It's been almost 30 years, but scientists working with the Very Large Telescope (VLT) in the Atacama Desert in Chile have now measured for the first time the unique orbit of a star orbiting the supermassive black hole that is believed to that it's lying in the center of our Milky Way. The path of the star (known as S2) traces a characteristic rosette-like pattern (similar to a spirograph) that corresponds to one of the central predictions of Albert Einstein's general theory of relativity. The international cooperation described their results in a new article in the journal Astronomy and Astrophysics.
"The general theory of relativity predicts that bound paths of one object around another are not closed as in Newtonian gravity, but go forward 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 proof of the general theory of relativity. One hundred years later we have now found the same effect in the movement of a star that the compact radio source Sagittarius A * (SagA *) in the center of the Milky Way. "
When Einstein developed his general theory of relativity, he proposed three classic tests to confirm its validity. One was the deflection of light by the sun. As massive objects warp and space-time warp, light follows a curved path around massive objects. This prediction was confirmed in 1919 with this year's solar eclipse, thanks to Sir Arthur Eddington's expedition to measure the gravitational deflection of starlight near the sun. The confirmation made headlines worldwide and Einstein became a household name.
Enlarge /. Mercury's perihelion precession.
Rainer Zenz / Wikimedia Commons
The general theory of relativity also predicted a red shift of light due to gravitation in the presence of strong gravitational fields. This was first confirmed in 1954 by measuring a red shift in the starlight of a white dwarf star.
The third test was the precession of Mercury's rather eccentric elliptical orbit around the sun. About every 100 years, the planet's perihelion, or the point closest to the sun, drifts by 0.001 degrees thanks to the attraction of other planets. In this way, astronomers finally discovered Neptune. Astronomers had noticed some strange disturbances in the orbit of Uranus, and 19th-century French mathematician Urbain Le Verrier was right to conclude that this was evidence of another planet. His forecast of 1845 was confirmed observingly in September 1846.
Le Verrier also attempted to model Mercury's orbit in accordance with Newton's gravity, which was tested during Mercury's transit of 1843. His model failed this test, and he suggested that the deviations could again be due to a hypothetical, yet undiscovered planet that is closer to the sun and will later be referred to as a volcano. In the decades that followed, however, there were no confirmed observations of such a planet. It was Einstein who showed that Newton's theory of gravitation was incomplete. The general theory of relativity explains exactly the observed precession of the Mercury orbit.
Enlarge /. This simulation shows the orbits of stars very close to the supermassive black hole in the heart of the Milky Way – a perfect laboratory for testing gravitational physics and in particular Einstein's general theory of relativity.
ESO / L. Calçada / spaceengine.org
If these important predictions of general relativity have already been confirmed experimentally, why are scientists so interested in further testing them? Well, there can be unique environments beyond our solar system – such as the extreme gravity of a supermassive black hole – in which the laws of physics may not be exactly the same. SagA * is the perfect laboratory to study this, especially given the dense star clusters that circle around it. One of these stars, S2, is of particular interest as it comes fairly close to the black hole the next time it approaches (less than 20 billion kilometers).
Enter the people behind the VLT, which first went online in 1998. The VLT team could see the faint glow around the black hole when S2 passed its first observations of the star. About two years later, in 2018, they successfully measured the redshift of gravity from S2, with the strong gravity of the black hole stretching the star's light over longer wavelengths as it progressed. Infrared observations – using the VLT's GRAVITY, SINFONIA and NACO instruments – showed that the shift in light exactly matches the predictions of general relativity.
Like the redshift effect, the orbit precession of S2 is tiny, meaning that longer observation times are required before astronomers can recognize it. S2 orbits every 16 years. The team finally collected enough data points on the position and speed of the star – a total of over 330 measurements – to accurately map its orbit. And just as general relativity predicts, every time S2 approaches the supermassive black hole it receives a gravitational kick that changes its orbit slightly so that the orbit path forms this pretty rosette shape.
Enlarge /. Artist's impression of the path of the star S2, which runs very close to the supermassive black hole in the middle of the Milky Way. When it approaches the black hole, the color of the star shifts slightly to red due to the very strong gravitational field. The color effect and size of the objects are exaggerated for clarity.
ESO / M. grain knife
"Our results to date have shown that the light emitted by the star experiences a general theory of relativity. Now we have shown that the star itself perceives the effects of general relativity," said Paulo Garcia of the Portuguese Center for Astrophysics and Gravitation, a leading scientist HEAVY.
The next phase will be based on the upcoming Extrem Large Telescope, which is intended to give scientists the opportunity to see much fainter stars near the supermassive black hole. "If we're lucky, we can catch stars so close that they actually feel the rotation, the spin of the black hole," said Andreas Eckart from the University of Cologne, another leading scientist on the project, and made it possible for astronomers, SagAs to measure define spin and mass as well as space and time around it. "That would be a completely different level of relativity testing."
DOI: Astronomy and Astrophysics, 2020. 10.1051 / 0004-6361 / 202037813 (About DOIs).