Evidence of a black hole was seen by the European Southern Observatory’s Very Large Telescope in Chile. (G.Hüdepohl (atacamaphoto.com)/European Southern Observatory)
In a patch of sky in the constellation Sagittarius, a small star, known as S2 or S0-2, cruises on the edge of eternity. Every 16 years, it passes close to a mysterious dark object that weighs some four million suns in the center of the Milky Way galaxy.
For the last two decades, two rival teams of astronomers have aimed their telescopes at the star, which lies 26,000 light-years away. They hope to confirm the existence of a monstrous black hole.
For several months this year, the star streaked through its closest approach, producing new insights into the behavior of gravity in extreme environments, and offering clues to the nature of the invisible beast in the center of the Milky Way.
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 yet that the entity is a supermassive black hole.
The evidence comes in knots of gas that appear to orbit the galactic center. Dr. Genzel’s team found that the gas clouds circle every 45 minutes or so, completing a circuit of 240 million kilometers at roughly 30 percent of the speed of light. Astrophysicists can’t imagine anything but a black hole that could be so massive, yet fit within such a tiny orbit.
The work goes a long way toward demonstrating that a supermassive black hole lurks in the heart of many galaxies.
The new data also help to explain how such black holes can wreak havoc. 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 also a triumph for the European Southern Observatory, a multinational consortium with headquarters in Munich and observatories in Chile. The organization’s facilities include the Very Large Telescope, an array of four giant telescopes in Chile.
Black holes — objects so dense that not even light can escape them — are a consequence of Albert Einstein’s general theory of relativity. When too much matter or energy is concentrated in one place, space-time can jiggle, time can slow and matter can shrink and vanish into those cosmic sinkholes.
Images of different galaxies adorn a wall at the Max Planck Institute in Germany. (Ksenia Kuleshova for The New York Times)
“We already 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?” Dr. Ghez is the leader of a separate team that is probing the galactic center. “What I like about the galactic center is that you get to see extreme astrophysics,” she said.
Two advances 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. Another was the development of optical techniques that could drastically increase the ability of telescopes to see small details by compensating for atmospheric turbulence.
These keen eyes revealed hundreds of stars in the galaxy’s blurry core, all buzzing around in a circle about a tenth of a light-year across. One of the stars, which Dr. Genzel calls S2 and Dr. Ghez calls S0-2, is a young blue star that follows an elongated orbit and passes within just 18 billion kilometers of the putative black hole every 16 years.
During these fraught passages, intense gravity on the star’s surface should slow the vibration of light waves, stretching them and making the star appear redder than normal.
This gravitational redshift was one of the first predictions of Einstein’s theory. The discovery of S2 offered astronomers a chance to observe the phenomenon.
The European team was aided by a new device, an interferometer named Gravity, that combined the light from the array’s four telescopes.
Meanwhile, Dr. Ghez was analyzing the changing spectra of light from the star, to determine changes in the star’s velocity. Events came to head this spring and summer when S2 made its closest approach to the black hole.
In July, Dr. Genzel announced that his team had measured the gravitational redshift.
A month later, Dr. Genzel explained that detecting the redshift was only the first step.
The big break came when his team detected evidence of hot spots, or “flares,” in the tiny blur of heat marking the location of the suspected black hole.
The star has finished its show for this year. Additional observations in the coming years may clarify the star’s orbit, and perhaps answer other questions, such as whether the black hole was spinning.