Fish is a mainstay at our house with breaded tilapia and salmon favorites. Looking around the sky fish appear to be just as popular with no fewer than four finned constellations across both hemispheres. We have two in the north — Pisces and Piscis Austrinus the Southern Fish — and two strictly southern hemisphere groups, Dorado the Goldfish and Volans the Flying Fish. There are also three dog constellations but interestingly, not a single cat. Makes you wonder.
Pisces is a large, faint group we’ll visit on another occasion. Today we’ll meet Southern Fish, which as you might gather, is also shared by southern hemisphere skywatchers. In fact, it’s rather low in the sky seen from mid-northern latitudes and would otherwise escape notice were it not for its brightest star, a first magnitude gem named Fomalhaut.
The next brightest star in the constellation shines dimly at 4th magnitude. Together with half a dozen other faint members you can force the outline of a fish, but in truth it looks more like a lasso. Piscis Austrinus is one of our most ancient constellations, first imagined by the Babylonians as a fish and passed on to the Greeks around 500 B.C. They pictured a Great Fish swallowing the water poured from the urn of Aquarius, the next constellation to the north.
The odd-sounding Fomalhaut (FOH-mal-owt) is an Arabic word meaning “mouth of the fish,” and that is precisely where the star lines up in the constellation. The star is about twice as big as our sun, one-and-a-half times as hot and just 25 light years away, making it one of the closest bright stars in the sky. It’s 10 times younger than the sun, born in a collapsing cloud of dust and gas about 440 million years ago.
In 1983, the orbiting infrared astronomy satellite IRAS detected the heat signal of an enormous disk of ice-coated dust surrounding Fomalhaut, four times the size of our planetary system. Embedded within that dusty disk astronomers discovered a new, extrasolar planet in Nov. 2008 temporarily named Fomalhaut b. It’s about twice as massive as Jupiter and revolves around its parent star once every 1,700 years at a distance that varies from 50 times Earth’s distance from the sun to 300 times!
Most exoplanets carry the name of their parent star followed by “b,” “c,” “d,” etc. in order of discovery. But the International Astronomical Union (IAU), the official naming authority of celestial objects, held a contest to name several exoplanets earlier this decade including Fomalhaut b. In December 2015, the IAU announced Dagon as the winner. Dagon is a Semitic deity represented as half-man, half-fish and one of the few extraterrestrial planets to get its photo taken. As you can see it appears as a tiny spot of light due to its great distance from Earth.
Fomalhaut is the only bright star shining in the lower half of the southern sky during the fall, the reason it acquired the nickname “The Lonely One.” Look for it at nightfall in the southeast. The solitary star climbs highest around 10:30 p.m. local time in mid-October. With the moon rising later now, you have a chance to not only find this lonely beacon but also get an eyeful of the complete fish. Use binoculars to help pick out the constellation’s fainter stars.
When you finally see Fomalhaut and friends, let it take you backwards in time to when our distant ancestors first imagined a fish here.
And now it’s time for dinner, and no kidding — we’re having salmon tonight.