Piscis Austrinus
What is Piscis Austrinus?
Constellations· Autumn evenings low south (N) · the lonely 1st-mag star. Home of Fomalhaut, the lone first-magnitude star of the autumn southern sky. The constellation is pictured as a fish drinking the water poured out by Aquarius.
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Stars of the figure
| Star | Magnitude |
|---|---|
| Fomalhaut | 1.2 |
The 1 brightest of the 6 stars used in the figure (6 lines). Lower magnitude means brighter.
Constellation basics
| Season | autumn evenings, low in the south (N) |
|---|---|
| Brightest star | Fomalhaut (α, 1.2) |
| How to find | the lone bright star below the Great Square |
| Area rank | 88 |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Fomalhaut is three — a family spread 8° across the sky
Fomalhaut looks like a lonely star, but it is really a family of three. The orange star Fomalhaut B, 2 degrees away in the sky, has long been known to travel through space alongside it. In 2013 a small red star in Aquarius, a full 5.7 degrees away, was confirmed as the third member, Fomalhaut C. That gap is more than ten times the width of the full Moon.
The three stars span 8 degrees of sky and about 2.5 light-years of real space, more than half the distance from the Sun to its nearest star. At such distances the gravity holding them together is very weak, and a star passing close by could scatter the family. Both A and C are surrounded by disks of dust, and one idea is that the distant companions have stirred up each other's disks.
The constellation in depth
Piscis Austrinus is one of Ptolemy's 48 constellations, and an old one: the Babylonians already drew a fish in this part of the sky. In Greek myth it is the great fish that rescued the Syrian goddess Derceto when she threw herself into a lake. One story even makes the two fish of the zodiac sign Pisces its offspring.
The star of the show is Fomalhaut. Its name is Arabic for ‘mouth of the fish’, and it sits right at the fish's mouth. It is a white star 25 light-years away, surrounded by several rings of dust. In 2008 the Hubble Space Telescope photographed what looked like a planet inside the dust, and it made headlines. Later studies showed that it was not a planet but a spreading cloud of dust from a collision between smaller bodies.
One of the Sun's close neighbours also hides here. Lacaille 9352 is a small red star about 10.7 light-years away. At magnitude 7.3 it is too faint for the naked eye and needs binoculars. In 2020 two planets were confirmed around it, one of them a little over four times Earth's mass. The star also moves across the sky quickly enough that photos taken a few decades apart show it shifting.
On evenings from September to November, drop straight down from the Great Square of Pegasus toward the southern horizon and you will meet a single bright star: Fomalhaut. The rest of the fish is 3rd to 4th magnitude and needs a dark sky. From New York it climbs only about 20 degrees, and from London under 10, so find a place with a clear view south. In the Sky view, look near the southern horizon on an autumn evening to see Fomalhaut marked.
Frequently asked
- When can I see Piscis Austrinus?
- Piscis Austrinus is best seen in autumn evenings, low in the south (N).
- How do I find Piscis Austrinus?
- To find Piscis Austrinus, look for the lone bright star below the Great Square.
- What is the brightest star in Piscis Austrinus?
- The brightest star in Piscis Austrinus is Fomalhaut, at magnitude 1.2.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.