On March 2, 2019, astronomers reported discovery of a dozen black holes gathered around Sagittarius A* (Sgr A*), the supermassive monster in the center of the Milky Way Galaxy, supporting a decades-old prediction. After conducting a cosmic inventory to calculate and categorize stellar-remnant black holes, the astronomers from the University of California concluded that there are probably tens of millions of the enigmatic, dark objects in the Milky Way – far more than expected.
Star S2 orbiting Sagittarius A*
Now, observations made with ESO’s Very Large Telescope (VLT) have revealed for the first time that a star, named S2, orbiting Sgr A* (artist impression above) –at the very edge of where space-time breaks down. The star moves in an egg-shaped orbit just as predicted by Einstein’s general theory of relativity. Its orbit is not like an ellipse as predicted by Newton’s theory of gravity. This long-sought-after result was made possible by increasingly precise measurements over nearly 30 years, which have enabled scientists to unlock the mysteries of the behemoth lurking at the heart of our galaxy.
“Every 16 years, the star’s orbit takes it within a cosmic whisker’s breadth — 11 billion miles — of the lip of what is believed to be the supermassive black hole Sagittarius A*, the ‘pothole in eternity’ at the center of the Milky Way galaxy,” reports Dennis Overbye for the New York Times. “That black hole has consumed mass equivalent to four million suns. During its fraught passages, the S2 star experiences the full strangeness of the universe, according to Einstein.”
“Einstein’s General Relativity predicts that bound orbits of one object around another are not closed, as in Newtonian Gravity, but precess forwards in the plane of motion. This famous effect — first seen in the orbit of the planet Mercury around the Sun — was the first evidence in favor of General Relativity. One hundred years later we have now detected the same effect in the motion of a star orbiting the compact radio source Sagittarius A* at the center of the Milky Way. This observational breakthrough strengthens the evidence that Sagittarius A* must be a supermassive black hole of 4 million times the mass of the Sun,” says Reinhard Genzel, Director at the Max Planck Institute for Extraterrestrial Physics (MPE) and the architect of the 30-year-long program that led to this result.
Genzel received the 2020 Nobel Prize in Physics (which he shared with Adrea Ghez of UCLA) for his discovery of a supermassive compact object at the center of our galaxy. “The most perfect macroscopic objects there are in the universe: the only elements in their construction are our concepts of space and time,” said Noble-Prize laureate Subrahmanyan Chandrasekhar, for whom NASA’s Chandra X-Ray Observatory was named about black holes.
An unexplored and extreme regime of gravity
Located 26 000 light-years from the Sun, Sagittarius A* and the dense cluster of stars around it provide a unique laboratory for testing physics in an otherwise unexplored and extreme regime of gravity. One of these stars, S2, sweeps in towards the supermassive black hole to a closest distance less than 20 billion kilometers (one hundred and twenty times the distance between the Sun and Earth), making it one of the closest stars ever found in orbit around the massive giant.
At its closest approach to the black hole, S2 is hurtling through space at almost three percent of the speed of light, completing an orbit once every 16 years. “After following the star in its orbit for over two and a half decades, our exquisite measurements robustly detect S2’s Schwarzschild precession in its path around Sagittarius A*,” says Stefan Gillessen of the MPE, who led the analysis of the measurements published in the journal Astronomy & Astrophysics.
First measurement of the Schwarzschild precession
Most stars and planets have a non-circular orbit and therefore move closer to and further away from the object they are rotating around. S2’s orbit precesses, meaning that the location of its closest point to the supermassive black hole changes with each turn, such that the next orbit is rotated with regard to the previous one, creating a rosette shape. General Relativity provides a precise prediction of how much its orbit changes and the latest measurements from this research exactly match the theory. This effect, known as Schwarzschild precession, had never before been measured for a star around a supermassive black hole.
S2 measurements follow General Relativity
“Because the S2 measurements follow General Relativity so well, we can set stringent limits on how much invisible material, such as distributed dark matter or possible smaller black holes, is present around Sagittarius A*. This is of great interest for understanding the formation and evolution of supermassive black holes,” says Guy Perrin and Karine Perraut, the French lead scientists of the project.
This result is the culmination of 27 years of observations of the S2 star using, for the best part of this time, a fleet of instruments at ESO’s VLT, located in the Atacama Desert in Chile. Because S2 takes years to orbit the supermassive black hole, it was crucial to follow the star for close to three decades, to unravel the intricacies of its orbital movement.
The research was conducted by an international team led by Frank Eisenhauer of the MPE with collaborators from France, Portugal, Germany and ESO. The team make up the GRAVITY collaboration, named after the instrument they developed for the VLT Interferometer, which combines the light of all four 8-metre VLT telescopes into a super-telescope (with a resolution equivalent to that of a telescope 130 metres in diameter).
The same team reported in 2018 another effect predicted by General Relativity: they saw the light received from S2 being stretched to longer wavelengths as the star passed close to Sagittarius A*.
“Our previous result has shown that the light emitted from the star experiences General Relativity. Now we have shown that the star itself senses the effects of General Relativity,” says Paulo Garcia, a researcher at Portugal’s Center for Astrophysics and Gravitation and one of the lead scientists of the GRAVITY project.
“Hotspots” orbiting Sagittarius A* at 30% of the Speed of Light
Back in October of 2018, before the release of the first image of the M87 black hole from the Event Horizon Telescope (EHT), astronomers announced that they found something orbiting the innermost possible orbit of the supermassive black hole. Their measurements suggest that these “hotspots” — perhaps made of blobs of plasma — are spinning not far from the innermost orbit allowed by the laws of physics.
The newly detected hotspots, reported Joshua Sokol in Quanta, “afford astronomers their closest look yet at the funhouse-mirrored space-time that surrounds a black hole. And in time, additional observations will indicate whether those known laws of physics truly describe what’s going on at the edge of where space-time breaks down.”
“It’s mind-boggling to actually witness material orbiting a massive black hole at 30% of the speed of light,” marveled Oliver Pfuhl, a scientist at the Max Planck Institute for Extraterrestrial Physics
For astrophysicists, this glimpse at plasma is interesting in and of itself. “We have a totally new environment, which is totally unknown,” said Nico Hamaus, a cosmologist at Ludwig Maximilian University in Munich, who also developed the early hot spot theory.
While some matter in the accretion disc — the belt of gas orbiting Sagittarius A* at relativistic speeds — can orbit the black hole safely, anything that gets too close is doomed to be pulled beyond the event horizon. The closest point to a black hole that material can orbit without being irresistibly drawn inwards by the immense mass is known as the innermost stable orbit, and it is from here that the observed flares originate.
Relativistic speeds are those which are so great that the effects of Einstein’s Theory of Relativity become significant. In the case of the accretion disc around Sagittarius A*, the gas is moving at roughly 30% of the speed of light.
“We were closely monitoring S2, and of course we always keep an eye on Sagittarius A*,” explained Pfuhl. “During our observations, we were lucky enough to notice three bright flares from around the black hole — it was a lucky coincidence!”
A Resounding Confirmation of the Massive Black Hole’s Existence
“This always was one of our dream projects but we did not dare to hope that it would become possible so soon.” Referring to the long-standing assumption that Sagittarius A* is a supermassive black hole, Genzel concluded that “the result is a resounding confirmation of the massive black hole paradigm.”
With ESO’s upcoming Extremely Large Telescope, the team believes that they would be able to see much fainter stars orbiting even closer to the supermassive black hole.
“If we are lucky, we might capture stars close enough that they actually feel the rotation, the spin, of the black hole,” says Andreas Eckart from Cologne University, another of the lead scientists of the project. This would mean astronomers would be able to measure the two quantities, spin and mass, that characterize Sagittarius A* and define space and time around it. “That would be again a completely different level of testing relativity,” says Eckart.
Avi Shporer, Research Scientist, MIT Kavli Institute for Astrophysics and Space Research via ESO, New York Times and Quanta Astronomers Creep Up to the Edge of the Milky Way’s Black Hole.
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