Not all black holes are black — and researchers found more than 75k of the brightest

When the most massive stars die, they collapse to form some of the densest objects known in the Universe: black holes. They are the “darkest” objects in the cosmos, as not even light can escape their incredibly strong gravity.

Because of this, it’s impossible to directly imageblack holes, making them mysterious and quite perplexing. But our new research has road-tested a way to spot some of the most voracious black holes of all, making it easier to find them buried deep in the hearts of distant galaxies.

Despite the name, not all black holes are black. While black holes come in many different sizes, the biggest ones are at the centers of galaxies and are still growing in size.

These “supermassive” black holes can have the mass of up to a billion Suns. The black hole at the center of our own Milky Way galaxy – called Sagittarius A*, whose discovery received the 2020 Nobel Prize in Physics – is fairly calm. But that isn’t the case for all supermassive black holes.

If material such as gas, dust, or stars gets too close to a black hole, it gets sucked in by the enormous gravitational force. As it falls towards the black hole, it heats up and becomes incredibly bright.

The light produced by these “bright black holes” can span the entire electromagnetic spectrum, from X-rays to radio waves. Another name for the bright black holes at the center of galaxies is “active galactic nuclei”, or AGN. They can shine trillions of times brighter than the Sun, and can sometimes even outshine all the stars in its galaxy.