We've all heard about black holes, the giant, galactic vacuum cleaners that suck in anything nearby. That's the science fiction version, anyway, and it's reasonably close to reality.
So what happens when you get sucked in? We're about to find out.
A giant cloud of gas is rocketing toward death, spiraling around our galaxy's black hole like water down a drain, but at an unbelievably giant scale.
"As the cloud plunges into a black hole, you're getting a beautiful opportunity to sample the environment near a black hole," says Dale Frail, an astronomer with the National Radio Astronomy Observatory in New Mexico, which was made famous in the movie "Contact." "These things happen once in a lifetime."
The gas cloud, now named "G2," isn't getting sucked in all at once. The front end – the end closest to the black hole – is being pulled many times faster than the back end, with the result that G2 is getting stretched thinner and thinner, like caramel on a hot day.
"The gas at the head of the cloud is now stretched over more than 160 billion kilometres around the closest point of the orbit to the black hole," wrote Stefan Gillessen, whose team discovered G2, in a statement released Wednesday.
His team's measurements, to be published in the Astrophysical Journal, show that G2 has gone from an egg-shaped blob in 2004 to a long ribbon of gas today, as can be seen above. Dr. Gillessen, who is based at the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, describes it this way: "Like an unfortunate astronaut in a science fiction film, we see that the cloud is now being stretched so much that it resembles spaghetti."
Eventually, G2 will be stretched too far, and snap. The surviving pieces will be "eaten" by the black hole, and Gillessen's team – along with hundreds of other astronomers around the world – will be watching to see exactly how it happens.
"When you fall into a black hole, the effects of gravity are so strong that they tear the atoms apart as they fall in. If you or I fall into the Grand Canyon, we'd just go splat. But if you fall into a black hole, you actually experience all kinds of interesting physical effects, including tidal forces. Atoms are getting ripped apart, but as they're ripped apart, they emit radiation," says Dr. Frail, who is the assistant director of the Very Large Array (VLA) and the Very Large Baseline Array (VLBA). "Material falls into the gravitational potential well and basically screams, 'Help, help, I'm being torn apart.' "
G2 hasn't started to scream yet, but Frail's radio telescopes are listening hard for those radiation emissions to "turn on."
"We'll be able to map it out in detail, when it turns on as a radio source," he says. The VLBA can resolve smaller objects than optical telescopes can – and believe it or not, even a gas ribbon billions of kilometers long is small when it's as far away as the center of the galaxy.
Astronomers have been watching G2's slow spiral towards destruction for over a decade. When Gillessen and his colleagues published their findings last year, they described it as a "a dusty cloud" with three times as much mass as Earth, but four times bigger than our solar system – the whole loop of Neptune's orbit. Even then, this unthinkably enormous but mostly empty cloud of gas was en route to the center of our galaxy, heading for the encounter that has now begun. Over the years, the German astronomers measured G2 accelerating: its front end doubled in speed from 2004 and 2011, rushing headlong towards its date with destiny. (Or at least density.)
The latest measurements, made in April, show that the front end is moving over 7 million miles per hour (3,000 km/s) – and it's heading back towards us, say the German scientists.
"The most exciting thing we now see in the new observations is the head of the cloud coming back towards us at more than 10 million km/h along the orbit – about 1% of the speed of light," said Reinhard Genzel, leader of the research group that has studied this region for nearly twenty years, in a press release. "This means that the front end of the cloud has already made its closest approach to the black hole."
Gillessen and Genzel won't give a particular date to the moment when the gas cloud rounded the bend, like a comet slingshotting around the sun. "The cloud is so stretched that the close approach is not a single event, but rather a process that extends over a period of at least one year," said Gillessen.
G2's origins are still unknown, though there are plenty of theories, which these new observations have been testing. Some astronomers thought that the gas cloud could have been created from stellar winds blowing from stars near the black hole. Others argued that the gas came from a jet at the galactic center. Many theorized that there must be a star at the heart of G2, like the frozen slushball at the heart of a comet. This last theory predicted less stretching than appears in the new observations, so Gillessen and his colleagues argue that it now seems unlikely.
Hundreds of astronomers around the world are watching to see what happens next. Will the accelerating gas cloud create a bow shock, like the bow of a boat? Will it fall into the black hole with '"a nice firework show?" asks Frail, whose radio telescopes are making monthly observations of the galactic center to watch the whole, slow-motion show.
With G2 spiraling toward its glorious demise, astronomers have a once-in-a-lifetime chance "to make predictions and actually test them," says Frail. "You get to make the measurements in real time to see if you were right or you were wrong. It's just a great opportunity."