Using a continent-spanning telescope, an international team of astronomers has peered to the edge of a black hole at the center of a distant galaxy. For the first time, they have measured the black hole’s “point of no return” — the closest distance that matter can approach before being irretrievably pulled into the black hole.

A black hole is a region in space where the pull of gravity is so strong that nothing, not even light, can escape. Its boundary is known as the event horizon.

“Once objects fall through the event horizon, they’re lost forever,” says lead author Shep Doeleman, assistant director at the MIT Haystack Observatory and research associate at the Harvard-Smithsonian Center for Astrophysics (CfA). “It’s an exit door from our universe. You walk through that door, you’re not coming back.”

The team examined the black hole at the center of a giant elliptical galaxy called Messier 87 (M87), which is located about 50 million light-years from Earth. The black hole is 6 billion times more massive than the sun. It’s surrounded by an accretion disk of gas swirling toward the black hole’s maw. Although the black hole is invisible, the accretion disk is hot enough to glow.

“Even though this black hole is far away, it’s so big that its apparent size on the sky is about the same as the black hole at the center of the Milky Way,” says co-author Jonathan Weintroub of the CfA. “That makes it an ideal target for study.”

According to Einstein’s theory of general relativity, a black hole’s mass and spin determine how close material can orbit before becoming unstable and falling in toward the event horizon. The team was able to measure this innermost stable orbit and found that it’s only 5.5 times the size of the black hole’s event horizon. This size suggests that the accretion disk is spinning in the same direction as the black hole.

The observations were made by linking together radio telescopes in Hawaii, Arizona, and California to create a virtual telescope called the Event Horizon Telescope, or EHT. The EHT is capable of seeing details 2,000 times finer than the Hubble Space Telescope.

The team plans to expand its telescope array, adding radio dishes in Chile, Europe, Mexico, Greenland, and the South Pole, in order to obtain even more detailed pictures of black holes in the future.

The work is being published in Science Express.