Astronomers are observing a remarkable star orbiting the supermassive black hole at the center of our Milky Way. These studies will help scientists examine relativistic effects more closely and test the general theory of relativity.
The Limits of Newtonian Physics and General Relativity
Although general relativity is an elegant theory of gravity, we do not often need to use it. For most gravitational interactions, Newton’s law of universal gravitation works perfectly well. The English physicist’s model is accurate enough to guide spacecraft through the Solar System and to describe most stars orbiting Sagittarius A*, the supermassive black hole at the center of the Milky Way, phys.org reports.
There are gravitational effects that Newton did not predict, such as gravitational waves, but for most phenomena there is practically no difference between the models of Newton and Einstein. The differences between Einstein’s model and alternative models of relativistic gravity are even subtler. Testing these limits is extremely difficult. However, a newly discovered star may help.
A Star Orbiting Sagittarius A*
The star has the modest name S301 because it is number 301 on the list of recognized S-stars orbiting Sagittarius A*. It is slightly more massive than the Sun and completes one orbit around Sagittarius A* every 8.7 years. This is the shortest known orbital period of any S-star, and S301’s orbit is extremely elliptical.
At its closest approach, it passes at a distance of approximately 140 Sagittarius A* radii, or about 24 astronomical units. In other words, if Sagittarius A* were located at the center of our Solar System, its event horizon would lie just inside Mercury’s orbit, while 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.
This makes S301 the most relativistic star we have 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 have also observed the precession of Mercury’s orbit, which was one of the classic tests of general relativity.
However, Mercury’s precession is very small. The difference between the motion predicted by Einstein and that predicted by Newton is shorter than the duration of a single human heartbeat over one orbit. For S301, the periapsis shifts by approximately 2° with each orbit.
Secondary Relativistic Effects
The star’s motion is so extreme that secondary relativistic effects begin to appear. These include gravitational redshift and the transverse Doppler effect. We have observed these effects in the laboratory, but S301 gives us an opportunity to study them in nature.
More importantly, over time we may be able to use S301 to investigate alternatives to general relativity.
One of the major problems with general relativity is that it does not fit well with quantum theory. Several models attempt to unify the two theories, but they differ from general relativity only under extreme conditions. Some of these differences may become observable at relativistic speeds because the effects are of the order of ((v/c)^2) or ((v/c)^3).
The effects of the black hole’s rotation and its interaction with the star’s rotation also appear at these orders. Future large telescopes, such as the Giant Magellan Telescope, will be able to observe S301’s spectrum with sufficient precision to measure second- and third-order effects.
Limitations of Observing S301
For now, however, observing S301 remains challenging. The center of our galaxy is obscured by clouds and dust, so we cannot observe it in visible light. Because S301 is a Sun-like star, it is not particularly bright even in the infrared.
We can observe its motion, but we are not yet able to obtain meaningful spectral data.
However, as with everything in relativity, it is only a matter of time.
