Astrophysicists have discovered the fastest known star orbiting the black hole at the center of the Milky Way. The star, designated S4716, completes an orbit around the supermassive black hole Sagittarius A* (Sgr A*) in just four years.
This means the star is moving at incredibly fast speeds of around 18 million mph (29 million km/h), or nearly 5,000 miles per second. During this rapid orbit of Sagittarius A*, which is estimated to be 14.6 million miles (23.5 million km) in diameter, S4716 comes within 92 million miles (150 million km) of the supermassive black hole. .
While that may seem incredibly far away, it’s only 100 times the distance between Earth and the sun, which is a relatively small distance in cosmic terms. For example, the sun orbits Sgr A* at a distance of 26,000 light-years, with each light-year equaling 9.5 trillion kilometers.
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S4716 is part of a dense, tight group of stars called the S cluster that orbits near the galactic center and the Milky Way’s supermassive black hole. These stars of the S cluster all move particularly fast but vary in luminosity and mass.
The discovery of a star so close to Sgr* could change our understanding of the evolution of our galaxy and especially of its fast-moving central stars.
“The compact short-period orbit of S4716 is quite puzzling,” Masaryk University Brno astrophysicist Michael Zajaček said in a statement. (opens in a new tab). “Stars cannot form so easily near the black hole. S4716 had to move inwards, for example by approaching other stars and objects in the S cluster, which significantly reduced its orbit .”
Arguably the most famous star in the S cluster is S2, which has an orbital period around Sgr A* of 16 years and only ever approaches the supermassive black hole 11 billion miles (18 million kilometres). But while S2 has been incredibly useful for studying Sgr A*, it’s not always helpful.
“S2 behaves like a tall person sitting in front of you in a movie theater – it blocks your view of what’s important. The view of the center of our galaxy is therefore often obscured by S2,” Florian Peissker, astrophysicist at the University of Cologne and co-author of the new research, said in a statement. “However, in brief moments, we can observe the surroundings of the central black hole.”
By continuously refining the analytical techniques over two decades and combining them with 20 years of observations, Peissker and his team were finally able to confirm the rapid orbital period of S4716.
Five telescopes have observed S4716: the Hawaii-based Keck Observatory instruments NIR2 and OSIRIS, and the Very Large Telescope instruments SINFONI, NACO and GRAVITY, providing detailed data on the star.
“For a star to be in such a close and fast stable orbit near a supermassive black hole was completely unexpected and marks the limit that can be observed with traditional telescopes,” Peissker added.
The team’s research was published Tuesday, July 5 in The Astrophysical Journal.
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