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| Brightest Stars on Sep 24, 2014

Fomalhaut had first visible exoplanet


The star Fomalhaut is an autumn star for the Northern Hemisphere (spring star for the Southern Hemisphere). From our northerly latitudes, it’s sometimes called the Loneliest Star. It’s also famous in astronomical science as the first star with a visible exoplanet. Follow the links below to learn more about the star Fomalhaut.

Fomalhaut’s debris disks and candidate planet

Fomalhaut as a star, and a word about the Main Sequence

How to see Fomalhaut

Fomalhaut history and mythology

This image shows the debris ring around Fomalhaut and the location of its first known planet. This is the actual discovery image – published in the journal Science in November 2008. View larger. Fomalhaut b was the first beyond our solar system visible to the eye in photographic images. Image via Wikimedia Commons.

Artists’ conception of what a planet orbiting Fomalhaut might have to endure as it plows through the dusty disk around this star. Image via NASA

Fomalhaut’s candidate planet. Fomalhaut, which is known to be surrounded by several debris disks, holds a special place in the search for planets beyond our solar system. In 2008, scientists announced a visible planet for Fomalhaut. It was the first extrasolar planet visible to the eye in photographic images. In 2008, when astronomers made this announcement, 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.

This planet, now called Fomalhaut b, was believed to have at least the mass of Neptune, and no more than three times the mass of Jupiter. Calculations reported in January 2013 indicated that the planet has a 1,700-year, highly-elliptical orbit.

And yet a controversy continues to surround this planet candidate.

That is, in 2012, the work of some astronomers showed that the emission observed from Fomalhaut b can be completely explained by starlight scattered by small dust. These astronomers argue that Fomalhaut b is indeed a planet, surrounded by dust in Fomalhaut’s observed dusty disks. Meanwhile, in the same year, the work of other astronomers suggested that the light of Fomalhaut b cannot be explained as due to direct or scattered radiation from a massive planet. One possibility they suggest is that, instead of being a real planet, Fomalhaut b might be the aftermath of a collision during the past 100 years of two small Kuiper belt objects, each with a radius of only about 50 kilometers.

Yet other groups of astronomers questioned whether a gas giant planet could exist within 40 AU of Fomalhaut. Spitzer Space Telescope imaging suggested that the object Fomalhaut b might merely be a dust cloud.

That same year, 2012, studies confirmed that, while Fomalhaut b does exist, it is shrouded by debris, so it may be a gravitationally-bound accumulation of rubble rather than a whole planet.

Time will tell, as more observations continue to be made of this fascinating system.

A comparison of the Fomalhaut system with our own solar system via Wikimedia Commons.

Fomalhaut as a star, and a word about the Main Sequence. 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.

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 the eye 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. This other star was recently found to be a companion to Fomalhaut, despite the fact that the two stars are separated by about a light-year. In 2013, researchers found that Fomalhaut is actually a triple star. It is one of the widest triple stars known. Read more about the third star in the Fomalhaut system.

View larger. | Fomalhaut is sometimes called the Loneliest Star because no other bright stars shine near it in the sky.  Photo by EarthSky Facebook friend Tony Gieracki.  Thank you, Tony!


Fomalhaut lies in the constellation Piscis Austrinus the Southern Fish. Its name means Mouth of the Fish in Arabic. This irregular circle of stars – visible in a dark sky – represents the fish’s open mouth.

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.

From the Southern Hemisphere. Those at far southern latitudes on Earth see Fomalhaut high in their sky, for more of the year, than we do on the northern part of Earth.

Water from the Water Jar of Aquarius going into the Mouth of the Fish. In the real sky, you can see a zig-zag line of stars representing this flow of water.

Fomalhaut history and mythology. 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″.