In a dark, dusty patch of sky in the constellation Sagittarius, a small star, known as S2 or, sometimes, S0-2, cruises on the edge of eternity. Every 16 years, it passes within a cosmic whisker of a mysterious dark object that weighs some 4 million suns, and that occupies the exact center of the Milky Way galaxy.
For the past two decades, two rival teams of astronomers, looking to test some of Albert Einstein’s weirdest predictions about the universe, have aimed their telescopes at the star, which lies 26,000 light-years away. In the process, they hope to confirm the existence of what astronomers strongly suspect lies just beyond: a monstrous black hole, an eater of stars and shaper of galaxies,
For several months this year, the star streaked through its closest approach to the galactic center, producing insights into the behavior of gravity in extreme environments, and offering clues to the nature of the invisible beast in the Milky Way’s basement.
One of those teams, an international collaboration based in Germany and Chile, and led by Reinhard Genzel of the Max Planck Institute for Extraterrestrial Physics, say they have found the strongest evidence that the dark entity is a supermassive black hole, the bottomless grave of 4.14 million suns.
The evidence comes in the form of knots of gas that appear to orbit the galactic center. Genzel’s team found that the gas clouds circle every 45 minutes or so, completing a circuit of 150 million miles at roughly 30 percent the speed of light. They are so close to the alleged black hole that if they were any closer they would fall in, according to classical Einsteinian physics.
Astrophysicists can’t imagine anything but a black hole that could be so massive, yet fit within such a tiny orbit.
The results provide “strong support” that the dark thing in Sagittarius “is indeed a massive black hole,” Genzel’s group writes in a paper expected to be published soon under the name of Gravity Collaboration, in the European journal Astronomy & Astrophysics.
The work goes a long way toward demonstrating what astronomers have long believed, but are at pains to prove rigorously: that a supermassive black hole lurks in the heart not only of the Milky Way, but of many observable galaxies. The hub of the stellar carousel is a place where space and time end, and into which stars can disappear forever.
The data help to explain how such black holes can wreak havoc of a kind that is visible from across the universe. Astronomers have long observed spectacular quasars and violent jets of energy, thousands of light-years long, erupting from the centers of galaxies.
The study is a major triumph for the European Southern Observatory, a multinational consortium with headquarters in Munich and observatories in Chile, which had made the study of S2 and the galactic black hole a major priority. The organization’s facilities include the Very Large Telescope, an array of four giant telescopes in Chile’s Atacama Desert (a futuristic setting featured in the James Bond film “Quantum of Solace”), and the world’s largest telescope, the Extremely Large Telescope, under construction on a mountain nearby.
Einstein’s bad dream
The existence of smaller black holes was affirmed two years ago, when the Laser Interferometer Gravitational-Wave Observatory, or LIGO, detected ripples in space-time caused by the collision of a pair of black holes a billion light-years away. But those black holes were only 20 and 30 times the mass of the sun; how supermassive black holes behave is the subject of much curiosity among astronomers.
“We know Einstein’s theory of gravity is fraying around the edges,” said Andrea Ghez, a professor at the University of California, Los Angeles. “What better places to look for discrepancies in it than a supermassive black hole?” Ghez is the leader of a separate team that, like Genzel’s, is probing the galactic center.
Seeing in the dark
Reinhard Genzel grew up in Freiburg, Germany, a small city in the Black Forest. As a young man, he was one of the best javelin throwers in Germany, even training with the national team for the 1972 Munich Olympics. Now he is throwing deeper.
He became interested in the dark doings of the galactic center in the 1980s, as a postdoctoral fellow at the University of California, Berkeley, under physicist Charles Townes, a Nobel laureate and an inventor of lasers. “I think of myself as a younger son of his,” Genzel said.
In a series of pioneering observations in the early 1980s, using detectors that can see infrared radiation, or heat, through galactic dust, Townes, Genzel and their colleagues found that gas clouds were zipping around the center of the Milky Way so fast that the gravitational pull of about 4 million suns would be needed to keep it in orbit. But whatever was there, it emitted no starlight. Even the best telescopes, from 26,000 light years away, could make out no more than a blur.
Two advances since have helped shed some figurative light on whatever is going on in our galaxy’s core. One was the growing availability in the 1990s of infrared detectors, originally developed for military use. Another was the development of optical techniques that could drastically increase the ability of telescopes to see small details by compensating for atmospheric turbulence.
In the wheelhouse of the galaxy
To conduct that experiment, astronomers needed to know the star’s orbit to a high precision, which in turn required two decades of observations with the most powerful telescopes on Earth. “You need 20 years of data just to get a seat at this table,” said Ghez, who joined the fray in 1995.
And so, the race into the dark was joined on two continents. Ghez worked with the 10-meter Keck telescopes, on Mauna Kea, on Hawaii’s Big Island. Genzel’s group benefited from the completion of the European Southern Observatory’s brand-new Very Large Telescope array in Chile.
The European team was aided further by a new device, an interferometer named Gravity, that combined the light from the array’s four telescopes. Designed by a large consortium led by Frank Eisenhauer of the Max Planck Institute, the instrument enabled the telescope array to achieve the resolution of a single mirror 130 meters in diameter. The name was an acronym for a phrase that included words such as “general,” “relativity,” and “interferometry,” Eisenhauer explained.
“All of the sudden, we can see 1,000 times fainter than before,” said Genzel in 2016, when the instrument went into operation. In addition, they could track the movements of the star S2 from day to day.
Meanwhile, Ghez was analyzing the changing spectra of light from the star, to determine changes in the star’s velocity. The two teams leapfrogged each other, enlisting bigger and more sophisticated telescopes, and nailing down the characteristics of S2. In 2012, Genzel and Ghez shared the Crafoord Prize in astronomy, an award nearly as prestigious as the Nobel. Events came to a head this spring and summer, during a six-month period when S2 made its closest approach to the black hole.
“It was exciting in the middle of April when a signal emerged and we started getting information,” Ghez said.
On July 26, Genzel and Eisenhauer held a news conference in Munich to announce that they had measured the gravitational redshift. As Eisenhauer marked off their measurements, which matched a curve of expected results, the room burst into applause.
“The road is wide open to black hole physics,” Eisenhauer proclaimed.
In an email a month later, Genzel explained that detecting the gravitational redshift was only the first step: “I am usually a fairly sober, and sometimes pessimistic person. But you may sense my excitement as I write these sentences, because of these wonderful results. As a scientist (and I am 66 years old) one rarely if ever has phases this productive. Carpe Diem!”
In early October, Ghez, who had waited to observe one more phase of the star’s trip, said her team soon would publish their own results.