The lower limit seems to be set by the mass at which the core of a star can support thermonuclear reactions...primarily helium or deuterium burning. This is about 0.04 times the mass of our Sun. The upper limit depends on the chemical abundances in the atmosphere of the star when it forms. If it has few heavy elements, there will not be much 'opacity' within the forming protostar gas, and a large mass above 100 times the Sun can form. For typical massive stars we know about, however, the limit seems to be near 50 solar masses, but many examples of extreme 'hyperstars' are known that are 100-200 times the mass of the Sun. Note, the range in mass from 0.04 to 50 solar masses, translates into a range of stellar luminosities from 0.00001 to over 1 million times the luminosity of the Sun!
The image on the left is that of the 'Pistol Star' near the center of the Milky Way as seen by the Hubble Space Telescope. It is one of the most massive stars known in our galaxy with a mass of 200 times that of the Sun. In the region known as Eta Carina, a cluster of stars with masses from 50-100 times that of the Sun can be found. Stars with this mass seem to be very common in the Large Magellanic Cloud.
The brown dwarf, called Gliese 229B (GL229B), represents the other extreem of masses, nearly 0.05 times the mass of the Sun. It is a small companion to the cool red star Gliese 229, located 19 light-years from Earth in the constellation Lepus. The above Hubble Space Telescope image on the right shows the bright emission from the main star, and the small spot to its right that is the Brown Dwarf. Estimated to be 20 to 50 times the mass of Jupiter, GL229B is too massive and hot to be classified as a planet as we know it, but too small and cool to shine like a star. At least 100,000 times dimmer than Earth's Sun, the brown dwarf is the faintest object ever seen orbiting another star.
Copyright 1997 Dr. Sten Odenwald
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