New Star Discovery Near Milky Way’s Supermassive Black Hole Could Reveal Its Spin and Origin
At the center of the Milky Way lies a supermassive black hole named Sagittarius A*, with a mass four million times that of our Sun and located about 27,000 light-years away. Despite its size and relative proximity, much about it remains unknown, including how it formed and how fast it spins. The black hole’s existence was confirmed only about 30 years ago, a discovery that earned Andrea Ghez and Reinhard Genzel the 2020 Nobel Prize in Physics. In 2022, the first image of the region around Sagittarius A* was published, yet many questions persist.
A recent breakthrough involving Israeli scientists may help answer these questions through the discovery of a previously unknown star, designated S301, orbiting extremely close to the black hole. Professor Haggai Perets of the Technion, one of the Israeli co-authors alongside Professors Zvi Piran and Ram Sari from the Hebrew University and others including Genzel, called this "the most important star discovery in the galactic center since the star S2," which was crucial in confirming the black hole’s presence. S301’s orbit will allow direct measurement of the black hole’s spin, a feat never before achieved.
The star S301 was detected using the European Southern Observatory’s Very Large Telescope in Chile, employing the GRAVITY instrument that combines four 8-meter telescopes to achieve the resolution of a 130-meter telescope. Observations in infrared wavelengths penetrated the dense dust near the black hole. S301 completes an elliptical orbit every 8.7 years, with its closest approach about 1.7 billion kilometers from Sagittarius A*. Its proximity subjects it to relativistic effects, enabling scientists to measure changes in its orbit caused by the black hole’s spin over one or two orbits, potentially within 10 to 16 years.
Sari explained that measuring the spin will shed light on the black hole’s formation: a rapid spin would suggest formation from the merger of two large black holes, while a slower spin would indicate multiple mergers of smaller black holes. The star’s presence so close to the black hole is puzzling, as star formation is unlikely in such an environment. The team hypothesizes that S301 was once part of a binary system disrupted by the black hole’s gravity, which ejected one star far from the center and captured the other in a tight orbit.
This discovery complements previous research by the Israeli team on hypervelocity stars ejected from the galactic center. The ongoing GRAVITY collaboration aims to find more stars near Sagittarius A*, which will improve measurements of the black hole’s properties, including mass, spin, and distance from Earth. These studies also offer a nearby laboratory to test general relativity under extreme gravity and to better understand the environments of supermassive black holes in other galaxies.
Professor Sari called the discovery "very exciting," as it confirms theories about binary star disruptions near black holes and promises to refine our understanding of the galactic center’s dynamics.
