Earthsky Tonight
Fomalhaut: Loneliest star
06-29-2009 - Brightest Stars
How to see it
Fomalhaut, sometimes called the Autumn Star, shines several degrees farther south than does Antares, summertime’s most prominent southern star. Fomalhaut appears in a part of the sky that is largely devoid of noteworthy stars. For this reason, Fomalhaut is often called the Lonely One or Solitary One.
Best seen in Autumn, whitish Fomalhaut is more or less opposite the sun in early September, and hence, shines in the sky all night long. In early September, the star 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.
Finding Fomalhaut as it culminates is simple. Just face south and look. Fomalhaut is the brightest star in front of you, and typically less than a third of the way up in the sky (higher from far southern Texas or Florida, lower from more northerly locations). Occasionally a bright planet could be in that general location, but planets are always considerably higher and most likely brighter than Fomalhaut.
Need confirmation it’s Fomalhaut you’re seeing? Two ‘pointer’ stars for Fomalhaut are the two stars on the western side of the Great Square of Pegasus, Alpha and Beta. A line drawn southward from Beta through Alpha and extended downward about three times the distance between them leads to Fomalhaut.
Fomalhaut is the 18th brightest star in the sky, and 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.
History and Myth
As mentioned above, Fomalhaut is sometimes called the Solitary One because it appears 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.
According to Richard Hinckley Allen, it 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 worshipped 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.
Until recently, it was considered the first magnitude star farthest from other first magnitude stars. It is just slightly more than 39 degrees from Achernar. However, proper motion of Antares, which causes a star’s position in the sky to change over long periods of time due to the star’s actual motion in the heavens, has now separated it from the nearest first magnitude star (Alpha Centauri) by a few second of arc more than the distance between Fomalhaut and Achernar.
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.
Science
As determined by an analysis of its light, Fomalhaut is classified as an “A3V” star, considerably hotter and heavier than our sun (as indicated by the “A3″). The “V” is called a luminosity class, and it designates the largest category of all, the “normal” stars in the mature and stable part of their life spans. The sequence, known as the Main Sequence, follows the rather odd progression of OBAFGKM, with Os being the hottest and most massive, and Ms being the coolest and least massive. 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, it is hotter and more massive. The other luminosity classes include I (supergiants), III (giants) and VII (white dwarfs) among others.
If it were possible to observe the sun and Fomalhaut from the same distance (astronomers like to use 10 parsecs or 32.6 light-years), Fomalhaut would outshine the sun in visible light by nearly 17 times. Its actual distance, 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 it 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. However, a disk of dust has been found around Fomalhaut, probably similar to the disk that formed earth and the other solar system planets. The disk is offset a bit, rather than being centered exactly on Fomalhaut, possibly indicating a massive planet that is pulling on it gravitationally. However, as of August 2008, no such planet has been observed directly.
Fomalhaut’s position is RA: 22h 57m 39s, dec: -29° 37′ 19″.
If this is the star that I think it is, it seems to “twinkle” more than any other that I have observed. I also see white, green (my wife sees blue), and red colors as it twinkles. Can you tell me if this is the star that I am describing?
David,
Without knowing when and where you are looking, I cannot say with absolute certainty, but chances are very good that you are referring to Sirius, the brightest star in the sky: http://earthsky.org/tonightpost/brightest-stars/sirius-the-brightest-star
This time of year it rises in mid to late evening in the southeastern sky, and sets around sunrise in the southwestern sky.
David, to follow up on my earlier comment, just last night (11/30) I observed Fomalhaut low in the southwestern sky and it did twinkle a lot, although not as much as Sirius. But what really struck me is that as it stood there all alone, it appeared to move back and forth and up and down just a little. It is a very strange sensation and can be confusing to many people. However, the star was not moving. Such apparent motions are not real, but are either the effects of atmospheric disturbances, or mire likely the natural tendency of the human eye to jump around a little. Our brain compensates for this most of the time, when there is a lot in view, but when there is an isolated point, the “jitteriness” of the human eye becomes apparent.
Can anyone tell me the hellical rise and set dates when Fomalhaut rose at 150 degrees and set at 210 degrees?
Larry Hancock,
If I am understanding your request, I do not know where information such as this might be available. Unless you have some specific criteria in mind, such as a particular anglular distance from the Sun or length of time before sunrise — that is, something that can be calculated — I don’t see that it would even be possible to tabulate anything since helical risings at least in ancient times depended on the visual acuity of the observer as well as local topography and weather. Time of helical rising also is dependent on geographic location and even the time period (e.g., now or thousands of years ago). Today Fomalhaut rises at about 150 degrees and sets at 210 degrees azimuth from the far North, roughly 55 degrees latitude. May I ask why you are interested in this information?