A star orbiting our galaxy’s supermassive black hole tests the limits of relativity

Although general relativity (GR) is an elegant theory of gravity, we don’t often need to use it. For most gravitational interactions, Newton’s theory of universal gravitation works just fine. Newton’s model is good enough to navigate spacecraft through the solar system and for most stars orbiting Sag A*, the supermassive black hole at the center of the Milky Way.

There are gravitational effects Newton didn’t predict, such as gravitational waves, but for most things, there is no effective difference between Newton’s and Einstein’s models. Even more subtle are the differences between Einstein’s model and alternative models of relativistic gravity. Testing these limits is extremely difficult. But a newly discovered star could help.

The star has the unassuming name S301 because it is No. 301 on the list of recognized S-stars that orbit Sag A*. It is a bit more massive than the sun and orbits Sag A* every 8.7 years. That is the shortest known period of any S-star, and S301 has an extremely elliptical orbit. At its closest approach, it comes within about 140 radii of Sag A*, or about 24 AU. In other words, if Sag A* were at the center of our solar system, its event horizon would be just inside the orbit of Mercury, and at closest approach, S301 would pass between the orbits of Uranus and Neptune. It would also be moving at more than 8% of the speed of light. The research is published on the arXiv preprint server.

This makes S301 the most relativistic star we’ve observed. It will allow us to test some of the limits of general relativity. For example, we have already observed that S301’s orbit precesses. We’ve observed orbital precession in Mercury’s motion. It was one of the classic tests of GR. But Mercury’s precession is tiny. The difference in motion between Einstein’s prediction and Newton’s is less than the span of a human heartbeat with each orbit. For S301, its perihelion advances about 2° with each orbit.

This motion is so extreme that secondary relativistic effects come into play. These include gravitational redshift and the transverse Doppler effect. We’ve observed these effects in the lab, but S301 gives us a chance to study them in nature. More significantly, over time, we can use S301 to study alternatives to general relativity.

One of the big difficulties with GR is that it doesn’t play well with quantum theory. Several models try to unify the two, but they differ from GR only in extreme cases. Some of these differences might be seen at relativistic speeds because the effects are on the order of (v/c)² or (v/c)³. The effects of the black hole’s spin and its interaction with the star’s rotation also come into play at these orders. Future large telescopes, such as the Giant Magellan Telescope (GMT), will be able to observe S301’s spectra with enough precision to measure second- and third-order effects.

For now, however, observing S301 remains a challenge. The center of our galaxy is veiled in clouds and dust, meaning we can’t observe it in optical light. Since S301 is a sun-like star, it isn’t particularly bright, even in the infrared. We can observe its motion, but we can’t yet gather any significant spectral data.

But as with all things relativity, it’s only a matter of time.

Who’s behind this story?

Swati Mestri

Swati Mestri holds a bachelor’s degree in Electronics Engineering and has worked as a content editor since 2019. She has experience editing research documents across technology, health care, and materials science, and has a particular interest in technology and space.

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Robert Egan

Bachelor’s in mathematical biology, Master’s in creative writing. Well-traveled with unique perspectives on science and language.

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