COROT is a space telescope and the first of its kind designed to search for transiting extrasolar planets. It detects planets by measuring the slight dimming caused when a planet happens to cross in front of its parent star. Besides discovering extrasolar planets, COROT has discovered what could be the smallest known hydrogen-burning star. C4780 was initially identified as a transiting planet candidate, but follow-up observations reveal that this object is an eclipsing binary system consisting of 2 stars. The primary star is an F-type star while the secondary star is a low mass red dwarf star which whizzes around the primary on a 20.7 day orbit. Each time the red dwarf star passes in front of the primary star, it causes the primary star to dim by a small amount and allows the size of the red dwarf star to be estimated at one-tenth the Sun’s diameter. This makes the red dwarf star similar in size to the planet Jupiter and slightly smaller than the next smallest red dwarf star with a measured diameter.
As the red dwarf star orbits the primary F-type star, the gravitational pull from the red dwarf star causes a measurable wobbling of the primary star. By measuring this wobbling, the mass of the red dwarf star is estimated to be less than one-tenth the Sun’s mass. This places the red dwarf star close to the minimum mass for a hydrogen-burning star. With its size (0.104 Sun’s diameter) and mass (0.096 Sun’s mass) known, the mean density of the red dwarf star is estimated to be 240 tons per cubic meter, or about 30 times the density of solid iron. Although similar in size to the planet Jupiter, this red dwarf star contains about 100 times the mass of Jupiter. Having such a high density, this red dwarf star is probably the densest hydrogen-burning star discovered so far. On the red dwarf star’s surface, the gravity will be crushing - about 250 times stronger than the surface gravity on Earth. For very low mass stars with masses less than 1/5 the Sun’s mass, only a handful have their masses and diameters measured. A larger sample of such stars with measured masses and diameters will certainly improve the understanding of stars in the lower end of the stellar mass spectrum.
Lev Tal-Or, et al. (2013), “CoRoT 101186644: A transiting low-mass dense M-dwarf on an eccentric 20.7-day period orbit around a late F-star”, arXiv:1302.5830 [astro-ph.EP]