Astronomers recently discovered the fastest and closest star ever seen orbiting Sagittarius A, the supermassive black hole at the heart of our Milky Way galaxy. This faint star, named S301, completes a full orbit in a mere 8.7 years. It zips past the colossal black hole at speeds up to 25,000 kilometers per second, or about 55 million miles per hour. This incredible speed is more than 8% of the speed of light. The discovery offers scientists a unique opportunity to study the extreme gravitational environment around Sagittarius A and test Albert Einstein's theory of general relativity.[space+4]
Record-Breaking Stellar Orbit
The star S301 travels on a highly stretched, elliptical path around Sagittarius A*, a black hole estimated to be 4.3 million times more massive than our Sun. At its closest point, S301 approaches the black hole at a distance of about 1.78 billion kilometers. This is roughly 12 times the distance between Earth and the Sun, or similar to the distance between the Sun and Saturn. This makes S301 the closest known star to the Milky Way's central black hole. Previous record holders had longer orbital periods, with another fast star, S2, taking 16 years for one lap.[livescience+7]
Researchers first spotted S301 in images captured in spring 2023 using the GRAVITY instrument on the European Southern Observatory's Very Large Telescope Interferometer (VLTI) in Chile. After identifying the star, the team looked through older observations, finding S301 in data from 2021 and 2017. This allowed them to trace out its complete, closed elliptical orbit over eight years.[livescience+2]
Felix Mang, a PhD student at the Max Planck Institute for Extraterrestrial Physics in Germany and a lead author of the study, highlighted the star's unique position. "What is special about this star is that it's orbiting Sagittarius A* on a very tight orbit, taking just 8.7 years to complete it, and is approaching the black hole at a mere 12 times the distance of Earth to the sun. That is unprecedented," Mang said.[space+2]
Unlocking Black Hole Secrets
S301's extremely close and fast orbit makes it an invaluable tool for studying the fundamental properties of black holes. Scientists predict that black holes, like many objects in the universe, spin. According to Einstein's general theory of relativity, a spinning black hole drags spacetime around with it, a phenomenon known as frame dragging or Lense-Thirring precession. This effect subtly warps the orbits of nearby stars.[eso+4]
Because S301 ventures so close to Sagittarius A*, its orbit is sensitive to this frame-dragging effect. Researchers believe that within about a decade, they will be able to measure the black hole's spin rate directly by observing subtle changes in S301's path. "For the first time, we would actually be able to measure very directly the spin of a massive black hole, which would be a key test of Einstein's theory," said Stefan Gillessen, a study co-author also from the Max Planck Institute.[livescience+5]
Previous observations of other stars, like S2, confirmed other predictions of general relativity, such as light losing energy as it escapes the black hole's gravity and the rotation of its elliptical orbit. However, these effects primarily depend on the black hole's mass. Measuring the black hole's spin requires a star that dives much closer, like S301.[livescience+1]
A Violent Cosmic History
The extreme and elongated orbit of S301 suggests a dramatic origin. Scientists propose that S301 might have once been part of a binary star system. This system could have wandered too close to Sagittarius A*. The black hole's powerful gravity then likely ripped the pair apart, capturing S301 while flinging its companion star out of the galaxy at tremendous speed.[livescience+2]
S301 itself appears to be an ordinary main-sequence star, about 1.5 times the mass of our Sun. Its compact nature means the black hole's tidal forces cannot easily tear it apart. The discovery of S301 adds to a densely packed group of stars known as the S cluster, which orbits close to the galactic center and the supermassive black hole. These S cluster stars move particularly fast, but S301 stands out for its record-breaking speed and proximity.[livescience+3]
Astronomers plan to continue monitoring S301 closely in the coming years. They hope to gather more data, especially during its next closest passage in 2031, to precisely measure the black hole's spin. This ongoing observation could reveal unprecedented details about the extreme physics at the heart of our galaxy. The insights gained from S301's orbit will significantly advance our understanding of how our galaxy evolved and how supermassive black holes influence their surroundings.[theguardian]
