See the 'eye' of the 'Southern Fish'

Bright star in night sky

Late these evenings, if it is clear, we will see a moderately bright, white star low in the sky near the south-eastern horizon.

That star is Fomalhaut (pronounced Fomalo - with the Os pronounced as in "post"), marking the eye of Piscis Austrinus, "The Southern Fish". This distinguishes it from the constellation Pisces, which just means “fish".

For me the appearance of Fomalhaut in the late evening heralds the arrival of autumn. This star stands out strongly in the sky because there are no bright stars nearby to compete with it.

Fomalhaut is a white star, with a surface temperature of just under 9,000 C. Our Sun has a surface temperature of 6,000 C. Fomalhaut has about double the mass of the Sun and is around 17 times brighter. By cosmic standards it is a close neighbour, being some 25 light years away. A light year is the distance light travels in a year—just under 10 trillion kilometres. It also means the light from that star takes 25 years to reach us.

Fomalhaut is the brightest of a partnership of three stars, orbiting around one another. It is often referred to as Fomalhaut A. The two companion stars, referred to as Fomalhaut B and C, lie at distances of 0.9 and 2.5 light years respectively from Fomalhaut A. Fomalhaut B is a much fainter, orange star, and Fomahaut C is a red dwarf.

Normally, double and triple stars find it hard to produce stable planets. Their interacting gravitational attractions can make planets hard to form, and if they do, they have very unstable and unpredictable orbits. This instability would make the surface conditions extremely variable. An inhabitable version of Tattoine is unlikely. However, the members of the Fomalhaut system lie far apart, so gravitational interference would be relatively small. There is some evidence that Fomalhaut A may have a planet or two.

Radio telescope observations, together with images from the Hubble Space Telescope and more recently the James Webb Space Telescope show the star to be surrounded by a great disc of gas, dust and larger bodies, such as asteroids.

Certainly the ingredients are there for making planets, but as yet we do not know for sure whether there are any. Fomalhaut is young, around 440 million years old. The Sun, along with the Earth and other planets, formed around 4.5 billion years ago. The oldest signs of life found so far are around 3.5 billion years old, so if Fomalhaut has any potentially life-bearing planets, life is likely not to have appeared yet, or maybe just started.

Fomalhaut has around twice the mass of the Sun, so it has twice the fuel supply. It is burning this fuel around 17 times as fiercely as the Sun. This suggests it will be able to keep shining only about one-eighth as long.

The Sun is around halfway through its life, so if we assume the Sun will last for 10 billion years, Fomalhout will run out of fuel in around 1.25 billion years. When the Earth was this old, there was life, but it was mostly single celled. It was not until 500 million years ago, around four billion years after our planet formed, that life really got going.

Even if there is life on a planet orbiting Fomalhaut, it won't survive long enough to evolve far. Hot, bright stars are poor places to look for life.

The science of astronomy is made more fascinating by its connections with our cultural roots. We put our stories and legends in the sky as constellations. Piscis Austrinus, Fomalhaut's home constellation, was known by the ancient Greeks as "The Great Fish". The two fish making up the constellation of Pisces were described as its offspring, drinking the water being poured by Aquarius, "The Water Carrier", and a nearby constellation.

Take time to look for that lone, white star near the south-eastern horizon. Fall is coming.


Saturn rises around 9 p.m. and Jupiter around 11 p.m. In hilly places they will appear later. The Moon will reach its last quarter on Aug. 6.

Ken Tapping is an astronomer with the National Research Council's Dominion Radio Astrophysical Observatory near Penticton.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.

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About the Author

Ken Tapping is an astronomer born in the U.K. He has been with the National Research Council since 1975 and moved to the Okanagan in 1990.  

He plays guitar with a couple of local jazz bands and has written weekly astronomy articles since 1992. 

Tapping has a doctorate from the University of Utrecht in The Netherlands.

[email protected]

The views expressed are strictly those of the author and not necessarily those of Castanet. Castanet does not warrant the contents.

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