The star Fomalhaut – an autumn star for the Northern Hemisphere, sometimes called the Loneliest Star – holds a special place in the search for planets beyond our solar system. Orbiting Fomalhaut is the first extrasolar planet visible to the eye in photographic images. By 2008, when Fomalhaut’s planet became visible, we knew other planets were out there, orbiting distant suns. But, prior to the images of Fomalhaut b, all extrasolar planets, or exoplanets, made their presence known indirectly, for example, in some instances, by their gravitational tugging on their stars.
Fomalhaut was also one of the first stars known to have a disk of dust around it, a sign that more planets might be forming there.
The image at left is lovely as well. It’s an artists’ conception of what a planet orbiting Fomalhaut might have to endure as it plows through the dusty disk around this star.
Does Fomalhaut have other planets in addition to Fomalhaut b? In 2012, astronomers announced the possibility of two low-mass planets orbiting this star.
As determined by an analysis of its light, Fomalhaut is classified as an A3V star. It’s considerably hotter and heavier than our sun, as indicated by the “A3.” Imagine you could place our sun and Fomalhaut side by side in space – say, at 10 parsecs or 32.6 light-years away. In that case, Fomalhaut would outshine our sun in visible light by nearly 17 times. The “V” in Fomalhaut’s classification is called a luminosity class, and it designates the largest category of all, ordinary stars like our sun in the mature and stable part of their life spans.
A word about the Main Sequence. These numbers and letters refer to what astronomers call the Main Sequence, a way of categorizing stars by mass and luminosity. The Main Sequence follows the rather odd progression of OBAFGKM. Os are the hottest and most massive stars, and Ms are the coolest and least massive stars. In addition, there is a numerical subdivision running from 0 to 9 with each letter. Our sun is a G2V, as is Alpha Centauri. Since Fomalhaut’s designation is significantly to the left of the sun’s in this sequence, you know it is hotter and more massive than our sun.
Fomalhaut’s actual distance from Earth, as determined by analysis of data from the Hipparcos mission, is 25 light-years. Fomalhaut’s mass and radius are, respectively, a little more than twice and a little less than twice solar values. Being hotter than the sun (about 8500 kelvins or nearly 15,000 degrees F, compared to 10,000 degrees F for the sun), this star burns its fuel faster and has a shorter lifetime. In fact, it is estimated that Fomalhaut may have a lifespan of only about a billion years, a tenth of that of our cooler sun. Currently, Fomalhaut is likely less than halfway through its life.
Fomalhaut appears to be a single star like the sun, but there is another faint star a couple of degrees below (south of) Fomalhaut that is at about the same distance and moving through space in the same manner. They are separated by about a light-year, although whether they are gravitationally bound as a binary star is unknown.
How to see Fomalhaut
Fomalhaut, sometimes called the Autumn Star by people in the Northern Hemisphere, appears in a part of the sky that is largely empty of bright stars. For this reason, in skylore, Fomalhaut is also often referred to the Lonely One or Solitary One or the Loneliest Star.
White Fomalhaut is more or less opposite the sun in early September, and so it shines in the sky all night long during the autumn months. Finding Fomalhaut from latitudes like those in the U.S. is simple. Just face south on an autumn evening and look. Fomalhaut is the brightest star in front of us on autumn evenings, as we face south. It is typically less than a third of the way up in the sky (higher from far southern Texas or Florida, lower from more northerly locations).
Fomalhaut is part of the faint constellation Piscis Austrinus the Southern Fish. It’s part of a round pattern of stars, supposedly the open mouth of the Fish. But don’t expect to see a fish in these stars.
In early September, Fomalhaut reaches its midnight culmination, meaning that it is highest in the sky to the south at local midnight. Finding Fomalhaut at the time it culminates is easiest, but this happens at different times on different dates. Here are just a few times and dates of culmination, but keep in mind that the times are only rough, although they are adjusted for Daylight Saving Time as needed:
July 15, 4 a.m.
August 15, 1 a.m.
September 15, midnight
October 15, 10 p.m.
November 15 , 7 p.m.
December 15, 5 p.m.
Fomalhaut is the 18th brightest star in the sky. It is probably is the most southerly bright star that many North Americans know. Granted, a few bright stars farther to the south are visible from tropical and subtropic northern latitudes, but these brighter stars lurk near or beneath the horizon as seen as from middle and far latitudes in the northern hemisphere. Fomalhaut can be seen from as far north as 60 degrees latitude (southern Alaska, central Canada, northern Europe), where it just skims the southern horizon. People in the Southern Hemisphere see Fomalhaut high in their sky, for more of the year, than we do.
History and Myth
As mentioned above, Fomalhaut is sometimes called the Loneliest Star. It really does look lonely because it’s as the only bright star in a large area of sky.
Fomalhaut is Alpha Piscis Austrinus (the brightest star in the Southern Fish), and the name Fomalhaut derives from the Arabic Fum al Hut, meaning Mouth of the Fish. Strangely, another Arabic title for the star means The First Frog.
In the sky visible from the Northern Hemisphere, the constellation Aquarius the Water Carrier resides above Fomalhaut’s constellation Piscis Austrinus. You can see a zig-zag line of stars that goes from Aquarius to Piscis Austrinus. In skylore, this line of stars represents water from the Jar of the Water Carrier, trickling into the open Mouth of the Fish, as shown in the illustration above.
According to Richard Hinckley Allen, Fomalhaut was one of the four guardians of the heavens to the ancient Persians, and given the name of Hastorang. (The other guardians were Aldebaran in Taurus, Antares in Scorpius, and Regulus in Leo.) Allen also says that Fomalhaut was a source of worship at the temple of Demeter at Eleusis in ancient Greece. In about 2500 BC, Fomalhaut helped mark the location of the winter solstice, meaning that it helped to define the location in the sky where the sun crossed the meridian at noon on the first day of winter.
Oddly, although Fomalhaut’s spectral type indicates a white to bluish white color, some writers have referred to it as red. There being little likelihood that this star has dramatically changed colors in historic times, the reference to a red Fomalhaut is puzzling. It may be due to a simple error, repeated unquestioning by later writers. Or it may be due to the reddening affects of Earth’s atmosphere, which filters out the bluer colors when an object is near the horizon. This causes, for example, red sunsets.
Bottom line: Look for the star Fomalhaut in the constellation Piscis Austrinus the Southern Fish on Northern Hemisphere autumn evenings (Southern Hemisphere spring evenings). Because it’s the only bright star in its region of sky, Fomalhaut is sometimes called the Loneliest Star. This star is famous for its planet, Fomalhaut b, which was the first extrasolar planet seen via images at wavelengths visible to the human eye.
Fomalhaut’s position is RA: 22h 57m 39s, dec: -29° 37′ 19″.