The discovery of a new star orbiting the Milky Way's central black hole at an astonishing speed has sparked excitement and intrigue in the scientific community. This star, named S301, completes an orbit around the black hole every 8.7 years, passing at a velocity of over 8% of the speed of light. What makes this finding particularly remarkable is the star's proximity to the black hole, reaching a distance of just 136 or 142 Schwarzschild radii during its closest approach. This is significantly closer than the previous record holder, S2, which had a closest approach of about 1,400 Schwarzschild radii.
The star's discovery was made possible by the GRAVITY+ instrument on the Very Large Telescope Interferometer, which has been observing the Galactic Center since 2017. The instrument uses a technique called interferometry to reconstruct images from data, allowing scientists to detect stars that would otherwise be missed by traditional fitting models. S301 was initially identified through its unique motion, which quickly indicated a tight orbit around the black hole.
One of the most intriguing aspects of this discovery is the star's extreme velocity. At 25,000 kilometers per second, it approaches the black hole at a speed that is more than 8% of the speed of light. This velocity is significant because it challenges our understanding of the black hole's spin. The paper estimates the effect of the black hole's spin on the star's orbit, suggesting that if the black hole were spinning at its maximum capacity, the star's pericenter would advance by 0.11 degrees per orbit. However, the current data is not precise enough to confirm this prediction.
The star's faintness and lack of a detected spectrum present additional challenges. It has a K-band magnitude of 19.3, making it five magnitudes fainter than S2. Without a spectrum, scientists cannot determine the star's mass or radial velocity, which are crucial for understanding its orbit. The paper suggests that the star is likely an early F-type main-sequence star with a mass between 1.1 and 1.5 solar masses, based on its magnitude and assumed distance.
The star's eccentricity of 0.98 is also noteworthy. Such a high eccentricity is difficult to achieve gradually, and the authors propose that S301 was likely delivered from elsewhere through the Hills process, where a tight binary star system is disrupted by the black hole's tidal field. This process could have occurred within the last few tens of millions of years, and the star's eccentricity would have thermalized, erasing any memory of its original orbit.
The discovery of S301 raises intriguing questions about the shape of spacetime around the nearest supermassive black hole. While it has helped pin down the black hole's mass, the measurement of its spin remains elusive. The paper highlights the challenges in determining the black hole's spin, including the need for more precise astrometric data and the potential influence of a stellar population around the orbit.
In conclusion, the discovery of S301 provides a unique opportunity to study the dynamics of a star orbiting a supermassive black hole at an extreme velocity. While it offers valuable insights into the black hole's mass and the star's orbit, the measurement of its spin remains a complex and challenging task. As scientists continue to observe and analyze S301, we can expect to uncover more fascinating details about the intricate dance between stars and black holes in the Milky Way's center.
