Procyon

• 1. Introduction

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  Procyon is a star whose very name is a forecast: in Ancient Greek, Prokyon means before the dog, because this star rises above the horizon just ahead of Sirius, the Dog Star, as Earth turns. At just 11.46 light-years away, Procyon is one of the closest stars to our own Sun, and it is usually the eighth-brightest star in the entire night sky, blazing at an apparent magnitude of 0.34. What looks like a single bright point of light is not, in fact, alone: Procyon is a binary system, paired with a faint companion so dim it went undetected for decades after astronomers first proved it had to exist. What is that hidden companion, and what will become of Procyon itself as it burns through the final stage of its life as an ordinary star?

• 2. In the Sky

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  Procyon reaches its highest point in the sky at midnight on the 14th of January, and shows up best for evening observers during late winter in the Northern Hemisphere. Alongside Sirius and Betelgeuse, it forms one of the three points of the Winter Triangle, an asterism stargazers use to orient themselves in the winter sky. A color index of 0.42 gives Procyon a faint yellow tinge, distinct from the blue-white glare of Sirius nearby.

• 3. The Two Stars of Procyon

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  What looks like one star is actually two, orbiting each other once every 40.84 years along a path with an eccentricity of 0.4, more elongated than Mercury's own orbit around the Sun. Procyon A shines at magnitude 0.34, while its companion, Procyon B, is far fainter at magnitude 10.7. Their orbital plane tilts 31.1 degrees relative to Earth's line of sight, and the pair's average separation of 15.0 astronomical units, a little less than the distance from the Sun to Uranus, actually swings between 8.9 astronomical units at their closest and 21.0 at their furthest.

• 4. A Star in Transition

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  Classified as spectral type F5IV-V, Procyon A is unusually bright for its class, a sign that it has nearly finished fusing the hydrogen in its core into helium and is beginning to evolve into a subgiant. Within an estimated 10 to 100 million years, it will swell to somewhere between 80 and 150 times its current diameter and shift toward a red or orange color as its nuclear reactions move outward from the core. Its atmosphere currently sits at an estimated 6582 degrees, giving it a white hue, and the star carries 1.5 times the Sun's mass, twice the Sun's radius, and seven times the Sun's luminosity, with both its core and outer envelope in convective motion, separated by a wide radiation zone.

  In late June 2004, Canada's orbital MOST satellite spent 32 days continuously monitoring the brightness of Procyon A, hoping to confirm the star's predicted solar-like oscillations and enable a technique called asteroseismology. It found none, leading the researchers to suggest the underlying theory of stellar oscillations might need revision, though other scientists argued the non-detection was still consistent with oscillations already measured from the ground. Photometric data gathered by NASA's Wide Field Infrared Explorer satellite in 1999 and 2000 had already shown evidence of surface granulation and solar-like oscillations that did agree with ground-based measurements, and a further MOST survey in 2007 finally detected the oscillations directly.

  On the 1st of April 1979, the Einstein Observatory's high-resolution imager detected an X-ray source associated with the Procyon system, positioned only about 4 arcseconds south of Procyon A, within the outer edge of the imager's 90 percent confidence circle. That placement pointed to Procyon A itself as the source, rather than Procyon B, which sat roughly 5 arcseconds to the north, about 9 arcseconds from where the X-rays were actually detected.

• 5. The Hidden Companion

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  German astronomer Friedrich Bessel first proposed the existence of Procyon B in 1844, based purely on wobbles in Procyon's motion through space, long before anyone actually saw it. Fellow German astronomer Arthur Auwers calculated its orbital elements in 1862 as part of his doctoral thesis, and it took until 1896 for anyone to actually see it: American astronomer John Martin Schaeberle spotted it at exactly the predicted position, using the 36-inch refractor telescope at Lick Observatory.

  At an estimated radius of 8,600 kilometres against 5,800 kilometres for Sirius B, Procyon B is considerably less massive than Sirius B yet physically larger, a paradox explained by the strange physics of degenerate matter, and the size matches white dwarf models built around a carbon core. Its atmosphere is dominated by helium with traces of heavier elements, earning it the classification DQZ, and for reasons that remain unclear, its mass is unusually low for that type of star. At a surface temperature of 7740 degrees, it also runs considerably cooler than Sirius B, the result of its lower mass and greater age: the star that became Procyon B likely had about 2.59 times the Sun's mass, burned through a main-sequence lifetime of 680 million years, and reached the end of its life some 1.19 billion years ago.

• 6. Names Across Cultures

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  In Greek mythology, Procyon was linked to Maera, a hound belonging to Erigone, daughter of Icarius of Athens. In July 2016, the International Astronomical Union's Working Group on Star Names formally approved Procyon as the star's official proper name.

  Babylonian astronomers knew Procyon as Nangar, the Carpenter, an aspect of the god Marduk associated with constructing the heavens, one of many names ancient civilizations gave the star. In Macedonian folklore, Procyon and Sirius became Volci, the wolves, circling hungrily around Orion, who was pictured as a plough pulled by oxen. Arabic tradition gave Procyon the name Al Shira, the Syrian sign, distinguishing it from Sirius, along with Elgomaisa, the bleary-eyed woman, in contrast to Sirius as the teary-eyed woman.

  In Chinese astronomy, Procyon anchors an asterism known as South River, and is individually known as the Third Star of South River, part of a larger constellation figure called the Vermilion Bird. Hawaiian navigators grouped it with Capella, Sirius, Castor, and Pollux into an asterism called Ke ka o Makali'i, the canoe bailer of Makali'i, surrounding the Pleiades star cluster, and call Procyon itself Puana, meaning blossom. In Tahiti, it was one of the mythical pillars propping up the sky, tied to the art of public speaking; it is also known as Puangahori, False Puanga, a name distinguishing it from Puanga-rua, the culturally vital star known by its western name Rigel.

  Procyon even appears on the flag of Brazil, representing the state of Amazonas. The Kalapalo people of Mato Grosso call Procyon and Canopus together Kofongo, Duck, with Castor and Pollux standing in for its hands, and its appearance in the sky signaled the start of the rainy season and a fresh harvest of manioc. To the Inuit, the star was Sikuliarsiujuittuq, the one who never goes onto the newly formed sea ice, the name of a man in a story who stole food because he was too heavy to hunt on the ice, and was ultimately killed by other hunters who lured him onto it; Procyon's tendency to glow red as it rises during the Arctic winter was tied directly to his bloody end.

• 7. Seen From Somewhere Else

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  Observed from the Procyon system, the Sun would appear as a magnitude 2.55 star sitting in the constellation Aquila, at coordinates exactly opposite Procyon's own position in Earth's sky, roughly as bright to hypothetical observers there as the star β Scorpii appears to us.

  Procyon's nearest known stellar neighbor is Luyten's Star, just 1.12 light-years away. Seen from a hypothetical planet orbiting Luyten's Star, Procyon would blaze as the brightest star in that alien night sky, at an apparent magnitude of minus 4.68. The view would work both ways: Luyten's Star itself would be visible from Procyon at magnitude 4.61, bright enough to see easily, unlike almost every red dwarf star visible from Earth, with the possible exception of Lacaille 8760.