Astronomers have been planning to use star S0-2 to test Einstein’s theory of general relativity because the star is in a truly unique position. It orbits Sagittarius A*, the supermassive black hole at the center of the Milky Way and it will make its closest approach this spring.

While the possibility of this test was exciting, researchers were concerned that S0-2 was a binary system. This would have made the analysis a lot more complex. However, the star is most likely single and, as reported in The Astrophysical Journal, the researchers have the go-ahead to continue with the test.

A team led by scientists at UCLA have used the Keck Observatory on Mauna Kea to look at the light spectrum of the stars. Their goal was to work out the influence that a potential second star might have on S0-2's light. They didn’t find any, so even if it has a companion, it’s not massive enough to matter.  

"This is the first study to investigate S0-2 as a spectroscopic binary," lead author Devin Chu of Hilo, from UCLA, said in a statement. "It's incredibly rewarding. This study gives us confidence that an S0-2 binary system will not significantly affect our ability to measure gravitational redshift."

The gravitational redshift is what the team, known as the Galactic Center Group, is going to measure to test relativity. As the star approaches the supermassive black hole, the wavelength of the emitted light will be stretched due to the incredible gravitational force present at the center of our galaxy. If there is a deviation between the theory and reality, it will be more obvious where gravity is strongest.

"It will be the first measurement of its kind," said co-author Tuan Do, deputy director of the Galactic Center Group. "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!"

Astronomers have been studying the stars at the center of the Milky Way for a long time. There are two decades of repeated observations that have already served to test relativity, but to finally see S0-2 getting to its closest approach is important. Researchers also hope to better understand this star and other similar objects around Sagittarius A*.

"S0-2 is a very special and puzzling star," added Chu. "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 a different way."

Various hypotheses and general relativity will be put to the test with the next round of observations for the object.