Accretion Disks

A black hole can be one of the darkest objects in the universe and one of the brightest. In fact, some black holes can outshine the entire galaxy of stars that surrounds them.

A black hole itself is truly black. Stuff can fall in, but nothing can come back out — including light. So from here on Earth, it’s almost impossible to spot a black hole that’s on its own.

Fortunately for science, many black holes have companions — “normal” stars or clouds of gas and dust. The black hole pulls in material from such a companion, heats it up, and makes it shine brilliantly. Much of what we know about black holes comes from studying this process.

As material flows toward the black hole, it doesn’t fall straight in. Instead, it spirals around the black hole, forming a wide, thin accretion disk. This flow is turbulent, so particles ram together. This creates friction, which generates heat. The hot material radiates energy into space.

The friction also causes the particles to spiral closer to the black hole, where they move even faster. That generates more friction and more energy, so the disk gets hotter and brighter.

As much as 40 percent of the material in a disk can be converted to energy — compared to less than one percent for the process that powers the stars. So if a big disk surrounds a giant black hole, it shines as a quasar — one of the brightest objects in the universe — brighter than an entire galaxy of stars.

Script by Damond Benningfield