In the fall of 2002, scientists calculated the mass of a truly gargantuan entity at the heart of our Milky Way galaxy. It was four million times the mass of our Sun — so big it almost certainly had to be a supermassive black hole.
The object "probably looks like Interstellar," says Stefan Gillessen, an astronomer at the Max Planck Institute for Extraterrestrial Physics in Germany, referring to the 2014 sci-fi film starring Matthew McConaughey and Anne Hathaway. "You have this dark shadow, this silhouette, where the absolute darkness is" surrounded by a shimmering halo of light.
There's a reason that some of our favorite images of black holes come from fiction: In reality, they're very hard to see. Black holes are, by their very nature, yawning enigmas — their gravitational pull is so powerful that not even light can escape — so to learn anything about them is like rescuing information from an abyss.
But astronomers may have found a luminary assistant to help them work around that challenge. In new research published in the journal Nature on Wednesday, Gillessen and his colleagues describe the fastest star ever observed, racing around Sagittarius A*, the black hole at the heart of the Milky Way galaxy.
Gillessen says that what's important about the discovery isn't the star's speed — but rather its proximity to the black hole. "Thereby we can use the star [as a probe] to visualize how gravity is acting on space and time," he says.
The black hole on our doorstep
Despite their gravitational dominance in the universe, researchers still have rather basic questions about black holes.
"We want to understand how black holes work and how black holes influence their direct environment," says Gillessen, including the fundamental role that they play in helping shape their home galaxies.
"And if you want to know about black holes," he adds, "the center of the Milky Way is the best laboratory we have. It's right in front of our doorstep."
Gillessen and his team used four of the European Southern Observatory's telescopes in Chile at the Very Large Telescope site to gather infrared light, which allowed them to see through cosmic dust and gas that might otherwise obscure their view.
"Just by using infrared light, you can peer into the most central part of the nucleus of the Milky Way," says Felix Mang, a Ph.D. student at the Max Planck Institute for Extraterrestrial Physics.
He first noticed something curious in the data a few years ago. "A small flicker of light, a very faint star came to our attention, which moved with a very high velocity," he remembers.
A stellar speed demon
Mang tracked the position of the star called S301 over the next couple of years and also looked back to see where it had been.
Using Newton's law of gravity with a slight correction from general relativity (thereby helping confirm an aspect of Einstein's theory), he determined the star's 8.7-year orbit took the form of a super elongated ellipse. The black hole sits at one end of that oval.
"The star comes in in one direction, getting faster and faster and faster, then gets a very sharp turn around the black hole," says Gillessen. That's when it's at its highest speed — over 15,000 miles per second, which is just shy of 8% of the speed of light.
This makes S301 the fastest star ever observed in the galaxy.
After whipping around the supermassive black hole, "it flies back out again and decelerates until it's at its furthest point," says Gillessen. "Once you know the orbit, it's pretty much like the Swiss railway system. Things are there precisely on time."
This discovery may teach researchers "about the gravitational potential of the black hole," he says. "The star, it feels how space and time are distorted by the black hole. And thereby we can use the star to visualize how gravity is acting on space and time."
Shedding light
"This is a really exciting result," says Erin Kara, an astrophysicist at MIT who wasn't involved in the study. "It kind of sets the stage in the next decade to make these kind of unprecedented measurements of black holes."
That includes determining how fast the black hole at the center of our galaxy is spinning, a behavior that can drag the surrounding space-time along with it. Previously, researchers have calculated the rotation of black holes that are actively growing in other galaxies by gobbling up gas. Using S301 to calculate the spin of Sagittarius A* (our local black hole) would represent the first such measurement for a quiescent galaxy, she says.
This kind of calculation can help scientists understand how the black hole emerged in the first place, which could yield insights into the formation of our galaxy.
"What I love about black holes," says Kara, "is that they're both exotic and where our understanding of physics breaks down. And at the same time, they're really fundamental for explaining why our galaxy looks the way that it does."
The notion of a star whizzing around a black hole tickles Gillessen's imagination. "You always work at the limit of what is possible," he says. He considers what it would be like to be an alien living on a hypothetical planet orbiting S301.
"Every nine years, you would get to see this black hole appearing big in the sky," he says. "I mean, it must be absolutely stunning to have that sort of experience."
He conjures the sight, gazing up at a pitch black disc enrobed in light — a portal into the very fabric of the universe.
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