After spending the better part of a decade closely observing the movement of the star S2, which has a mass about 15 times that of the Sun, a team of astrophysicists concluded in 2002 that it orbited the black hole at the center of the galaxy. They came to this conclusion because the orbital dynamics of S2 were extreme, with an orbital period of about 15.5 years and a maximum velocity of 2.5 percent the speed of light. These observations provided the final proof astronomers needed to confirm that the compact radio source Sagitarrius A* was, in fact, that black hole.

Since then, astronomers have been keenly interested in the star S2 because its movement can tell them a lot about the supermassive black hole at the galaxy's center. They can also test some ideas about general relativity when the star makes its closest approach to the black hole, within about 17 light hours, in 2018. And now they have a powerful instrument to do just that.

The European Southern Observatory says its GRAVITY instrument has made its first observations, and they were successful. The instrument employs four 8.2-meter telescopes that are part of the "Very Large Telescope" facility in Chile. By using the telescope's interferometer capabilities, astronomers will be able to obtain ultra-precise positions of the orbiting star.

It is not a simple matter to point a telescope into the night sky and observe stars at the center of the galaxy, which is obscured by gas and dust between our Sun and the bright core of the galaxy. The GRAVITY instrument essentially allows the astronomers to lock onto the position of a "stable" object in the night sky and then track the movement of another object with a higher relative velocity. This allows for longer exposure times that can bring fainter objects into view.

Within minutes of turning on the instrument, astronomers were able to find S2 and narrow down its location. "It was a fantastic moment for the whole team when the light from the star interfered for the first time—after eight years of hard work," said GRAVITY’s lead scientist Frank Eisenhauer from the Max Planck Institute for Extraterrestrial Physics in Garching, Germany. "First we actively stabilized the interference on a bright nearby star, and then only a few minutes later we could really see the interference from the faint star—to a lot of high-fives.”