The Lonely Star of Autumn


NightSky_PNGJust as our seasons come and go so to do many of the constellations in our night sky. The exceptions being certain circumpolar constellations such as Ursa Major, Ursa Minor, Cassiopeia, Cepheus, Draco, and Camelopardalis, all seen in our northern sky and which all appear to rotate throughout the year around the “Pole Star”, Polaris, in Ursa Minor.

On autumn evenings you can see that the constellations of the southern summer sky: Sagittarius, Libra, and Scorpius, have all shifted from their more usual muggy, early summer evening positions in the south and southeast towards the southwest. Horning in on them from the southeast is a new and decidedly fishy lot of constellations. Well, maybe “fishy” isn’t totally accurate but they all have in common a distinct water theme. Aquarius (the water bearer), Pices (the fish), Cetus (the sea monster), Capricornis (the sea goat- what the heck is a sea goat anyways?), and Piscis Austrinus (the southern fish) are all inhabitants of the southern sky at this time of the year.   Sadly, all of these aquatic star patterns are rather faint and not easily seen, especially in our overly illuminated nighttime urban environment. There is however one bright star among them and if you step outside at around 10PM or 11PM and face south you can catch a glimpse of it low along the southern horizon.   Its name is Fomalhaut and it is the alpha star (the brightest visible star) in the constellation of the southern fish, Piscis Austrinus. You can pronounce “Fomalhaut” in one of two ways: “FOAM-ah-low” or Foam-a-lot”.

fomalhaut autumnFacing south on cool autumn evenings you will see that there is an area of sky, low along the horizon that seems oddly bereft of bright stars. The exception of course is lonely Fomalhaut, shining nicely as our 18th brightest star in the night sky at a very respectable apparent magnitude of 1.2.

The ancient Greek astronomer Hipparchus devised the apparent (how bright the star looks from Earth, not its actual brightness) stellar magnitude scale and he decreed that the very brightest stars in our nighttime skies are “of the first magnitude”. We’ve revised the scale a bit to include the sun, moon, and planets but the bottom-line-thing-to-know is that the higher the magnitude number the dimmer the object, while the lower the number the brighter the object. Our revisions now include negative numbers so the brightest star in our sky, Sirius, has a magnitude of -1.4, while the Moon (the brightest object in our night sky) is at -12.6 during full phase. The dimmest stars you can potentially see naked eye are shining at magnitude 6.

The name “Fomalhaut” means something along the lines of “the mouth of the fish” and is an Arabic translation of the description that the last of the great Greek astronomers, Ptolemy, had given it.   The constellation that Fomalhaut belongs to, Piscis Australis is just one of the 48 constellations that Ptolemy lists in his classic compendium of Greek astronomy knowledge and lore, “The Almagest”, written around 150 A.D.

The fish lies just to the south of Aquarius and is often depicted as drinking from the stream of water being poured out of the vase being held aloft by the water bearer.Sidney_Hall_-_Urania's_Mirror_-_Aquarius,_Piscis_Australis_&_Ballon_Aerostatique

While the translation of “Fomalhaut” may be “the mouth of the fish” we amateur astronomers have a special nickname for it: “The Lonely One”, or “The Lonely Star of Autumn”. We give it this moniker because of its seeming isolation as a bright star in our southern sky. But, as we shall see in just a moment, Fomalhaut is not quite as lonely as the name suggests.

Fomalhaut is about 25 light years away and is twice the size of our own Sun and some 17 times as bright. It’s also very hot. The Sun has a toasty surface temperature of around 10,000 degrees Fahrenheit but Fomalhaut is a much warmer 15,000 degrees Fahrenheit. It’s also a fairly young star at 440 million years of age. That’s old to you and me but in terms of the lifespans of the average star Fomalhaut is just a pup.   But there is an important caveat to add here: hot, massive stars live fast and die young and since Fomalhaut has twice the mass of our Sun its total life expectancy is figured to only be about a billion years or so.   The Sun’s total life expectancy is around 11 billion years so both it and Fomalhaut are actually middle aged.

Now, if that nickname of “The Lonely Star” has you feeling all weepy for Fomalhaut you can put aside the tissues because it just isn’t so.   We now know that Fomalhaut has a couple of other stellar companions. One is an orange-colored dwarf star (Fomalhaut B) about 1 light year away from the primary and the other is a red dwarf star (Fomalhaut C), the most common type of star in the universe, located some 2.5 light years from Fomalhaut A.   These are very wide separations for multiple star systems and it made things a bit tough for astronomers to get the data they needed to confirm that the stars are indeed gravitationally connected with each other.

fomalhaut b berkley univsersityBut wait, there’s more! We’ve known for some time that Fomalhaut is surrounded by disks of gas and dust (making it look like the Eye of Sauron from Tolkien’s The Lord of the Rings), the stuff that planets are formed out of, and in 2008 astronomers announced that they had found an extrasolar planet candidate amidst the debris disks.   Designated Fomalhaut b (astronomers use lower case letters for planet designations and upper case letters for the parent star) this alien exoplanet became the first ever world outside our own solar system to be imaged directly by photographic means. The planet is still shrouded in dust and may be as massive as our own Neptune or it could contain three times the mass of Jupiter. It also has a rather weird, highly elliptical orbit that takes the planet on a 1,700 year trip around its parent star! That’s a long year by anyone’s standards.

And not to be outdone, it was announced earlier this year that a small debris disk also surrounds Fomalhaut C.   We don’t know yet whether or not there is anything planet-like within this disk but we do know one thing for certain: Fomalhaut is not so lonely after all and it still continues to surprise us.


Posted in: Night Sky

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