A newly discovered star is so close to our galaxy’s central supermassive black hole that it completes an orbit in just four years.
It is the shortest orbit to date for one of the stars that winds around Sagittarius A *. It is an oval-shaped journey around the black hole that takes the star to orbital speeds above 2.5 percent of the speed of light.
The discovery adds fascinating new information about the strange dynamics in the center of the Milky Way.
Although our galactic center is fairly quiet compared to other galaxies, the environment around Sgr A * is an extreme kind of place. The black hole is a monster that is about 4 million times the mass of the sun. Before astronomers confirmed its existence with a direct imageresearchers concluded its presence and calculated its mass based on a star locked in orbit around Sgr A *.
That star, called S2, is just one of a group of stars called S stars, which follow long, sharply elliptical orbits around Sgr A *, with the black hole at one end of the ellipse. The end where the star is closest to the black hole is periapseand how the stars change their velocities as they move in and out of the periapse is one of the tools that helped “weigh” the black hole.
But S2 is far from the only star at the party.
A team of astrophysicists led by Florian Peissker at the University of Cologne in Germany has been looking to see what else they can emerge in this strange, high-speed treasure trove.
“S2 behaves like a big person sitting in front of you in a movie theater: It blocks your view of what’s important,” Fireplaces explained. “The view into the center of our galaxy is therefore often obscured by S2. But in brief moments we can observe the surroundings of the central black hole.”
Scientists discovered this star, called S4716, thanks to the development of observation and analysis techniques. This was clearly seen in data from five different instruments on its neck-breaking path around Sgr A *.
The team estimated that its circumference was about 15 billion kilometers (9.3 billion miles) from the supermassive black hole, about 100 times the distance between the earth and the sun. As it approaches and enters the periapse, the star reaches a speed of about 8,000 kilometers (4,970 miles) per second.
It is not the closest, nor the fastest, S-star in the galactic center. That honor belongs to a star named S4714, also discovered by Peissker and his colleagueswhich comes as close to Sgr A * as 1.9 billion kilometers, and reaches speeds of up to 24,000 kilometers per second.
However, S4714 has an orbital period of 12 years. S4716, with its four-year orbit, has the shortest mean distance to the black hole throughout its orbit of any of the S stars discovered so far.
“That a star would be in a stable orbit so close and fast near a supermassive black hole was completely unexpected and marks the limit that can be observed with traditional telescopes.” said Peissker.
The discovery sorts out several oddities in previous observations attributed to other S-stars. S4716, however, presents something of a new mystery: It is not entirely clear how it ended up there. This, the researchers said, could take a little more work to solve.
“The short, compact orbit of the S4716 is quite puzzling,” and astrophysicist Michael Zajaček at Masaryk University in the Czech Republic.
“Stars cannot form so easily near the black hole. S4716 had to move inward, for example by approaching other stars and objects in the S cluster, which caused its orbit to shrink considerably.”
The research has been published in The Astrophysical Journal.
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