The Black Hole Next Door

Superfast stars could trace back to a behemoth in a neighboring galaxy

- PHIL PLAIT

AN ASTONISHING fact known for only the past few decades is that every big galaxy in the universe has a supermassive black hole at its heart. Scientists suspected this was the case in the 1980s, and observations from the Hubble Space Telescope, which has peered deep into the cores of galaxies all across the sky, confirmed it. The “normal” kinds of black holes made when stars explode range from five to about 100 times the mass of our sun, more or less. But the central galactic monsters are millions of times more massive, and some have grown to the Brobdingnagian heft of billions of solar masses.

A lot of mysteries remain, of course, such as how these black holes formed early in the history of the universe, how they grew so humongous so fast and what role they played in their host galaxy’s formation. One odd question in particular nags at astronomers: What’s the galaxy-size cutoff where this trend stops? In other words, is there some lower limit to how massive a galaxy can be and still harbor one of these beasts?

The inklings of an answer are emerging from a surprising place: studies of rare stars moving through our galaxy at truly ludicrous speeds.