“We have been waiting 16 years for this,” said Devin Chu with UCLA’s Galactic Center Group in 2018. “We are anxious to see how the star will behave under the black hole’s violent pull. Will S0-2 follow Einstein’s theory or will the star defy our current laws of physics? We will soon find out!”
“Origin a Mystery” –the S-Star Cluster
The Keck Observatory 2018 study sheds light on the strange birth of S0-2 and its stellar neighbors in the S-Star Cluster –a small cluster of high velocity stars that surround the pothole in eternity known as Sagittarius A*, the supermassive black hole at the center of the Milky Way. The fact that these stars exist so close to the supermassive black hole is unusual because they are so young; how they could’ve formed in such a hostile environment is a mystery.
“Star formation at the Galactic Center is difficult because the brute strength of tidal forces from the black hole can tear gas clouds apart before they can collapse and form stars,” said Tuan Do, deputy director of the Galactic Center Group.
“S0-2 is a very special and puzzling star,” said Chu of the star that every 16 years orbits 11 billion miles from Sagittarius A*. “We don’t typically see young, hot stars like S0-2 form so close to a supermassive black hole. This means that S0-2 must have formed in a different way.”
Astronomers had the “all-clear” for an exciting test of Einstein’s Theory of General Relativity, thanks to the discovery about S0-2’s star status.
SO-2 –”A Binary Star?”
Up until then, it was thought that S0-2 may be a binary, a system where two stars circle around each other. Having such a partner would have complicated the upcoming gravity test. About 70% of massive stars in normal Galactic clusters have stellar companions within 10 astronomical units, corresponding to the orbit of Saturn.
The orbit of S0-2 (light blue) located near the Milky Way’s supermassive black hole will test Einstein’s Theory of General Relativity and generate potentially new gravitational models.
But the study published in The Astrophysical Journal, the team of astronomers led by a UCLA scientist from Hawaii has found that S0-2 does not have a significant other after all, or at least one that is massive enough to get in the way of critical measurements that astronomers need to test Einstein’s theory.
The researchers made their discovery by obtaining spectroscopic measurements of S0-2 using W. M. Keck Observatory’s OH-Suppressing Infrared Imaging Spectrograph (OSIRIS) and Laser Guide Star Adaptive Optics.
Not A Spectroscopic Binary — Confirming Einstein
“This is the first study to investigate S0-2 as a spectroscopic binary,” said lead author Chu, at the time an astronomy graduate student with UCLA’s Galactic Center Group. “It’s incredibly rewarding. This study gives us confidence that a S0-2 binary system will not significantly affect our ability to measure gravitational redshift.”
Einstein’s Theory of General Relativity predicts that light coming from a strong gravitational field gets stretched out, or “redshifted.” Researchers expect to directly measure this phenomenon as S0-2 makes its closest approach to the supermassive black hole at the center of our Milky Way galaxy.
This allowed the Galactic Center Group to witness the star being pulled at maximum gravitational strength – a point where any deviation to Einstein’s theory is expected to be the greatest.
“It will be the first measurement of its kind,” said co-author Tuan Do. “Gravity is the least well-tested of the forces of nature. Einstein’s theory has passed all other tests with flying colors so far, so if there are deviations measured, it would certainly raise lots of questions about the nature of gravity!”
The Follow Up –Consistent With General Relativity
General relativity predicts that light emitted by an object in a strong gravitational field—for example, close to a black hole—should be shifted to longer wavelengths. This gravitational redshift does not exist in the Newtonian theory of gravity. In 2019 Tuan Do et al. monitored the position and spectrum of S0-2 as it passed Sagittarius A*, the supermassive black hole at the center of the Milky Way. Around the closest part of S0-2’s 16-year orbit, they detected the effect of gravitational redshift on its spectrum. These results are more consistent with general relativity than Newtonian gravity at the 5σ (99.99994%) confidence level.
There are several theories that provide a possible explanation, with S0-2 being a binary as one of them. “We were able to put an upper limit on the mass of a companion star for S0-2,” said Chu. This new constraint brings astronomers closer to understanding this unusual object.
“Stars as massive as S0-2 almost always have a binary companion. We are lucky that having no companion makes the measurements of general relativistic effects easier, but it also deepens the mystery of this star,” said Do.
The Galactic Center Group plans to study other S-Stars orbiting the supermassive black hole, in hopes of differentiating between the varying theories that attempt to explain why S0-2 is single.
Image at top of page: The center of our Milky Way Galaxy shown in the NRAO image is anchored by a black hole that is nearly 5 million times the mass of our Sun. Surrounding it is a chaotic city of stars, gas, and dust that we call Sagittarius A.
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