Sagittarius A, the supermassive black hole at the heart of the Milky Way, is orbited by three massive young stars that have been dubbed “S-stars” and circle it at incredibly fast speeds.
These strange objects have sparked a lot of interest some years back, when astronomers were still trying to figure out how these stars ended up within such a short distance from Sagittarius A — a violent place exerting a massive gravitational pull, Phys.org reported a while ago.
One of these stars, known as S2 (or S-02), has been avidly monitored by the European Southern Observatory (ESO) over the past 26 years, in an effort to understand its motion through the extreme gravitational field around Sagittarius A.
All this effort has finally paid off, Science Daily reported earlier today, yielding the first-ever direct observation of a phenomenon called gravitational redshift — the stretching of the star’s light to longer wavelengths as a result of the black hole’s gravity, which shifts its color slightly to the red.
“The change in wavelength of light from S2 (or S0-2) agrees precisely with that predicted by Einstein’s theory of general relativity,” proving that the eminent theoretical physicist was right, ESO announced on Thursday.
This is the first time that this particular effect of Einstein’s gravity theory was observed in the motion of a star going around a supermassive black hole, and the third major experiment in the last month to prove the 100-year-old General Theory of Relativity is correct.
Earlier this month, the Inquisitr reported that the theory’s “Strong Equivalence Principle” was proven right in a triple star system located 4,200 light-years from our planet. In late June, another gravity experiment conducted on a nearby galaxy confirmed that Einstein’s General Theory of Relativity is the correct theory of gravity on an astronomical scale, per an Inquisitr report.
Spotting Gravitational Redshift With The VLT
Described in a study published last week in the journal Astronomy & Astrophysics, this incredible breakthrough belongs to an international team of scientists led by Reinhard Genzel of the Max Planck Institute for extraterrestrial physics (MPE) in Garching, Germany.
By studying the star orbiting Sagittarius A with three highly sensitive instruments mounted on ESO’s Very Large Telescope (VLT), namely GRAVITY, NACO, and SINFONI, the team obtained their first observations of S2 back in 2016 and then again on May 19 of this year, when the star reached pericenter — or the orbital point closest to the black hole’s center of gravity.
The exceptional environment around Sagittarius A — which represents the strongest gravitational field in the entire galaxy, notes ESO — offered the scientists the perfect opportunity to test gravitational physics, and Einstein’s gravity theory in particular.
Sure enough, the VLT observations allowed the team to see how the immense gravitational pull of the supermassive black hole accelerated S2 at a staggering 25 million kilometers per hour (15.5 million mph), reaching almost three percent of the speed of light, as the star passed less than 20 billion kilometers (nearly 12.5 billion miles) from Sagittarius A at pericenter on May 19.
According to Science Alert, S2 is one of the two “S-stars” orbiting the Milky Way’s central black hole which come closest to Sagittarius A at pericenter. The star circles the supermassive black hole on a 16-year-long elliptical orbit and comes within just 17 light-hours from the heart of the Milky Way during its closest approach to Sagittarius A. That’s about four times the distance between the sun and Neptune, and roughly 150 times the distance between Earth and the sun.
“We have been preparing intensely for this event over several years, as we wanted to make the most of this unique opportunity to observe general relativistic effects,” Genzel said in a statement.
“Our first observations of S2, about two years ago, already showed that we would have the ideal black hole laboratory,” added VLT’s GRAVITY co-principal investigator Frank Eisenhauer, also from the MPE.
But getting a glimpse of S2 redshifting near Sagittarius A was no easy task. This supermassive black hole, packing four million solar masses, lies 26,000 light-years from our planet and is shrouded in a thick veil of dust clouds which hinder any visible light observations.
The team was able to capture the first direct detection of a star’s redshift near a supermassive black hole only thanks to infrared observations with the VLT’s three instruments.
“This is the second time that we have observed the close passage of S2 around the black hole in our galactic center. But this time, because of much improved instrumentation, we were able to observe the star with unprecedented resolution,” said Genzel.
“During the close passage [on May 19], we could even detect the faint glow around the black hole on most of the images, which allowed us to precisely follow the star on its orbit, ultimately leading to the detection of the gravitational redshift in the spectrum of S2,” explained Eisenhauer.