Observations made with ESO’s Very Large Telescope (VLT) have revealed
for the first time that a star orbiting the supermassive black hole at the
centre of the Milky Way moves just as predicted by Einstein’s general theory of
relativity. Its orbit is shaped like a rosette and 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.
“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 favour 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 centre 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) in Garching, Germany and the architect of the
30-year-long programme that led to this result.
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 kilometres (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.
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.
The study with
ESO’s VLT also helps scientists learn more about the vicinity of the
supermassive black hole at the centre of our galaxy. “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,” say 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. The number of data points marking the star’s position and
velocity attests to the thoroughness and accuracy of the new research: the team
made over 330 measurements in total, using the GRAVITY, SINFONI and NACO
instruments. 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 Centre
for Astrophysics and Gravitation and one of the lead scientists of the GRAVITY
project.
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
characterise Sagittarius A* and define space and time around it. “That would be
again a completely different level of testing relativity,” says Eckart.
No comments:
Post a Comment