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Bigger Than a Million Suns
A primary characteristic of supermassive black holes is, of course, their supermassive size. Unlike a stellar black hole that may have the mass of 100 Suns, a supermassive black hole has the mass of a million or more Suns. The Schwarzschild radius, or event horizon, is similarly proportional. A typical black hole might have an event horizon of 30 kilometers across. But a black hole of a million solar masses would have an event horizon of about three million kilometers, or four times bigger than our sun, according to University of Richmond physics professor, Ted Bunn. This, he says, is, "actually not so big by astronomical standards."
Galaxy Centers
While stellar black holes may be scattered throughout the galaxies of the universe, scientists now believe supermassive black holes are at the centers of most galaxies, according to Dr. Jeffrey E. McClintock, senior astrophysicist at the Smithsonian Astrophysical Observatory. The relationship between a galaxy and its black hole is unclear. Whether the galaxy gives rise to the black hole or the black hole gives rise to the galaxy is a debated issue, but a 2009 study by astronomers from France, Germany, and Belgium suggested that "supermassive black holes can trigger the formation of stars, thus 'building' their own host galaxies."
Some Are Quasars
Quasars, the brightest objects in the universe, consist of supermassive black holes and the superheated, brightly glowing material falling into them (called an accretion disk). A single quasar can shine with the brightness of a hundred billion stars, though it may be smaller than our solar system. In a 2005 study, a precursor to the study noted above, 19 of 20 quasars four billion light years away were discovered to be supermassive black holes at the centers of galaxies. The lone quasar, though, was shooting radio jets and gas into a nearby galaxy that is producing stars at an enormous rate. This led French astrophysicist Dr. David Elbaz and his colleagues to the hypothesis that "a quasar can drive the formation of stars." They believe the quasar and the galaxy will eventually merge, and the "naked" quasar will eventually be "dressed" in its own host galaxy.
Higher Mass, Lower Density
While stellar black holes are extremely dense because their mass is concentrated into a smaller volume, supermassive black holes are spread out over a larger space, perhaps the size of a solar system. But because density and volume are inversely proportional, the larger a black hole, the lower its density and the less its gravitational, or tidal, force in the vicinity of the event horizon. American Museum of Natural History astrophysicists Steven Soter and Neil deGrasse Tyson say it's even "conceivable that our entire observable universe is a supermassive black hole within a larger universe," perhaps on the edge of an event horizon.