ESO Sees a Star’s Orbit Precess Around a Supermassive Black Hole

The European Southern Observatory has observed the way a star’s orbit changes as it circles the supermassive black hole at the centre of our galaxy – an effect called precession. The star doesn’t follow the same path each time it orbits around the black hole; instead, its orbit moves around a little each time. This doesn’t come as a surprise (the astronomers were expecting to see this), but it is the first time this effect has been seen in the orbit of a star rather than a planet (it was famously first observed in the orbit of Mercury and explained by general relativity).

This work is the result of nearly 30 years’ worth of observations. The star in question, known as S2, takes 16 years to orbit the black hole at the centre of the galaxy (which is known as Sagittarius A*). So, after observing S2 for so long astronomers have still seen less than two full orbits. That’s enough for the precession effect to be measurable, though.

So why go to the effort of observing S2 over the course of three decades? The reason is it presents a great opportunity to test general relativity. S2 orbits roughly 970 times further from Sagittarius A* than the Earth does from the Sun. That might sound like it’s far away, but it’s actually really close for a black hole of that size, and that gave astronomers the chance to test relativity’s predictions of precession on a much bigger scale than they could otherwise.

As usual, relativity’s predictions turned out to be exactly right. Chalk up another one for Einstein.

Featured image credit: ESO/L. Calçada

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