Wednesday, 24 January 2018


Procyon, bright and yellowish, is the brightest star in the winter constellation of Canis Minor, the small dog, and the eighth brightest star in our sky. Not that it's a big and impressive star as such, but its relative brightness merely originates in its proximity. At a distance of only 11.4 light-years, Procyon is the 14th closest star to our Solar System. Let's say it lies right under our doorstep, in astronomical terms. For the rest it's nothing out of the ordinary, with a mass s.5 times that of our Sun and a diameter twice as big. It also boasts a solar-like corona which it heats up to 1.6 million °C. Procyon's much younger than our Sun though, but after 1.7 billion years it has already depleted its entire hydrogen supply and fusion has started to expand outside of its nucleus. It's preparing to evolve into a much bigger, helium-fusing red giant, which will happen in the next 10 to 100 million years.

Nothing out of the ordinary, you'd say. Yet, this inconspicuous, bright star hides a little secret. It had already been suspected in the early 19th century due to irregularities in the star's proper motion, but it was not confirmed visually until 1896. Procyon is indeed double, the main star orbited by a tiny companion that's extremely difficult to observe. Procyon B lies 4.3 arc-seconds from Procyon A, which should be doable also in small telescopes, but the problem is that A shines 15,000 times more brightly than B and therefore the little one disappears into A's glare. Even with my binoscope I had a really hard time separating the two and I had to wait for that moment of perfect seeing to detect the companion, as I've tried to reflect in my sketch.  

B revolves around A in a highly eccentric orbit which takes it as close as 9 AU (Astronomical Unit - the distance between the Earth and the Sun) and as far as 21 AU over a 40-year period. It's a white dwarf only 30% larger than the Earth but contains 60% of the Sun's mass. Its average density is therefore a whopping two tonnes per cubic inch! Scientists believe that B was once bigger and hotter than A and therefore it evolved a lot faster. When it got older, it evaporated much of its mass onto A, which gradually became the dominant star in the system. We find that A is indeed quite rich in heavier elements, byproducts of advanced nuclear fusion in B. 

No evidence for planets has been found to date and even if there were, they would probably not be suitable to sustain life due to the distortion and radiation of this extreme double star system. However, a large ring of dust has been detected. 

PS: The image doesn't show well in Blogger, but B lies slightly to the bottom-right of A...

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