Insanely fast star puts Einstein’s theory to the test

U. TORONTO (CAN) — A star orbits the massive black hole at the Milky Way’s center in 11.5 years—less time than Jupiter takes to orbit the Sun.

It’s the shortest known orbit of any star near this black hole. The previous record-holding star, known as S0-2, orbits in 16 years.

The newly discovered star, known as S0-102 (the “S” is for Sagittarius, the constellation containing the galactic center and the black hole), could offer a unique opportunity to test Einstein’s theory of general relativity, says Tuan Do, an astronomer at the University of Toronto’s Dunlap Institute and a member of the team that made the discovery. They report their findings in Science.


Einstein’s theory predicts that mass distorts space and time and therefore not only slows down the flow of time but also stretches or shrinks distances.

The results are the latest from the 17-year effort of the UCLA Galactic Center Group, led by Andrea Ghez, a professor at UCLA. It was by studying the stars in the Milky Way Galaxy’s central region that Ghez and her team previously discovered the supermassive black hole in the heart of our galaxy.

Known as Sgr A* (“Sagittarius A-star”), the prodigious object contains the mass of four million stars equal in mass to our Sun. “Having proved that black holes exist,” says Ghez, “our research today aims to understand their nature and how they warp space and time.”

There are thousands of stars within a few light-years of our galaxy’s center. As Do describes it, “This region is the most extreme environment in the galaxy. It has the highest density of stars—equivalent to having over a million stars between the Sun and our next closest star, Alpha Centauri.

“The stars closest to Sgr A* are traveling at over 4,000 kilometers (about 2,500 miles) per second. That’s 1 percent the speed of light.”

According to general relativity, the elliptical orbits of objects like S0-2 and S0-102 should themselves “rotate,” creating a rosette-pattern over time. This motion is known as precession and is most easily observed in bodies orbiting close to massive objects.

But the mass of other stars near the galaxy’s center creates a different type of precession that is difficult to separate from precession caused by general relativity. By studying the orbits of S0-02 and S0-102 together, the Galactic Center Group will be able to distinguish between the two precessions.

And according to Ghez, “It is conceivable that we will be able to observe deviations from Einstein’s theory in regions where S0-102 and other short period stars reside.”

S0-102 was discovered using images taken with the twin 10-metre telescopes of the Keck Observatory on Mauna Kea in Hawai’i, the largest optical telescopes on the planet. These included observations with the Keck II telescope using adaptive optics and laser guide-star technology that corrects for distortions caused by the Earth’s atmosphere.

With a resolution greater than that of the Hubble Space Telescope, the observations allow Ghez, Do, and the group to resolve individual stars in the crowded region.

Sources: University of Toronto and UCLA