Star spotted with spirograph orbit around supermassive black hole

In space, most orbiting objects will have circular- or elliptical-shaped orbits. But now, almost 30 years of observations has revealed that a star in the center of our galaxy orbits the supermassive black hole Sagittarius A* (Sgr A*) in a rosette, or spirograph shape. The find once again confirms a prediction made by Einstein’s General Relativity.

If you were to look down at the “plane” of the planets orbiting the Sun, their paths would mostly appear as a series of irregular circles. Sure, Earth’s orbit wobbles from an almost perfect circle to more egg-shaped over 100,000 years or so, but it still centers on the Sun.

But things are very different for S2. This star orbits the supermassive black hole at the heart of the Milky Way on an oval-shaped path – but the oval isn’t centered on the black hole. Instead, it’s located at one end. Thanks to the extremely powerful gravitational pull of Sgr A*, S2 speeds up as it falls towards the black hole, before it’s slingshotted away, slows back down, and is eventually pulled back towards the black hole.

Astronomers have been studying S2 for decades, and its unusual orbit was actually one of the first compelling pieces of evidence that there is a supermassive black hole at the center of the Milky Way.

And now, scientists have found an even stranger aspect of this star’ orbit. The ellipsis isn’t the same every time – the intense gravity throws S2 off in a slightly different direction every time. The path shifts forwards, and then next time it loops back around it shifts again. Over many iterations the star’s orbit comes to resemble a rosette or a spirograph. This phenomenon is known as Schwarzchild precession, and this was the first time it had been seen in a star orbiting a black hole.

It took a long time to make these observations however – S2 takes 16 years to complete one lap around Sgr A*. To confirm that its orbit was on a precession, astronomers had to watch it for 27 years, making over 330 measurements of the star’s position and velocity. This was done using multiple instruments on the Very Large Telescope (VLT) in Chile.

Interestingly, the team says that this observation once again confirms elements of Einstein’s Theory of General Relativity. At the same time it also gives new evidence of the properties of Sgr A* and the environment around it.

“Because the S2 measurements follow General Relativity so well, we can set stringent limits on how much invisible material, such as distributed dark matter or possible smaller black holes, is present around Sagittarius A*,” say Guy Perrin and Karine Perraut, lead scientists on the project. “This is of great interest for understanding the formation and evolution of supermassive black holes.”

The research was published in the journal Astronomy & Astrophysics. An animation of S2’s odd orbit can be seen in the video below.

Artist’s animation of S2’s precession effect

Source: European Southern Observatory

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