Yesterday at the American Astronomical Society's conference in Los Angeles, evidence for the smallest black hole ever observed was presented. It comes in at a mere 3.8 times the Sun's mass and is only 24 kilometers across. New Scientist has a story about it.
There has been a lot of discussion in recent days about microscopic black holes that could theoretically be created at the Large Hadron Collider. The black holes observed in the universe are entirely different creatures. A stellar black hole, made from a collapsing star, accretes nearby surrounding matter, growing larger over time. In contrast, the microscopic black holes at a collider would "evaporate" in a fraction of a second, unable to suck in any matter. So leaving aside the microscopic variety, are there limits to how small a black hole can be?
For a stellar black hole to form, two things need to happen: There must be a high enough mass and it must be in a small enough volume. So to look for the mass limits, we have to think about the densest stars we know: white dwarfs and neutron stars. These compact stars remain stable up to a certain point at which the mass gets enough to cause collapse to a black hole.
That mass limit for neutron stars is called the Tolman-Oppenheimer-Volkoff limit after the physicists who came up with it. Modern estimates of this limit are 1.5-3.0 times the mass of the Sun.
What this means is that the black hole just discovered is really pushing down to the lowest limits of stellar black hole mass and we are unlikely to find many smaller.
