micrometer. This involved placing two extremely thin threads within the eyepiece (Bessel used strands from spiders’ webs!), positioning one of them over the star whose position was being measured and the second over a reference star. Repeated observations over time would reveal very slight changes in the first star’s (in this case, Procyon’s) position. By 1862, mathematician Georg Friedrich Julius Arthur von Auwers of the University of Gottingen had determined the unseen companion’s basic orbital elements using Bessel’s astrometric data. But no one had yet confirmed visually that such an object existed. This was due to two factors; the first being the extremely close distance between the two components of the stellar system. Procyon B orbits the primary A star once each 40.8 years at a distance never greater than 21 AU (a little further out than Uranus is from the Sun), and as little as 8.9 AU (somewhat less than Saturn’s distance from the Sun). The second reason is the extreme difference in luminosity between the A and B components. Procyon A shines with a brightness of nearly 7 Suns, whilst Procyon B has a luminosity of 0.00055 Suns – or one fifteen thousandth of the primary! It wasn’t until 52 years after Bessel first realized such a companion existed that in 1896 the German-born American astronomer John Martin Schaeberle finally caught sight of the elusive target, using the 36 inch refractor at Lick University in California. I wouldn’t try splitting this double using amateur equipment!

     Procyon B currently has a mass of only 0.6 Suns, but astronomers calculate that it began its life about 2 billion years ago as a star of at least 2.5 solar masses. Quickly running through its main sequence phase (remember, the greater the mass, the faster the evolution) after about 400 million years, Procyon B swelled up to a red giant and expelled most of its original material into interstellar space long ago. Now a white dwarf, it packs all its remaining mass into a volume somewhat greater than that of the Earth (its diameter being about 10,700 miles).

     In another 100 million years or so, Procyon A will have followed its B companion in itself becoming a second white dwarf, resulting in an interesting pairing for future astronomers.