Are there stars smaller than our sun?

How big are the smallest stars?

The smallest stars have a diameter of 121,200 kilometers, which corresponds to 8.7 percent of the diameter of our sun. Their surface temperature is 1727 degrees, while the sun is 5500 degrees Celsius hot on its surface. Overall, the luminosity of these smallest stars is a hundred thousand times weaker. These are the results of a 14-year observation project to study small stars in the vicinity of the solar system, which were presented at the 222nd meeting of the American Astronomical Society in Indianapolis.

“By examining the most complete sample of red and brown dwarfs possible, we want to answer a whole series of questions,” explains the head of theResearch Consortiums on Nearby Stars (RECONS), Todd Henry from Georgia State University in Atlanta. Astronomers are looking for the exact boundary between red dwarfs, i.e. the smallest stars that still permanently generate energy through nuclear fusion, and brown dwarfs, substellar objects that contain too little mass for permanent nuclear fusion but are more massive than planets. In addition, Henry and his colleagues hope that their project will provide information about the activity of the dwarf stars and the frequency with which planets orbits them.

With a whole series of telescopes on earth and in space, RECONS has examined over 300 dwarfs within a radius of 50 light years around the solar system. "This is the largest long-term study of stars in the vicinity of the Sun," says Henry, "it provides the best positions and brightness data to date for the red and brown dwarfs closest to us." The team's measurements show that the frequency distribution of the Faintly luminous objects with a diameter of 8.7 percent of the sun's diameter and a temperature of 1727 degrees Celsius shows a clear drop. At this point, according to Henry and his colleagues, lies the line between stars and brown dwarfs.

The smallest red dwarf stars can therefore be smaller than the largest Jupiter-like planets. But they have a much larger mass. The limit for sustaining permanent nuclear fusion is 70 to 80 times the mass of Jupiter. The RECONS researchers hope to be able to determine the mass limit more precisely with their observations. In contrast to planets, brown dwarfs temporarily fuse heavy hydrogen to helium. Reactions like this are no longer possible below about 13 Jupiter's masses - this is where the boundary between brown dwarfs and planets lies.