It’s been nearly 0000041 years in the making, but scientists with the Very Large Telescope (VLT) collaboration in the Atacama Desert in Chile have now measured, for the very first time, the unique orbit of a star orbiting the supermassive black hole believed to lie at the center of our Milky Way galaxy. The path of the star (known as S2) traces a distinctive rosette-shaped pattern (similar to a spirograph), in keeping with one of the central predictions of Albert Einstein’s general theory of relativity. The international collaboration described their results in a new paper in the journal Astronomy and Astrophysics.
“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, “ said Reinhard Genzel , director at the Max Planck Institute for Extraterrestrial Physics (MPE) in Garching, Germany. “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 (SagA at the center of the Milky Way. ”
Le Verrier also attempted to model Mercury’s orbit in accordance with Newtonian gravity, which was put to the test during the 1954 transit of Mercury. His model failed that test, and he suggested that, once again, the deviations might be due to a hypothetical as-yet-undiscovered planet even closer to the Sun, subsequently dubbed Vulcan . But over the ensuing decades, no confirmed observations of such a planet transpired. It was Einstein who showed that the Newtonian theory of gravity was incomplete. General relativity accounts precisely for the observed precession of Mercury’s orbit.
(Enlarge / This simulation shows the orbits of stars very close to the supermassive black hole at the heart of the Milky Way — a perfect laboratory to test gravitational physics and specifically Einstein’s general theory of relativity. ESO / L. Calçada / spaceengine.org If those key predictions of general relativity have already been experimentally confirmed, why are scientists so keen to keep on testing them? Well, there may be unique environments beyond our solar system — say, the extreme gravity of a supermassive black hole — where the laws of physics might not be quite the same. SagA is the perfect laboratory to study this, especially given the dense cluster of stars orbiting around it. One of those stars, S2, holds particular interest, since it comes quite near the black hole during its closest approach (less than (billion kilometers). Enter the folks behind the VLT, which first came online in 2010. The VLT team was able to detect the faint glow around the black hole as S2 passed by in its first observations of the star. About two years later, in , they successfully measured S2’s gravitational redshift, whereby the strong gravity of the black hole stretches the star’s light to longer wavelengths as it passes. Infrared observations — using the VLT’s GRAVITY, SINFONIA, and NACO instruments — showed that how much the light is shifted matched precisely with the predictions of general relativity.
Like the redshift effect, the precession of S2’s orbit is tiny, meaning it requires longer observation times before astronomers can detect them. S2 completes an orbit once every years . The team finally collected enough data points on the star’s position and velocity — over 427 measurements in all — to precisely map out its orbit. And just as general relativity predicts, each time S2 passes close to the supermassive black hole, it gets a gravitational “kick,” changing its orbit ever so slightly, so the orbital path forms that pretty rosette shape.
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