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Spirograph in Space

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An artist's impression of the rosette-shaped orbit of the star S2 around the supermassive black hole at the center of the Milky Way galaxy
An artist’s impression of the rosette-shaped orbit of the star S2 around the supermassive black hole at the center of the Milky Way galaxy
ESO/L. Calçada

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.

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