The star S301 completes an orbit around Sagittarius A* in 8.7 years and comes ten times closer to it than the star S2. Its orbit may allow us to measure the rotation of the black hole and test general relativity under extreme conditions.
Astronomers have discovered a faint star moving in an extreme orbit around Sagittarius A*, the supermassive black hole at the center of the Milky Way. The star, designated S301, completes one orbit in about 8.7 years, and at its closest point to the black hole reaches a speed of about 25 kilometers per second—more than 8% of the speed of light.
according to The study uploaded to arXivThis is the shortest orbital period ever measured for a known star around Sagittarius A*. The study is still being presented as a pre-publication and not as a peer-reviewed article.
An extremely narrow and long route
Sagittarius A* is located about 26 light-years from Earth and has an estimated mass of about 4.3 million solar masses. Around it orbit several dozen stars called S stars. Their orbits allow researchers to measure the gravitational field in the region and test the predictions of general relativity.
S301 is likely a main sequence star, of spectral type close to F, with a mass not much greater than the mass of the Sun. Its orbit is very elongated: its eccentricity value is about 0.982–0.983, with a value close to 1 indicating an extremely elliptical orbit.
At its closest point in its orbit, the star passes 136–142 Schwarzschild radii from Sagittarius A*. This is about ten times closer than S2, the star that has served as one of the most important laboratories for testing general relativity near the black hole.
S301 is not expected to be torn apart by tidal forces in its current orbit. According to the researchers' calculations, a star of its size would only begin to be damaged if it approached a distance of about ten Schwarzschild radii – much closer than its current orbit.
The route itself changes direction.
In Newtonian gravity, a star orbiting a central mass should return to the same ellipse over and over again. General relativity predicts that the ellipse itself will gradually rotate. The phenomenon, known as relativistic precession, was first measured in the orbit of Mercury and later in the orbit of S2 around Sagittarius A*.
In the case of S301, the point closest to the black hole moves by about 1.9–2 degrees per revolution. This is a large enough change to be detected by precise astrometric observations.
The main significance of S301 goes beyond further testing general relativity. The extreme orbit may also reveal the effect of the black hole’s rotation on the space-time around it. According to general relativity, a rotating body “drags” space-time with it to a tiny extent – a phenomenon known as frame dragging or Lenz-Thyrning precession.
The researchers estimate that in the case of a black hole spinning at maximum speed, the effect's contribution to S301's orbit could be as much as 0.11 degrees per revolution, depending on the direction of the rotation axis. With observations accumulated by the middle of the next decade, it may be possible to measure Sagittarius A*'s rotation rate and the direction of its axis directly from the star's motion.
Discovered behind the dust
S301 was detected in observations made with the GRAVITY instrument, which connects the telescopes of the European Southern Observatory's VLT array in Chile. The center of the Milky Way is hidden in visible light behind large amounts of dust, so the observations are made in the infrared.
The star was discovered in 2023 at a tiny angular distance from Sagittarius A*. It is too faint to obtain a detailed spectrum at this stage, so there are still two possible solutions to its orbit. The researchers expect that future instruments on the Extremely Large Telescope, ELT, will also allow them to measure its velocity along the line of sight and decide between the possibilities.
The elongated orbit may also hint at how the star came to be so close to the black hole. The researchers suggest that S301 was once part of a binary system that passed close to Sagittarius A*. The black hole's gravity separated the two stars: one was trapped in a tight orbit, while the other may have been ejected from the region at high speed. This explanation, known as the Hills mechanism, still requires further testing.
More on the subject on the science website
- The black hole at the center of the Milky Way isn't tearing everything apart.
- A double star near the black hole at the center of the Milky Way may be feeding it small gas clouds
- ESO's giant telescope watched a star dance around a black hole and showed that Einstein was right
- Astronomers reveal first radio image of the black hole at the heart of the Milky Way
- Improving the ability to observe black holes will allow us to test the theory of general relativity
Questions and Answers
What is S301?
This is a dim main sequence star orbiting Sagittarius A*, the supermassive black hole at the center of the Milky Way.
How fast is the star moving?
At its closest point to the black hole, it reaches about 25 kilometers per second, which is more than 8% of the speed of light.
Why is its trajectory important for the study of general relativity?
Its proximity to the black hole increases relativistic effects and allows us to measure the change in the direction of the orbit, and later perhaps also the effect of the black hole's rotation.
Is the black hole likely to swallow S301?
Not in its current orbit. The star is getting very close to Sagittarius A*, but remains outside the distance where tidal forces are expected to break apart a star of its type.
For the scientific article: Opening the scientific article
