A newly discovered star is so close to our galaxy’s central supermassive black hole that it completes one orbit in just four years.
This is the shortest orbit yet for any of the stars orbiting Sagittarius A*. It’s an oval-shaped journey around the black hole that takes the star to orbital speeds in excess of 2.5 percent the speed of light.
The discovery adds intriguing new information about the strange dynamics of the Milky Way’s center.
Although our galactic center is fairly quiet compared to other galaxies, the area around Sgr A* is an extreme place. The black hole is a monster that has about 4 million solar masses. Before astronomers confirmed its existence with a direct image, scientists inferred its presence and calculated its mass based on a star orbiting Sgr A*.
This star, called S2, is just one of a group of stars known as S stars that 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 the periapse, and the way stars change speed as they move in and out of the periapse is one of the tools that helped have to “rock” the black hole.
But S2 is far from the only star at the party.
A team of astrophysicists led by Florian Peissker of the University of Cologne in Germany has been looking to see what else they might find in this strange, high-speed treasure chest.
“S2 behaves like a large person sitting in front of you in a cinema: it blocks your view of the essentials,” explains Peissker. “The view into the center of our galaxy is therefore often obscured by S2. However, in brief moments we can observe the surroundings of the central black hole.”
The orbits of several S stars in the galactic center. (Peissker et al., ApJ, 2022)
Researchers discovered this star, named S4716, thanks to advances in observation and analysis techniques. It was clearly visible in data from five different instruments in its breakneck orbit around Sgr A*.
The team calculated that its periapse was about 15 billion kilometers (9.3 billion miles) from the supermassive black hole, about 100 times the distance between Earth and the Sun. As it approaches and enters the periapse, the star will reach a speed of about 8,000 kilometers (4,970 miles) per second.
This is neither the closest nor the fastest S star in the galactic center. That honor goes to a star called S4714, also discovered by Peissker and his colleagues, which comes within 1.9 billion kilometers of Sgr A* and can reach 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 over its entire orbit of any S star discovered to date.
“For a star in a stable orbit to be so close and fast to a supermassive black hole was completely unexpected and marks the limit that can be observed with conventional telescopes,” Peissker said.
The discovery clears up several oddities in previous observations attributed to other S stars. However, S4716 presents something of a new mystery: it’s not entirely clear how it got there. This, the researchers say, could take some more work to solve.
“S4716’s brief, compact orbit is quite puzzling,” said astrophysicist Michael Zajaček of Masaryk University in the Czech Republic.
“Stars cannot form so easily near the black hole. S4716 had to move inward, for example by getting closer to other stars and objects in the S cluster, causing its orbit to shrink significantly.”
The research was published in The Astrophysical Journal.