In an epic battle between a powerful supermassive black hole and an unassuming gas cloud, scientists naturally bet on the black hole.
And they bet wrong, according to a paper published Thursday in the journal Astrophysical Journal Letters.
In 2006, a team of astronomers first discovered a gas cloud travelling through space on a clear-cut path toward the supermassive black hole at the heart of our galaxy, the Milky Way.
The gas cloud, called G2, reached its closest approach to the black hole last May, and the team was certain that the black hole would stretch the cloud into a thin, stringy strand as it slurped the gas into oblivion. And they had good reason to suspect such a brutal outcome.
Black holes are extremely dense, massive objects with a gravitational pull so powerful that nothing escapes if it passes the point of no return, called the event horizon. And supermassive black holes are the most powerful kind. The one at the heart of our galaxy is estimated to be 4.3 million times more massive than the sun.
Therefore, when an object passes close enough to a supermassive black hole, even if it doesn’t cross over the event horizon, the black hole’s gravitational pull will play a dangerous tug-of-war game on the object that usually spells disaster.
Case in point: In 2012, a collection of space and ground-based telescopes discovered the remains of a star that a supermassive black hole in a distant galaxy had ripped apart. Below is a short simulation of how the star’s gas spiraled down into the black hole as it ate:
The gas cloud, G2, is less compact than a star and smaller, too. In 2011, the international team of scientists monitoring G2 determined that the cloud is roughly three times more massive than Earth. (For comparison, the sun is about 300,000 times more massive than Earth.)
Because of its small size and relatively low density, G2 should have been an easy meal for our galaxy’s supermassive black hole. And since astronomers rarely get the chance to see a feeding black hole in action — both because many active black holes are too far away to watch and supermassive black holes don’t feed often — the team made sure they didn’t miss the show.
Over the last eight years, they used the Very Large Telescope in Chile to snap a series of sharp pictures of the cloud, located about 25,000 light years away.
Below is a compilation of the photos that were taken in 2006, 2010, 2012, and then twice in 2014: before and after its closest approach. The black hole, which is invisible, is marked by a plus sign. Have a close look at the final shot, taken in September of last year:
That last picture is undeniable proof that G2 mysteriously slipped past the black hole intact. If it had not survived the event, G2 should have looked more elongated, and the team would have detected a flare of energy coming from the black hole as it gobbled the gas. No such flare was detected.
Not only did G2 survive, it got a nice boost in speed and is now moving away from the black hole at 7.5 million miles per hour, 1.5 million miles per hour faster than when it was approaching. The change in the gas cloud’s direction was a clear sign that it had swung around the black hole, which had brought dangerously close, to within 25 billion miles of the event horizon.
“It was amazing to see that the glow from the dusty cloud stayed compact before and after the close approach to the black hole,” said Monica Valencia-S., a post-doctoral researcher at the University of Cologne and co-author of the paper, in a European Southern Observatory press release.
How did the cloud escape? The answer is not clear, the team reports. They propose that G2 might be hiding a star at its core, which would make it more dense than originally thought, and therefore harder for the black hole to slurp. Below is an animation of the cloud’s path between 2006 and September of last year:
Another reason could be that they misjudged the trajectory on which G2 was moving, which would mean it didn’t get as close to the black hole as they expected.
Although the orbital mechanics that makes Earth orbit the sun and G2 orbit our galaxy’s supermassive black hole is well understood, the mass of the black hole and G2 is not as concrete, making these kinds of calculations incredibly difficult.
For example, their original estimate was that G2 would reach its closest approach to the black hole in 2013, which they later revised to May of 2014.
Now that it’s out of the woods, G2 is going to be harder for astronomers to track because a high-speed star is moving into the field of view of the cloud’s path.
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