In a few months, astronomers will point their telescopes at Sagittarius A *, the supermassive black hole in the center of our galaxy. A star is in fact dangerously close to it, enough to test – once again – Einstein’s theory of general relativity.
Named S0-2, the star is part of a class of stars known as “S-Stars” (not to be confused with S-type stars) that gravitate around Sgr A *, the black hole, the supermassive stationed in the center of our galaxy. The mass of the latter is estimated at about 4.3 million suns, for a diameter equivalent to that of Mercury. The star stands out today more precisely because its orbit is leading it ever closer to the cosmic ogre. Enough to allow astronomers to study the effects of its gargantuan gravitational attraction.
According to general relativity, light affected by a strong gravitational field is stretched. The orbit borrowed by the star — a priori about 15 times more massive than the Sun — must also be slightly modified, accelerating to 3% of the speed of light and leading it as close as 17 light-hours from the center of the Galaxy. That’s about four times the distance between the Sun and Neptune. The researchers will then carefully examine whether these changes actually occur. If so, then they will once again confirm the law of general relativity.
“This will be the first step of its kind,” said astronomer Tuan Do, deputy director of the Galactic Center Group (USA), who will oversee the analyzes. “Gravity is the least tested of the forces of nature: Einstein’s theory has passed all other tests brilliantly, so if discrepancies are measured, it would certainly raise many questions about the nature of gravity!”
Researchers have been observing S0-2 since 1992, which means that its closest orbit has already been observed 16 years ago. It is thanks to it that astronomers have been able to prove the existence of Sgr A *. But at the time, the instrumentation used was not sensitive enough to observe a gravitational shift of the light of the star. Today, the means have evolved.
“We waited 16 years for that,” notes one of Devin Chu’s team members. “We are anxious to see how the star will behave under the effect of a black hole’s violence: will S0-2 follow Einstein’s theory or will the star challenge our current laws of physics ?”. This summer, we’ll find out.