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An artist’s impression of the star S2 passing close to the supermassive black hole at the center of the Milky Way

M. Kornmesser/ESO

Star’s black hole encounter puts Einstein’s theory of gravity to the test

For more than 20 years, a team of astronomers has tracked a single star whipping around the supermassive black hole at the center of our galaxy at up to 25 million kilometers per hour, or 3% of the speed of light. Now, the team says the close encounter has put Albert Einstein’s theory of gravity to its most rigorous test yet for massive objects, with the light from the star stretched in a way not prescribed by Newtonian gravity. In a study announced today, the team says it has detected a distinctive indicator of Einstein’s general theory of relativity called “gravitational redshift,” in which the star’s light loses energy because of the black hole’s intense gravity.

“It’s really exciting. This is such an amazing observation,” says astronomer Andrea Ghez of the University of California, Los Angeles (UCLA), who heads a rival group that is also tracking the star. “This is a direct test [of relativity] that we’ve both been preparing for for years.”

The star, called S2, is unremarkable apart from a highly elliptical orbit that takes it within 20 billion kilometers, or 17 light-hours, of the Milky Way’s central black hole—closer than any other known star. A team led by Reinhard Genzel at the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany, has been tracking S2 since the 1990s, first with the European Southern Observatory’s (ESO’s) 3.6-meter New Technology Telescope in Chile’s Atacama Desert and later with ESO’s Very Large Telescope (VLT), made up of four 8-meter instruments. Ghez’s team at UCLA also began to observe the star around the same time with the twin 10-meter Keck telescopes in Hawaii.

Studying stars close to the galactic center is difficult because of clouds of dust and gas that block much of the stars’ light. But by tracking their orbits at infrared wavelengths, the teams have been able to estimate the mass of the central black hole, known as Sagittarius A* (Sgr A*), at about 4 million times that of our sun. The teams also figured that when S2 swooped closest to Sgr A* in its 16-year orbit, the extreme gravity would make the effects of general relativity detectable.

When S2 last made a close approach to Sgr A*, in 2002, telescopes were not precise enough to make the necessary measurements. This time, the teams were ready. Both Keck and the VLT are now equipped with adaptive optics, flexible mirrors that change shape in real time to compensate for distortions caused by Earth’s atmosphere. The European team also had the benefit of using the four VLT scopes as an interferometer, combining their light in a way that they can achieve the equivalent resolution of a telescope 130 meters across.

We were always running up against the deadline. It was extremely stressful for the team.

Reinhard Genzel, Max Planck Institute for Extraterrestrial Physics

For Genzel’s team, it proved to be a race to get its interferometric instrument, known as GRAVITY, ready in time. “We were always running up against the deadline. It was extremely stressful for the team,” he says.

Since March, the teams have been monitoring S2 regularly both before and after the closest approach on 19 May. With images they could track the star’s apparent path across the sky and with spectrometers they could measure its speed in the other dimension—its “radial velocity” toward or away from Earth—via its Doppler shift. They were hoping to see two effects predicted by Einstein. The first, gravitational redshift, is a reduction in energy of photons as they work to escape from the black hole’s intense gravitational field. The second, called the relativistic transverse Doppler effect—predicted by Einstein’s earlier theory of special relativity—is a redshift that occurs when an object is moving tangentially to the line of sight.

In a paper published today in Astronomy & Astrophysics, Genzel’s group reports seeing the combined action of the relativistic effects, with the black hole’s gravity redshifting S2’s radial velocity by 200 kilometers per second, a small fraction of its overall speed. The results match closely with the predictions of relativity and are inconsistent with Newtonian gravity. “We were extremely happy and delighted with the result,” says team member Stefan Gillessen of MPE. Ghez says her team made a deliberate decision to hold off on announcing its results. “We have a result and will publish in September. We want to see all phases” of the close approach, she says. 

General relativity has been tested many times before, both in earthbound experiments and astrophysical observations, and always passed with flying colors. The most rigorous tests involve pairs of neutron stars and the recent gravitational wave observations of merging black holes. But these cases involve objects that weigh at most a few dozen solar masses. Tests involving Sgr A* moves into the realm of extreme gravitational fields. “This mass scale is untested,” Gillessen says, and physicists are eager to see how relativity stands up under such conditions.

Both teams predict further confirmations of Einstein’s theory. “This is the first step on a ladder of tests of general relativity,” Gillessen says. Over the next year or two, they hope to see S2’s path begin to diverge very slightly from the one it followed 16 years ago. This is because of a phenomenon predicted by relativity called Schwarzschild precession in which the axis of the star’s orbit is shifted by a tiny amount on each circuit. “We’re beginning to see it, but it will take another year or two to make [the data] robust,” Genzel says.

Next up will be attempts to find other stars even closer to Sgr A* than S2. Tracking their orbits could enable researchers to measure the spin rate of the black hole. And increasingly sensitive instruments may be able to detect material falling into the black hole at half the speed of light and jets blasting out from its poles. “We can only dream of the possibilities,” Gillessen says.