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At the centre of the Milky Way, Einstein’s laws rule. Measurements of light from a star that orbits close to our galaxy’s central supermassive black hole cannot be explained by classical views of gravity and instead require Einstein’s general relativity.

General relativity predicts that starlight should lose some energy as it travels through the powerful gravitational field of a black hole. That loss in energy is expected to��stretch the wavelength��of light from stars near huge black holes, making them look more red. Standard, or Newtonian, gravitational theory doesn’t predict this.��No one expects Einstein’s theory to be wrong at this scale, but if its predictions were off by even a little, it might point towards new physics.

To test between the two theories, Tuan Do at the University of California, Los Angeles, and his colleagues used 24 years of observations of a star called S0-2, which orbits relatively close to our galaxy’s central supermassive black hole, Sagittarius A*. The data included measurements of the star’s colour and position. The team calculated its velocity from measurements��of its position over time.

“When the star becomes redder, it also looks like it’s moving away from us faster than it would without this gravitational redshift,” says Do. “That is why it took 24 years to do this.” The researchers needed enough data to nail down the star’s orbit and make sure this wasn’t tainting the colour measurements.

There have been similar studies in the past, but this work includes new measurements of the star’s closest approach to the black hole in 2018, which increased their confidence in its orbit. Do and his colleagues found that S0-2 appeared significantly redder than we’d expect if its light wasn’t being stretched by relativistic effects. “Einstein was right, for now,” says Do.

S0-2 isn’t the closest star to Sagittarius A*, but it is the only one that was bright enough to observe with the equipment available when the observations began in 1995. Do and his team, as well as other research groups, have started observing dimmer stars that are even closer to the black hole, but don’t yet have enough data to test general relativity more precisely with them.

Journal reference:��Science,��