Fomalhaut: Actually Triple Star, Astronomers Say

In a new study accepted for publication in the Astronomical Journal (arXiv.org), astronomers show that a previously known red dwarf star called LP 876-10 is part of the Fomalhaut system

This artist’s impression shows the Fomalhaut A planetary system. Image credit: David Hardy.

Fomalhaut, the 18th brightest star visible in night sky, is located in the constellation Piscis Austrinus about 25 light-years from Earth. The star is twice as massive as the Sun and 20 times brighter.
The name Fomalhaut derives from the Arabic name for this star – Fum al Hut, meaning ‘the Fish’s Mouth.’
Fomalhaut has been featured in science fiction novels by writers Isaac Asimov, Stanislaw Lem, Philip K. Dick, and Frank Herbert.
Despite being a well-studied system, it was only recently confirmed that Fomalhaut was a binary star – two stars (Fomalhaut A and B) that orbit each other – although it had been first suggested in the 1890s.
In the new study, lead author Dr Eric Mamajek of the University of Rochester and his colleagues have found the triple nature of Fomalhaut through a bit of detective work.
“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,” Dr Mamajek said.
“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.”
By carefully analyzing astrometric and spectroscopic measurements, the astronomers were able to measure the distance and speed of the third star. They concluded that a star known as LP 876-10 is part of the Fomalhaut system, making it Fomalhaut C.

 

 

This image taken with SMARTS 0.9-m telescope shows Fomalhaut C. Image credit: Eric E. Mamajek et al.

“Fomalhaut C looks quite far apart from the big, bright star that is Fomalhaut A when you look up at the sky from Earth,” Dr Mamajek said.
Fomalhaut C is a red dwarf star – the most common type of star in the Universe. From the vantage point of a hypothetical planet orbiting Fomalhaut C, Fomalhaut A would appear to be a brilliant white star nine times brighter than Sirius appears from Earth, similar to the typical brightness of the planet Venus. Fomalhaut B would appear to be an otherwise unremarkable bright orangish star similar in brightness to Polaris. The age of the trio is about 440 million years – roughly a 10th of the age of our Solar System.
“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,” Dr Mamajek explained.

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