The Galaxy’s Fastest Star to Test Einstein’s Theory of Relativity

In a few months, the orbit of a star deep in the core of the Milky Way will cause it to pass within 18.4 billion kilometers (11.4 billion miles) of the supermassive black hole at the center of the galaxy, Sagittarius A*. This event is of particular interest to physicists, as the plummeting star, S0-2, will speed up to 3 percent of the speed of light as it slingshots around its supermassive parent–a prime opportunity to test Einstein’s Theory of General Relativity.

As S0-2 nears its perigee, or the closest point in its orbit to Sagittarius A*, the acceleration caused by the supermassive black hole’s mass–4.3 million times that of the Sun–is expected to stretch out the light being emitted from the star ever so slightly toward the infrared, a Doppler effect referred to as "redshift". The effect, provided relativistic theory holds, should be that the light coming from the star will be of a slightly lower wavelength than when we see coming from it at other points in its 16-year orbit.

"It will be the first measurement of its kind," explains deputy director of the Galactic Center Group, 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!"

S0-2 itself is part of a cluster of very young stars found at the center of the Milky Way, meaning that they likely formed there rather than having migrated into the core after forming closer to the galactic rim. This presents a puzzle for scientists, considering that the gravitational forces in Sagittarius A*’s vicinity should prevent stars from forming, let alone a star of S0-2’s mass of 15 Suns. S0-2 wasn’t discovered until 1992, meaning that although at least one of its perigees have been observed, the technology that will be used to measure any potential redshift experienced by the star wasn’t available at the time.

"We have been waiting 16 years for this," explains study lead author Devin Chu. "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!"