Third Star Confirmed in Bright Fomalhaut Star System

Fomalhaut, the most massive and widest star system that's relatively nearby Earth, contains more stars than astronomers previously believed.

Astronomers had long thought that Fomalhaut was a binary star system, but recent observations and a bit of serendipity have revealed that the are actually three stars in the system, according to a paper scheduled for publication in the Astronomical Journal, which reveals that a third star nearby the binary pair is itself part of the star system.

The triple star system of Fomalhaut is one of the widest triple star systems known; the distance between the main star, dubbed Fomalhaut A -- a behemoth double the size of the Sun -- and the third star, Fomalhaut C, is more than 150,000 times the distance from Earth to the Sun. Fomalhaut A is the 18th brightest star visible in the night sky.  

Eric Mamajek, associate professor of physics and astronomy at the University of Rochester, said he noticed the star that would become Fomalhaut C a few years ago when he was plotting the motions of starts in the vicinity of Fomalhaut for another study.

Later, by pure chance, Mamajek was in a hotel in Chile at the same time as Todd Henry, the director of the Research Consortium On Nearby Stars (RECONS) team at Georgia State University.

"Eric was playing detective on this third star and I just happened to be sitting there with an observing list that contained the unpublished parallax," Henry said, referring to a type of measurement astronomers use to determine distances.

The third star was, at the time, known as LP876-10.

"A student at the time, Jennifer Bartlett at the University of Virginia, was working with us on a sample of potentially nearby stars for her Ph.D. thesis, and LP876-10 was on it," Henry said. "Eric and I got to talking, and here we are with a cool discovery."

After measuring the movements, temperature and radial velocity of the third star, the astronomers concluded that ti was indeed a part of the Fomalhaut system.

"Fomalhaut C looks quite far apart from the big, bright star that is Fomalhaut A when you look up at the sky from Earth," Mamajek said, noting that an observer on Earth would see roughly 11 full Moons worth of distance between the two stars.

That they appear so far apart explains why the third star was initially overlooked as a member of the star system. But now that astronomers have concluded Fomalhaut is a triple star system, it joins a special class. There are only 11 star systems closer to the Sun than Fomalhaut that have three or more stars. Alpha Centauri, the closest star system, counts among those.

The astronomers' calculations show that the Fomalhaut system is the most massive and widest among these nearby star systems.