Closest Star Found Orbiting Milky Way's Central Black Hole

Researchers have identified a star on an extremely eccentric orbit that brings it closer to Sagittarius A*, the supermassive black hole at the center of the Milky Way, than any previously known object. This discovery, detailed in the March 13, 2024, issue of the journal Nature, offers a unique opportunity to measure the spin of Sagittarius A* for the first time. Scientists have been studying the stars orbiting Sagittarius A* since the 1970s to understand its mass and size. These stars act as crucial instruments, allowing astronomers to probe the environment around the black hole, which was only directly imaged in recent years. By analyzing the brightness and spectral features of these stars, researchers can estimate their masses. Combining these mass estimates with reconstructed orbital data spanning several years has enabled scientists to determine that Sagittarius A* has a mass of approximately 3.7 million times that of the Sun, or nearly 10^37 kilograms. The newly discovered star's orbit is particularly noteworthy for its high eccentricity, meaning it deviates significantly from a perfect circle. This extreme orbit will allow it to approach Sagittarius A* more closely than other observed stars. The proximity of this star to the black hole is expected to produce observable effects, such as relativistic beaming or gravitational redshift, that are directly influenced by the black hole's spin. Measuring these effects could provide the first direct empirical data on how fast Sagittarius A* is rotating. Understanding the spin of supermassive black holes is a key goal in astrophysics, as it provides insights into their formation history and how they interact with their surrounding environments, including the accretion of matter and the launching of powerful jets. The Milky Way's central black hole, Sagittarius A*, is located approximately 26,000 light-years from Earth. Its existence was inferred from observations of stellar orbits in the galactic center, and its properties have been refined over decades of study. The Event Horizon Telescope collaboration released the first image of Sagittarius A* in May 2022, providing visual confirmation of its presence and structure. This new stellar discovery complements previous observational efforts by offering a new avenue for detailed physical characterization of the black hole itself, moving beyond inferential methods to direct measurement of its rotational dynamics.