Bright Double Star Fomalhaut is Actually a Triple: Three Stars, Not Two

The nearby star system Fomalhaut was once thought to possess a double star. Now, though, astronomers have discovered that this double star is actually one of the widest triple stars known. The findings reveal a little bit more about this nearby system, which is especially interesting due to its unusual exoplanet and dusty debris disk.

"I noticed this third star a couple of years ago when I was plotting the motions of stars in the vicinity of Fomalhaut for another study," said Eric Mamajek, one of the researchers, in a news release. "However I needed to collect more data and gather a team of co-authors with different observations to test whether the star's properties are consistent with being a third member of the Fomalhaut system."

In order to get a better look at the star, the researchers carefully analyzed astrometric and spectroscopic measurements of the star. They then measured the distance and speed of the third star and were able to conclude that it was actually part of the Fomalhaut system. While it was called LB 876-10 in the past, the new findings make the star into Fomalhaut C.

"Fomalhaut C looks quite far apart from the big, bright star that is Fomalhaut A when you look up at the sky from Earth," said Mamajek.

There are about 5.5 degrees between the two stars, which means that it's as if they're separated by roughly 11 full moons for an observer on Earth. Because Fomalhaut is just 25 light years from Earth, these stars appear to be farther away from one another than they would if they were located much further afield.

"Fomalhaut A is such a massive star, about twice the mass of our sun, that it can exert sufficient gravitational pull to keep this tiny star bound to it--despite the star being 158,000 times farther away from Fomalhaut than the Earth is from the Sun," said Mamajek.

The tiny star Fomalhaut C is actually a red dwarf star, one of the most common types of stars in the universe. Yet what is most interesting is how this star might affect the gravitational pull being exerted on the Fomalhaut "b" exoplanet and the debris disk orbiting Fomalhaut A. Currently, researchesr are working on pinning down the orbit of Fomalhaut C to learn a little bit more about the system.

The findings are published in the Astronomical Journal.