By Stacy Palen

Astronomers have long been on the hunt for “intermediate-mass” black holes. These are black holes with masses between a few hundred and a few ten-thousands of solar masses. It was thought that these should exist in globular clusters. While looking for these, astronomers have instead found a swarm of smaller black holes, forming a mini-cluster in the center of a globular cluster!

Below are some questions to ask your students based on this article.

1). What is the approximate range of masses for an intermediate-mass black hole?

Answer: Tens to hundreds of thousands of solar masses.

2). How old is this globular cluster? 

Answer: This globular cluster is almost as old as the universe itself, so nearly 13.7 billion years old.

3). How do astronomers find the age of a globular cluster?

Answer: They make an H-R diagram and find the main-sequence turnoff.

4). How does the team of astronomers from the IAP know that there is not one single black hole at the center, but rather a swarm of black holes?

Answer: The shape of the orbits of nearby stars shows that the mass at the center is extended in size, rather than point-like.

5). How do they know that those masses in the core are black holes and not stars?

Answer: They used the theory of stellar evolution, combined with the fact that the mass is invisible.

6). Why are all the black holes in the cluster found near the core?

Answer: Because of dynamical friction, where they lose momentum to other less-massive stars.

7). How might astronomers further test this idea about the core of this globular cluster?

Answer: Mergers of these black holes might be detected by LIGO/VIRGO.