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 numeroud 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. These 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. As for the supermassive ones, here's what a recent Hubble Space Telescope press release has to say about their masses:
Astronomers are concluding that monstrous black holes weren't simply born big but instead grew on a measured diet of gas and stars controlled by their host galaxies in the early formative years of the universe. These results, gleaned from a NASA Hubble Space Telescope census of more than 30 galaxies with its powerful "black hole hunting" spectrograph, are painting a broad picture of a galaxy's evolution and its long and intimate relationship with its giant central black hole.
Though much more analysis remains, an initial look at Hubble evidence favors the idea that titanic black holes did not precede a galaxy's birth but instead co-evolved with the galaxy by trapping a surprisingly exact percentage (0.2%) of the mass of the bulbous hub of stars and gas in a galaxy. This means that black holes in small galaxies went relatively undernourished, weighing in at a mere few million solar masses. Black holes in the centers of giant galaxies, some tipping the scale at over one billion solar masses, were so engorged with infalling gas that they once blazed as quasars, the brightest objects in the cosmos. The bottom line is that the final mass of a black hole is not primordial; it is determined during the galaxy formation process.
In most cases the black holes not only bulked up through the accretion of gas in isolated galaxies, but also through the mergers of galaxies where pairs of black holes combined. The black hole feeding that makes the black hole's mass grow is also what makes the quasar shine. A quasar is the brilliant signature of the fueling and building of the central black hole. The results also explain why galaxies with small bulges, like our Milky Way, have diminutive central black holes of a few million solar masses, while giant elliptical galaxies house billion-solar-mass black holes, some still smoldering from their days as quasars.
An alternative but less favored idea is that black holes came first, all packaged in a standard size, namely 0.2 percent of the mass of the first galaxy fragments that formed. Then mergers of small galaxies made bigger galaxies, and the standard black hole mass fraction was preserved because, when two galaxies merge, their black holes merge too. This idea is not favored by the new observations. The results do not shed light on how seed black holes originate. They are just required to be in place early in the galaxy formation process so that they can grow and shine as quasars. Nor do astronomers know why the galaxy formation process makes a black hole with such a precisely correlated mass.
Evidently, the process that decides how much mass gets fed to black holes produces almost the same result, largely independent of the details of galaxy formation. Hubble is astronomy's preeminent "black hole hunter" because of its unique ability to use its Space Telescope Imaging Spectrograph (STIS) camera to precisely measure the speed of gas and stars around a black hole. Hubble is the best way to find lots of black holes without selection biases. The findings reported at the AAS meeting are based on two types of Hubble observations. Several teams measured the black holes' masses by recording the whirling speeds of disks of gas trapped around the black holes, like water swirling around a drain. Other teams measured the motions of stars around the galaxies' hubs like a swarm of bees hovering around a beehive. The more massive the bulge, the greater the speed of the stars.
Return to Dr. Odenwald's FAQ page at the Astronomy Cafe Blog.