Astrophysicists have identified the fastest known star moving around a galaxy’s black hole. In just four years, the star S4716 completes one orbit around the supermassive black hole Sagittarius A*.
This indicates that the star is moving at about 18 million miles per hour (29 million kilometers per hour), or about 5,000 miles per second. S4716 comes as close as 9.2 billion miles (15 billion kilometers) to the supermassive black hole during this accelerated orbit of Sagittarius A*, which has an estimated diameter of 14.6 million miles (23.5 million kilometers).
Although this may seem incredibly distant, it is just 100 times the distance between Earth and the Sun, which is a relatively short distance in cosmological terms. For example, the Sun orbits Sgr A* at a distance of 26,000 light-years, with each light-year being equal to 5.9 trillion miles (9.5 trillion km).
S4716 is part of a dense, tightly packed group of stars called the S cluster that orbits close to the Milky Way’s center and the Milky Way’s supermassive black hole. These S cluster stars all move particularly fast but vary in brightness and mass.
“S4716’s short-period, compact orbit is quite puzzling,” astrophysicist Michael Zazek of Masaryk University in Brno said in a statement. “Stars cannot form so easily near a black hole. S4716 had to move inward, for example by approaching other stars and objects in the S cluster, causing its orbit to shrink significantly.
The discovery of a star so close to Sgr* could change our understanding of how our galaxy has evolved, and in particular its fast-moving central stars.
The most famous star in the S-cluster is arguably S2, which has an orbital period of 16 years around that of Sgr A* and only comes as close to the supermassive black hole as 11 billion miles (18 million km). But while S2 has been incredibly useful for the study of Sgr A*, it is not always helpful.
“S2 behaves like a large person sitting in front of you in a movie theater – it blocks your view of what is important. So the view at the center of our galaxy is often obscured by S2,” says Florian Peisker, University of Cologne an astrophysicist in the U.S. and a co-author on the new research, said in a statement. “However, in brief moments we can observe the surroundings of the central black hole.”
By continually refining analytical techniques over two decades and combining them with 20 years of observations, Peissker and his team were finally able to confirm S4716’s rapid orbital period. Five telescopes observed S4716: the Hawaii-based Keck Observatory instruments NIR2 and OSIRIS, and the Very Large Telescope Instruments SINFONI, NACO and GRAVITY, which provide detailed data on the star.
“For a star to be in a stable orbit so close and in the vicinity of a supermassive black hole was completely unexpected and marks the limit to what can be observed with conventional telescopes,” Pesker said.
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