A star hurtling towards the supermassive black hole at the centre of the Milky Way has helped astronomers test Albert Einstein’s theory of general relativity, showing that the theory holds up even under some of the most extreme conditions found within our galaxy.
Astronomers at the European Southern Observatory observed a star very near Sagittarius A*, the supermassive black hole at the centre of our galaxy. The star, called S2, passes less than 20 billion kilometres from the black hole, or about four times further than Neptune is from the sun.
“The galactic centre is an ideal laboratory,” says team member Reinhard Genzel of the Max Plank Institute for Extraterrestrial Physics in Germany. This is the because extreme gravity experienced by the star at this distance creates the perfect opportunity to see effects of general relativity that don’t exist in other places.
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The team spent the ten years observing S2 before a close approach in 2002, but the telescope available at the time meant there were limits to what they could learn about the star and black hole. Â They knew the star would pass just as close sixteen years later, in May 2018, and started planning an upgrade.
Powerful telescope
In the end, they used four telescopes in Chile together to create, in effect, a much larger one – the equivalent of a 120 metre diameter telescope. That is powerful enough to see objects on the moon that are only centimetres across.
Armed with this new technology, they observed the star passing close to the black hole again. They were able to carefully measure the light from the star, and found a slight change in its colour as it moves.
This alone wouldn’t be too exciting. Objects often change colour in space, through something known as the Doppler effect. Anything moving towards an observer has its light waves squished together, making them shorter and, therefore, bluer. Objects moving away, however, have their light stretched a bit to longer, redder wavelengths.
In this case, the astronomers saw something else shifting the star’s colour: the gravitational pull of the black hole. This gravitational redshift occurs when the waves of light from an object are stretched by the gravity of another object.
Feryal Ozel of Arizona State University says, “Gravitational redshift is a core element of general relativity. It has been previously measured for the sun, nearby stars, and white dwarfs, but it is the first time we see it from an object so far away in the galaxy and one that is orbiting a black hole.”
In this case, the redshift was exactly as predicted by general relativity. However, the researchers point out that this doesn’t mean relativity is absolutely correct. “We know Einstein’s theory must break down at very small scales and extreme conditions around the universe,” says Genzel. But, for now, the theory is robust enough to hold up in the face of a supermassive black hole.
Astronomy & Astrophysics
Article amended on 3 August 2018
We corrected the size of the orbit of S2
