Ask Dr. Universe – Black Holes

Dr. Universe: How many black holes are in the galaxy and the universe?
-Krisha, 9, New Jersey

Dear Krisha,

While we can’t see black holes with our eyes, astronomers have figured out how to spot these objects in our universe.

One astronomer who is really curious about understanding black holes is my friend Sukanta Bose, a researcher at Washington State University.

First, he told me there are different kinds of black holes. Supermassive black holes can be millions to billions of times the mass of the Sun. We have a supermassive black hole in our own Milky Way galaxy called Sagittarius A*, which is pronounced as Sagittarius A-star.

Scientists think supermassive black holes may be found in the center of most large galaxies.

If you are anything like me, you might be wondering: why not just count all the different galaxies to find the number of black holes?

“Of course, we cannot see every galaxy,” Bose said. “We see many galaxies that are closer because they are brighter.”

For galaxies that are farther away, you have to use very powerful telescopes, he adds.

That also means we have to make an inference about the number of galaxies in the universe. An inference is an educated guess based on evidence and current knowledge about how things work.

Using telescopes, math and their inference skills, astronomers estimate there are hundreds of billions of galaxies and likely hundreds of billions of supermassive black holes— that’s just in the observable universe.

Bose told me there’s another kind of black hole that sometimes forms when a star dies and collapses in on itself. We call these stellar mass black holes.

The Sun is a star, but it is far too small to become a black hole. Only heavier stars make black holes. When it comes to stellar mass black holes, astronomers estimate there are ten million to a billion right here in the Milky Way galaxy.

On the hunt for these massive objects, scientists often look for different interactions among stars or gases, clues that there may be a black hole in the neighborhood.

For instance, when a black hole and a companion star are in a tight orbit, their interaction can sometimes create high energy light we can’t see, but that scientists can detect with their high-tech tools.

“When you open a new way of probing the universe, you see objects that challenge your previous wisdom or theories,” Bose said.

Bose and fellow researchers have been able to spot black holes because of a new way to detect something called gravitational waves. When two black holes collide, they can create a kind of wave that brings information to Earth about its source and helps us learn more about the universe.

It’s a bit like listening for sound waves from particular instruments in an orchestra, Bose said. But instead of picking out the sound of a cello or a flute, they are listening for gravitational waves from those colliding black holes.

Who knows, maybe one day you can help us learn more about black holes and discover ways to help astronomers count them all.

Dr. Universe

Thanks to all our kid readers who voted for this question in our recent poll. Stay tuned for future polls at

Know a kid with a science question?
With help from my friends at Washington State University, we’re investigating tough and smart questions from curious kids around the world.
Submit a question