Gravitational traps in space, black holes, come in different sizes. Or more correctly, different masses, because they are all infinitely small. The first black hole ever discovered, in 1971, weighed in at 21 times our Sun's mass. It was formed by the explosion and collapse of a star. Examples of a completely different class of black hole were identified in the 1960s-1970s. They weighed in at millions to billions of times our Sun's mass. Like all supermassive black holes, those monsters dwell in the center of major galaxies.

So, black holes can be super-big or super-small. The missing link is an intermediate-mass black hole, weighing roughly 100 to 1,000 times our Sun's mass. A handful have been found in other galaxies. Perhaps they are on the road to growing into supermassive black holes.

The cores of globular star clusters are hunting grounds for intermediate-mass black holes. They are smaller than galaxies and should have correspondingly smaller black holes. Over 150 of these snow-globe-shaped collections of hundreds of thousands of stars orbit our Milky Way galaxy, like artificial satellites whirling around Earth. Searches for intermediate-mass back holes in these clusters have been elusive. The suspected central black hole can't be directly observed, of course. Astronomers gather circumstantial evidence by watching stars swarming around the black hole, like bees around a hive. Based on their speeds, the invisible central mass can be calculated using straightforward Newtonian laws of physics.

Tracking the stars is meticulous work that's cut out for the Hubble Space Telescope's sharp resolution and longevity. Astronomers looking through over a decade of Hubble observations of the nearby globular star cluster Messier 4 calculated there is a very dense central object of about 800 solar masses. It is so compact, the observations tend to rule out alternative theories as to what's happening in the heart of the cluster.