Gerhard Hüdepohl

The black hole at the center of our galaxy has been on a near-starvation diet for almost a million years—but now it's time for a snack.

Scientists in Garching, Germany, are closely watching a rare event some 26,000 light years away: a supermassive black hole in the act of devouring a huge gas cloud. It's providing the first-ever glimpse of how a black hole uses its massive gravitational power to pull in and consume interstellar materials—a little understood phenomenon.

"The cloud is being torn apart," said Stefan Gillessen of the Max Planck Institute for Extraterrestrial Physics in Garching, who first brought the event to the world's attention in 2011.


A black hole can form when a large star dies and its matter collapses into a much smaller volume. It's as if the mass of the Earth were squeezed into a ball the size of a marble. The resulting gravity is so intense that even light cannot escape.

What is known about black holes is largely theoretical, and scientists are eager to gather more real-world data. The best telescopes aren't up to the job because most black holes are millions of light years away. Indeed, it was only a decade ago that scientists established that a supermassive black hole lurked surprisingly close by, in the Milky Way.

Now, by a stroke of fortune, our neighboring black hole is offering up some unexpected celestial fireworks. The Milky Way monster has drawn in bits of stray material for nearly a million years. Stars closely orbiting the black hole indicate that it has the gravitational pulling power of four million suns. That gravity is now starting to act on the gas cloud, which itself is about 37 billion miles long.

Using data from the European-funded Very Large Telescope, or VLT, perched high up in Chile's Atacama Desert, Dr. Gillessen's team recently concluded that the front of the gas cloud is traveling 310 miles per second faster than its tail. About 10% of the cloud has already been dragged to the far side of the black hole.

Scientists hope to observe the cosmic drama for at least a year. They have many questions: Will the cloud get shredded to bits and sucked in, or will much of it stay in orbit around the black hole? Will the black hole get brighter as it consumes its prey? Will Einstein's theory of relativity, which has never been experimentally tested under such extreme conditions, hold up?

"In astronomy, you rarely have the chance to catch something in the act," said Andrea Ghez, an astrophysicist who leads a rival group observing the event at the University of California, Los Angeles. "We just don't know what will happen."

In 2006, Dr. Ghez's team noticed an object drifting in the vicinity of the Milky Way's black hole. But it didn't look especially interesting, "so we didn't jump on it," she said.

Five years later, Dr. Gillessen was using a telescope in Chile to observe stars circling the black hole. Studying old data, he stumbled upon the same object seen by Dr. Ghez. "It was a little blob—not a hot star, but something cold," recalled Andreas Burkert, a theoretical physicist at Max Planck and a colleague of Dr. Gillessen. "I said [the observation] was probably a mistake."

In January 2012, the Max Planck team published its findings in the journal Nature. They concluded that the blob was a gas cloud, of unknown origin, in the gravitational clutches of the Milky Way's black hole.

Until a few years ago, scientists could barely observe objects in the center of our galaxy partly because of the blurring caused by the Earth's atmosphere, the same effect that causes stars to twinkle. And because dust at the center of the galaxy absorbs most of the optical light, optical telescopes are largely useless.

Dr. Gillessen and others instead rely on telescopes that view the galactic core in the infrared range of light. A more recent innovation is a technique known as adaptive optics, which can correct for the blurring effect. This year, researchers have made more than 40 proposals to observe the black hole's encounter with the gas cloud.

On a recent morning in his office, Dr. Gillessen used a computer to show a succession of images taken by the VLT. In 2002, the gas cloud, represented by a tiny blue blob, appears to be moving closer to the Milky Way's black hole. "It's not going in a straight line, which means some object is pulling on it," said Dr. Gillessen. Data from 2008 show the gas cloud traveling at about 800 miles per second—a lot faster than Earth, which moves at about 19 miles per second. Tellingly, the most recent findings indicate that the cloud is hurtling at almost 1,250 miles per second. It means that the black hole's tug is getting stronger and stronger.

A separate slide, titled "Total Disruption in Front of Our Eyes," showed part of the gas cloud stretched out to at least 10 times its 2004 length.

Dr. Gillissen then headed to a lab where he and other Max Planck colleagues are assembling a roughly 2-yard-long device known as the Gravity Machine, at a cost of almost $27 million. When finished, by the summer of 2014, the machine will combine the light signals from VLT's four separate telescopes to provide far more accurate measurements.

Periodically, the black hole flares up, just as Earth's atmosphere briefly brightens from flashes of lighting. Dr. Gillessen says that the encounter with the gas cloud may cause flares from the black hole to become more frequent or violent. In that case, the gravity machine should pick up those signals more easily, giving scientists more information about how the black hole operates.

Like many astronomical observations, this one has a touch of the surreal. The Milky Way's black hole is 26,000 light years away. That means that the gas cloud event actually occurred 26,000 years ago, and it has simply taken that long for the light to reach us. "We're out in the suburbs of the galaxy, so the movie only got to us now," said Dr. Ghez. "But it's an amazing movie."