Stars span an enormous range of sizes as the figure above shows. Credit:Wikipedia-Dave Jarvis:
The minimum size for a star is believed to be near 0.04 times the mass of the Sun or about 80 times the mass of Jupiter. An object called a brown dwarf is really a large planet which was not massive enough for thermonuclear fusion to get ignited in the core. The difference between a brown dwarf and a planet is believed to be about 13 times the mass of Jupiter. The closest red dwarf is Proxima Centauri with a masss of about 130 times Jupiter. The closest brown dwarf to our sun as of 2014 is about 7.5 light years away and is called WISE J085510.83-071442.5, and is now the record-holder for the coldest brown dwarf, with a temperature between minus 54 and 9 degrees Fahrenheit (minus 48 to minus 13 degrees Celsius) and a mass of about 3-10 times Jupiter. The exciting thing about red dwarf stars is that they burn their nuclear fuel so slowly that they exist as stars for 10 times longer than our own sun!
The largest star is probably about 150-200 times the mass of the Sun. There are only a handful of these hyperstars in our own Milky Way which has over 200 billion stars in it. The Eta Carina nebula appears to have several dozen, mostly unstable stars with masses between 50 and 200 times the sun’s mass. These stars are so masssive that they run through their nuclear fuel in a few million years and explode as hypernovae, many times brighter than ordinary supernovae.
Although hyperstars are rare in a galaxy as large as the Milky Way, brown dwarfs and red dwarfs are not. In fact current searches for exoplanets favor the more numerous red and brown dwarf stars.
Black holes can have any mass from 0.00001 grams to 10 billion times the mass of the Sun…or more. The supermassive black holes are found in the cores of ‘active galaxies’ and quasars. Astronomers have never seen a black hole that is much smaller than the mass of our Sun yet, so we don’t know if they really exist.