How big can the mass of a star be? Stars that are more massive than the Sun are rare. None of the stars within 30 light years from the Sun has a mass more than four times that of the Sun. A long-distance search for the sun led to the discovery of some stars with a mass 100 times the mass of the sun, and a handful of stars (some in billions) can have masses of up to 250 solar masses. Most stars, however, have less mass than the sun.
According to theoretical calculations, the smallest mass that a true-star can have is about 1/12 of the Sun. By a “true” star, astronomers mean a star that becomes hot enough to fuse protons into helium. Objects with a mass between about 1/100 and 1/12 the mass of the Sun can generate energy for a short time through nuclear reactions with deuterium, but they do not get hot enough to fuse protons. These objects have an intermediate mass between stars and planets. and they got the name Brown Dwarfs. Brown dwarfs are similar to Jupiter in radius, but have a mass about 13 to 80 times greater than that of Jupiter.
Still, smaller objects with masses less than about 1/100 the mass of the Sun (or 10 masses of Jupiter) are called planets. They can radiate energy generated by the radioactive elements they contain, and they can also radiate heat generated by slowly compressing them under their own weight. (a process known as gravitational contraction), however, interior will never reach temperatures high enough for nuclear reactions to take place. For example, Jupiter, whose mass is about 1/1000 the mass of the Sun, is undoubtedly a planet. In the 1990s we could only detect planets in our own solar system, but now we have thousands of them elsewhere as well.