Explainer: What are black holes?

A black hole isn’t truly a hole. It’s quite the opposite. A black hole is a place in space containing an enormous amount of mass packed very tightly together. And it has the capacity to draw in more mass all the time. These objects have so much mass — and therefore gravity — that nothing can escape them, not even light. That makes them some of the most extreme objects in the universe.

And they aren’t just massive, but also dense. Density is a measure of how tightly mass is packed into a space. Imagine a black hole the size of New York City. It would have as much mass and gravity as our sun.

Most black holes form after a giant star, one at least 10 times as massive as our sun, runs out of fuel and collapses. The star shrinks and shrinks and shrinks. Eventually, it forms a tiny dark point. This is known as a stellar-mass black hole. Now much smaller than the star that made it, this black hole still has the same mass and gravity.

Our galaxy the Milky Way may have some 100 million such black holes. Astronomers estimate a new one forms every second. (Note that small- and medium-sized stars, such as the sun, cannot form black holes. When they run out of fuel, they just become small, planet-sized objects called white dwarfs.)

Stellar-mass black holes may be common, but they also are relative shrimps. At the other end of the spectrum are giants. Called supermassive black holes, they may have as much mass as a million — or even a billion — stars. These rank among the most powerful objects in the known universe. And their gravity holds together the millions or billions of stars that form a galaxy. In fact, a supermassive black hole called Sagittarius A* holds together our galaxy. It was discovered more than 40 years ago.

As nothing can escape a black hole — not visible light, X-rays, infrared light, microwaves or any other form of radiation — black holes are invisible. So astronomers have had to “observe” them by studying how they affect their surroundings.

For example, black holes often form powerful, bright jets of gas and radiation that are visible to telescopes. Physicists can use the size of that jet to estimate the size of the black hole responsible for the jet.

Jonelle Walsh is an astronomer at Texas A&M University in College Station. Astronomers continue to find and observe more black holes all the time. Several years back, Walsh told Science News for Students: Those observations can help untangle the complicated relationships black holes have with stars, galaxies and clusters of galaxies. One day, she explains, that research “will push us toward understanding how everything [in the universe] works together and forms and grows.”