Why Are Black Holes So Bright

And why is the black hole at the center of our own galaxy so dim?

Liz Kruesi | NAUTILUS

Black holes, by definition, are so dense that not even light can escape. But ask any astrophysicist, and they’ll report that black holes are among some of the brightest objects in the universe. What’s going on here?

The answer, in part, is that black holes don’t live alone. The monster black holes at the centers of galaxies are typically surrounded by searing clouds of hot gas. As this material funnels toward the black hole, it can create cosmic auras around the darkest place in the galaxy.

Strangely, the black hole at the center of the Milky Way galaxy isn’t nearly as bright as it should be. Figuring out the mystery of why this black hole is so dim, relatively speaking, will help clarify the connection between the light that we see and what falls in.

Why Black Holes Are So Bright

There are a number of ways that black holes can appear to glow. When gas from nearby stars falls toward the black hole, the gas spirals like water going down a drain. As it does so, it rubs together and gets hot. It’s essentially the same process that Boy Scouts use to light a fire with two sticks, except here the temperature of this gas can reach millions of degrees.

Gas that hot pulls its own atoms apart, creating a sea of positive ions and negative electrons. These churning charged particles generate turbulent magnetic fields, which funnel the gas into two jets pointing in opposite directions. If one of these jets happens to be angled so that it’s pointing toward Earth, we see a brilliantly bright black hole.

But sometimes we don’t have to be located directly in the path of the fire hose. Those jets can also stream out and slam into nearby clouds of gas or even a neighboring galaxy. The collision generates a distinct glow.

Our Dim Monster

At the center of our galaxy lies a surprisingly calm monster. “As a black hole, as an energetic system, it’s almost dead,” said Geoffrey Bower of the Academia Sinica Institute of Astronomy and Astrophysics in Hilo, Hawaii.

The question is why. “We know that there’s some sort of mechanism for preventing material from reaching the center, or reaching the black hole itself and falling in,” said Lía Corrales, an astrophysicist at the University of Michigan, “but we haven’t figured out exactly what that mechanism is.”

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