The fastest star ever seen is moving at 8% the speed of light

In the center of our galaxy, hundreds of stars orbit a supermassive black hole. Most of these stars have orbits large enough that their motion is described by Newtonian gravity and Kepler’s laws of motion. But some orbits are so close that their orbits can only be accurately described by Einstein’s theory of general relativity. The star with the smallest orbit is known as S62. The closest approach to the black hole is that it moves more than 8% of the light speed.

The supermassive black hole of our galaxy is known as Sagittarius A * (SgrA *). It’s a mass of about 4 million suns, and we know this because of the stars that rotate it. For decades, astronomers have followed the motion of these stars. By calculating their orbits, we can determine the mass of SgrA *. In recent years, our observations have become so accurate that we can measure more than the mass of the black hole. We can test whether our understanding of black holes is correct.

The most studied star orbiting SgrA * is known as S2. It is a bright, blue giant star that revolves around the black hole every 16 years. In 2018, S2 made its closest approach to the black hole, giving us an opportunity to observe an effect of relativity, known as gravity change. When you throw a ball in the air, it slows down when it comes up. When you shine a ray of light in the air, the light does not slow down, but gravity takes away some of its energy. As a result, a ray of light turns red as it climbs out of gravity. This effect has been observed in the lab, but S2 gave us a chance to see it in the real world. Sure enough, with the close approach, the light shifted from S2 to red just as predicted.

For years it was thought that S2 was the closest star to SgrA *, but then S62 was discovered. As one team recently discovered, it’s a star about twice as massive as the sun orbiting the black hole every 10 years. By their calculations, on the closest approach, their speed approaches 8% of the speed of light. That’s so fast that time dilation comes into play. An hour on S62 would take about 100 Earth minutes.

Due to its nearby SgrA *, S62 does not follow a Keplerian orbit. Instead of being a simple ellipse, it follows a spirographic motion that moves its orbit about 10 degrees with each cycle. This kind of relativistic advance was first observed with the orbit of Mercury, but only as a small effect.

In the autumn of 2022, S62 will take another close approach to SgrA *. It should allow astronomers to test the effects of relativity even more accurately than S2’s narrow approach.