Skip to main content

Star's motion around Milky Way's monster black hole proves Einstein right yet again

Einstein's theory of general relativity just passed a dramatic black-hole test with flying colors.

The motion of a star orbiting Sagittarius A*, the supermassive black hole at the heart of our Milky Way galaxy, precisely matches that predicted by general relativity, a new study reports.

"Einstein's general relativity predicts that bound orbits of one object around another are not closed, as in Newtonian gravity, but precess forward 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," study co-author Reinhard Genzel, director of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, said in a statement. 

"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," Genzel added. "This observational breakthrough strengthens the evidence that Sagittarius A* must be a supermassive black hole of 4 million times the mass of the sun." 

Related: Images: black holes of the universe

Observations made with the European Southern Observatory’s Very Large Telescope in Chile have revealed for the first time that a star orbiting the supermassive black hole at the center of the Milky Way moves just as predicted by Einstein's theory of general relativity. Its orbit is shaped like a rosette and not like an ellipse as predicted by Newton's theory of gravity. This effect, known as Schwarzschild precession, had never before been measured for a star around a supermassive black hole. This artist's impression illustrates the precession of the star's orbit, with the effect exaggerated for easier visualization.

Observations made with the European Southern Observatory’s Very Large Telescope in Chile have revealed for the first time that a star orbiting the supermassive black hole at the center of the Milky Way moves just as predicted by Einstein's theory of general relativity. Its orbit is shaped like a rosette and not like an ellipse as predicted by Newton's theory of gravity. This effect, known as Schwarzschild precession, had never before been measured for a star around a supermassive black hole. This artist's impression illustrates the precession of the star's orbit, with the effect exaggerated for easier visualization. (Image credit: ESO/L. Calçada)

The motion Genzel mentioned, called Schwarzschild precession, describes a sort of rotation in an object's elliptical orbit. The location of the object's closest-approach point changes with each lap, so the overall orbit is shaped like a rosette rather than a simple, static ellipse.

Astronomers had never measured Schwarzschild precession in a star zooming around a supermassive black hole — until now.

The research team used the European Southern Observatory's (ESO) Very Large Telescope (VLT) in Chile to track a star called S2 as it looped around Sagittarius A*, which lies about 26,000 light-years from Earth. Over the course of 27 years, the astronomers made more than 330 measurements of S2's position and velocity using multiple VLT instruments. (One of those instruments is called GRAVITY, which gives the research team its name: the GRAVITY collaboration.)

Such a long observational window was necessary to pick up S2's precession, for the star takes 16 Earth years to complete one orbit around Sagittarius A*. 

The observed precession matched the predictions of general relativity exactly, which could lead to further discoveries down the road, the researchers said.

“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*," team members Guy Perrin and Karine Perraut — of the Paris Observatory-PSL and the Grenoble Institute of Planetology and Astrophysics in France, respectively — said in the same statement. 

"This is of great interest for understanding the formation and evolution of supermassive black holes," they added.

This simulation shows the orbits of stars very close to the supermassive black hole at the heart of the Milky Way. One of these stars, named S2, orbits every 16 years and is passing very close to the black hole in May 2018. (Image credit: ESO/L. Calçada/spaceengine.org)

The new study, which was published online today (April 16) in the journal Astronomy & Astrophysics, may presage even more exciting black-hole insights to come. For example, coming megascopes such as ESO's Extremely Large Telescope could allow astronomers to track stars that get even closer to Sagittarius A* than S2 does, the researchers said.

“If we are lucky, we might capture stars close enough that they actually feel the rotation, the spin, of the black hole," said study team member Andreas Eckart of Cologne University in Germany. "That would be again a completely different level of testing relativity."

Mike Wall is the author of "Out There" (Grand Central Publishing, 2018; illustrated by Karl Tate), a book about the search for alien life. Follow him on Twitter @michaeldwall. Follow us on Twitter @Spacedotcom or Facebook

OFFER: Save 45% on 'All About Space' 'How it Works' and 'All About History'!

For a limited time, you can take out a digital subscription to any of our best-selling science magazines for just $2.38 per month, or 45% off the standard price for the first three months.View Deal

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: community@space.com.

  • Lovethrust
    Is Einstein being right yet again really surprising any more?
    Reply
  • rod
    So far, Einstein Special Relativity and General Relativity are not called, the law of Relativity or the law of General Relativity like Kepler's laws, Newton's laws of motion, law of gravity. It took nearly two hundred years or more of observation and testing, showing the math worked elsewhere outside of Earth before these became laws. Einstein showed his math in 1905 and 1916, astronomers still working on it :)
    Reply
  • dfjchem721
    "Einstein's theory of general relativity just passed a dramatic black-hole test with flying colors."

    What an enormous surprise!

    Does anyone out there have a list of all of the observations made that conform to GR or SR?

    Just a few off the top:

    Eddington's observation during a solar eclipse demonstrated warping of space by the sun .

    Cosmic rays have shown time dilation (increase in "resting" half-life at the bench) when entering the atmosphere at high speeds.

    A pair of atomic clocks were synced and one flown around while the other was stationary also showed time dilation.

    Likely tons more most have never heard of.

    Or how about a list that doesn't (above sub-atomic)? Even one real observation not conforming?
    Reply
  • rod
    FYI, I think this is the central point of this test reported. "Einstein's general relativity predicts that bound orbits of one object around another are not closed, as in Newtonian gravity, but precess forward 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," study co-author Reinhard Genzel, director of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany, said in a statement. "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," Genzel added. "This observational breakthrough strengthens the evidence that Sagittarius A* must be a supermassive black hole of 4 million times the mass of the sun."

    I have this Mercury test in my trusty Einstein book, Relativity, The Special and The General Theory, A Clear Explanation That Anyone Can Understand, 1961, p. 126. On page 103, "On the basis of the general theory of relativity, it is found that the ellipse of every planet round the sun must necessarily rotate in the manner indicated..."

    Looks like confirmation of this prediction from GR rather than Mercury observations in the solar system without other solid measurements too. It is detecting this same effect in the motion of a star that is important. Moving in the direction of GR becoming a law of science vs. theory :)
    Reply
  • rod
    FYI, GR predictions for the center of a black hole is a singularity but QM contradicts this.
    Reply
  • dfjchem721
    I was not even aware of the Mercury precession issue with GR. And I am way out of touch with this story since they detected a gravitational red-shift on closest approach.

    It appears they had to track S2 for quite a while to get the data. Is this data set from the same for the red-shift, only over longer periods of time? Maybe a different instrument. Cannot keep up with all this.

    Think the RS data from the VLT was shown on a Nova special, and the Kecks also have shown similar results, last time I looked.

    It is still amazing they can even see this stuff. Maybe I will try my C-11 with the 12mm Naglers! Looking at the center of the Milky Way, the optical density of "stuff" renders it more opaque (at least to visible) by about 25 orders of magnitude than looking away from it. High tech stuff to be sure.
    Reply
  • rod
    FYI, S2 star moving around Sgr A* black hole, has been measured for many years. In 2018, the gravitational redshift predicted by GR was observed, Star Swings Around Black Hole, Tests Gravity
    Now the orbital precession according to GR is validated as the new report shows. Black Hole Changes Star’s Orbit in Gravity Test
    The star, S2 and Sgr A* black hole offer a combination punch here supporting Einstein GR :) Sgr A* is considered about 4E+6 solar mass black hole. The Schwarzschild radius is about 11.813E+7 km, the diameter then is about 0.16 AU.
    Reply
  • mviswanathan
    Admin said:
    Einstein's theory of general relativity just passed a dramatic black-hole test with flying colors.

    Star's motion around Milky Way's monster black hole proves Einstein right yet again : Read more
    Interestingly, when the the planet motions are calculated just applying the Newton's Laws also produces precesion in orbit. For example when i tried just the Earth, Moon and the Sun, the orbit of the Moon was precessing at about one rotation of the Major axis in about 9 years. Is this different from what is expected with Einstein theory. I just want to understand.

    I have to check if any precession results considering just the Sun and Earth
    Reply
  • IG2007
    mviswanathan said:
    Interestingly, when the the planet motions are calculated just applying the Newton's Laws also produces precesion in orbit. For example when i tried just the Earth, Moon and the Sun, the orbit of the Moon was precessing at about one rotation of the Major axis in about 9 years. Is this different from what is expected with Einstein theory. I just want to understand
    Einstein's theory is just more accurate and more perfect than Newton's. The thing is Newton's theory doesn't work well outside our solar system.
    Reply
  • Think twice
    Admin said:
    Einstein's theory of general relativity just passed a dramatic black-hole test with flying colors.

    Star's motion around Milky Way's monster black hole proves Einstein right yet again : Read more
    Lovethrust said:
    Is Einstein being right yet again really surprising any more?
    Actually isn't it the case that Le Verrier noted apsidal procession centuries ago and Einstein only tried to explain it. Anyone can fiddle up a formula to "predict" precession, as long as you know what it is already. As Albert already knew. He certainly didn't predict any orbital precession.
    Reply