Procyon's hidden companion was predicted 52 years before anyone saw it
In 1844 Friedrich Bessel concluded from Procyon's wobbling motion that an unseen partner must be tugging on it. Not until 1896 did John Martin Schaeberle, peering through Lick Observatory's 36-inch refractor, spot the faint white dwarf exactly where calculations said it would be.
Procyon, the brightest star in Canis Minor, usually ranks eighth among the stars of the night sky. It sits just 11.46 light-years away, one of the Sun's nearest neighbours, and together with Sirius and Betelgeuse it marks a corner of the Winter Triangle. Its name comes from Greek for before the dog, because from most northern latitudes it rises ahead of Sirius, the Dog Star. Greek myth linked it to Maera, the hound of Erigone.
The bright component, Procyon A, is a white F-type star with 1.5 times the Sun's mass, twice its radius and seven times its luminosity, glowing at about 6,582 kelvin. It is unusually bright for its class, a sign it has almost exhausted the hydrogen in its core. Within 10 to 100 million years it should swell to between 80 and 150 times its present diameter and turn red or orange. Its surface oscillations caused a stir in 2004, when Canada's MOST satellite watched it for 32 days and found none. Ground-based measurements and NASA's WIRE satellite had suggested otherwise, and later MOST observations in 2007 did detect the pulsations.
Procyon B, the white dwarf, orbits its partner every 40.84 years on an elongated path whose separation swings from 8.9 to 21.0 astronomical units. Arthur Auwers worked out its orbit in 1862, still decades before it was seen. Glimpsing it is harder than spotting Sirius B, because it is fainter relative to its primary and sits closer to it in the sky.
The dwarf holds only 0.6 solar masses yet, thanks to the odd physics of degenerate matter, it is wider than Sirius B, with a radius near 8,600 kilometres against 5,800. Its mass is puzzlingly low for its type. The star that became it was roughly 2.6 times the Sun's mass and ended its life about 1.19 billion years ago.
Source: Procyon
