But Proxima Centauri is so small that astronomers have not been able to accurately measure it. Proxima Centauri is a very-low-mass star, in fact barely massive enough to burn hydrogen to helium in its interior. It is about seven times smaller than the Sun, and the surface temperature is "only" about 3000 degrees, about half of that of our own star. Consequently, it is also much fainter -- the intrinsic brightness is only 1/150th of that of our Sun.

Proxima Centauri is barely massive enough to produce the thermonuclear fusion that powers a star. Slightly less massive objects fail to generate the reaction and are called brown dwarfs.