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Event Horizon Telescope Collaboration, European Southern Observatory
The first photo of a black hole, the supermassive one at the center of the M87 galaxy, as imaged by the Event Horizon Telescope, eight radio telescopes scattered around the world that combined their data.

Editor's note: A version of this was previously published on the author's blog.

Judging by Hollywood's always ludicrous treatment of black holes, scientific knowledge about them is sadly scarce. Probably most know that a black hole is a system in space where so much mass is packed into such a small region that its tremendous gravity can shred and pull in a star that wanders too close; also, that nothing can escape once inside.

That may be most of the information many have about black holes. But with the April 10 release of the first photograph of one, now is a good time to expand on the basics.

On Astronomy Day, May 11, Paul Ricketts of the University of Utah's physics and astronomy program did just that, speaking in the South Physics Building on campus before a crowd of several dozen attentive listeners. Some of the most perceptive questions came from young children.

Joe Bauman
Silhouetted against a projected photo of a globular star cluster, Paul Ricketts explains black holes. He spoke during Astronomy Day, May 11, in the University of Utah's South Physics Building. Ricketts is director of the South Physics Observatory. Besides the university itself and the observatory, the free public demonstrations and lectures were sponsored by the U.'s Physics and Astronomy program, College of Science and departments of Chemistry and Mathematics.

Ricketts outlined the basic types of black holes, including the more common stellar-mass objects created when large stars end their lives, objects that can be from five to "several-tens" times as massive as the Sun, and supermassive ones at the center of many galaxies, which according to NASA can be "millions, if not billions, of times as massive as the Sun."

Scientists can detect black holes and estimate their size by observing the orbits of nearby stars or gas clouds. If some invisible point around which stars orbit contains enormous mass but nothing can be seen at that point, it's probably a black hole.

Other than star motion, black holes sometimes can be pinpointed by gigantic jets of energy that shoot from the centers of galaxies. This is the case of the object in M87, whose jet is so enormous — at least 1,000 light-years long — that I was able to photograph it with my 12-inch-diameter telescope.

The center of the black hole, where the mass is concentrated, is termed a singularity. Surrounding it is the event horizon, which marks the region from which nothing can escape.

The photon sphere, a bright disk just outside the event horizon, is light and energy emitted by nearby material. Light photons do escape from this sphere. Their energy was picked up by the Event Horizon Telescope, having traveled to Earth from the black hole region. If light from the energetic material beyond it approaches the photon sphere at certain angles, it will be swept up in the whirl and may escape. "If it comes in at a slighter steeper angle, it'll get pulled in," Ricketts said.

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The innermost stable orbit close to the event horizon is the nearest where anything with mass can hold its orbit without going into the black hole. But a great deal of material falls through this region and inward, never to emerge. The innermost stable orbit is at the inside edge of what's called the accretion disk.

Relativistic jets are enormous flares of charged material shooting outward from the accretion disk in opposite directions, fueled by twisted magnetic fields and friction from the super-hot orbiting material. The jets are called relativistic because they approach the utmost speed limit as defined by the theory of relativity, that of light. They can stretch for thousands of light-years, sometimes extending beyond their host galaxies.