09 September 2016 by Brian Jones
Observers at mid-northern latitudes will find the distinctive ‘M’ or ‘W’ form of Cassiopeia at or near the overhead point during autumn evenings, with the prominent line of stars forming Andromeda located a little way to the south. The westernmost star in Andromeda is Sirrah, which also marks the north-eastern corner of the adjoining and visually striking quadrilateral of stars forming the Square of Pegasus (the winged horse). One of the largest constellations in the sky, Pegasus is also a guide to Piscis Austrinus (the southern fish), our celestial target this time around. Fomalhaut, the brightest star in Piscis Austrinus, can be located by following a line drawn from Scheat through Markab, both in the Square of Pegasus. Projecting this line southwards will eventually lead you to Fomalhaut.Piscis Austrinus can be viewed in its entirety from anywhere south of latitude 53°N, and those observing from Argentina, South Africa, Australia and similar latitudes during October evenings will see brilliant Fomalhaut at or near the overhead point. However, ramblers, trekkers and backyard astronomers in the UK and at other mid-northern latitudes will see Piscis Austrinus lying quite low over the southern horizon, and managing to locate it at all may present something of a challenge unless the sky is particularly dark and clear.
As you follow the line southwards from Scheat through Markab you will pass through the neighbouring constellation Aquarius (the water carrier) before reaching Fomalhaut (click here for more about star names). Old star charts depict the southern fish lying on its back drinking the liquid being poured into its mouth by Aquarius. This is a rather strange thing for a fish to do, albeit one that is reflected in the name Fomalhaut, a title derived from the Arabic fam al-hut al-janubi meaning ‘the mouth of the southern fish’. Fomalhaut itself shines at magnitude 1.17 from a distance of 25 light years (click here for more about magnitudes).
The rest of the stars forming Piscis Austrinus are all relatively faint in comparison, the next brightest being magnitude 4.18 Epsilon, located just to the north-west of Fomalhaut and whose light has taken nearly 500 years to reach our planet. Somewhat closer to us than Epsilon is the yellow giant star Delta, which shines at magnitude 4.20 from a distance of around 150 light years (click here for more about star colours). Marking the westernmost point of the southern fish is Iota, a white giant star shining at magnitude 4.35 from a distance of just over 200 light years and the light from which set off on its journey towards us at around the time of the Battle of Waterloo.
Although Piscis Austrinus offers little to the backyard astronomer other than identification of the constellation itself, its leading star Fomalhaut is fairly conspicuous, the area of sky immediately around it being devoid of other bright stars. Have a go at locating Fomalhaut yourself, bearing in mind that, as far as observers at mid-northern latitudes are concerned, your southern horizon will need to be fairly free of mist, cloud and light pollution, and that a pair of binoculars may be needed to help you pick out the glow of this star. However, those of you located at more southerly latitudes will see the southern fish swimming higher in the sky. Wherever you are… happy stargazing!
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