A newly discovered star is so close to the central supermassive black hole in our galaxy that it completes an orbit in just four years.
This is the shortest orbit to date for one of the stars orbiting Sagittarius A *. It is an oval-shaped journey around the black hole that takes the star to orbital speeds in excess of 2.5 percent of the speed of light.
The discovery adds fascinating new information about the strange dynamics of 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 place. The black hole is a monster, with a mass about 4 million times that of the Sun. Before astronomers confirmed its existence with a direct image, scientists inferred its presence and calculated its mass from a star locked in orbit around Sgr A *.
This star, called S2, is just one of a group of stars known as S stars, which follow long, pronounced elliptical orbits around Sgr A *, with the black hole at one end of the ellipse. missing. This end, at which the star is closest to the black hole, is the periapsis, and the way stars change their velocities as they enter and exit the periapsis is one of the tools that helped “weigh “the black hole.
But S2 is far from the only star of the party.
A team of astrophysicists led by Florian Peissker of the University of Cologne in Germany has been looking for what else they could find in this strange, high-speed treasure.
“S2 behaves like an elderly person sitting in front of you in a movie theater – it blocks your view of what’s important,” Peissker explained. “The view of the center of our galaxy, therefore, is often obscured by S2. However, in brief moments we can observe the surroundings of the central black hole.”
The orbits of various S stars in the galactic center. (Peissker et al., ApJ, 2022)
Researchers discovered this star, called S4716, thanks to the evolution of observation and analysis techniques. It was clearly seen in the data of five different instruments in their dizzying orbit around Sgr A *.
The team estimated that its periapsis 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 periapsis, the star reaches a speed of about 8,000 kilometers (4,970 miles) per second.
This is neither the nearest nor the fastest star S in the galactic center. This honor belongs to a star called S4714, also discovered by Peissker and colleagues, which approaches Sgr A * up to 1.9 billion kilometers, reaching 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 average distance to the black hole along its entire orbit from any of the S stars discovered so far.
“That a star was in a stable orbit so close and fast in the vicinity of a supermassive black hole was completely unexpected and marks the limit that can be observed with traditional telescopes,” Peissker said.
The discovery addresses several curiosities in previous observations attributed to other S stars. However, S4716 presents a bit of a new mystery: it is not entirely clear how it got there. This, the researchers said, may need more work to solve.
“The compact, short-period orbit of S4716 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, approaching other stars and objects in the S cluster, which caused its orbit to shrink. significantly “.
The research has been published in The Astrophysical Journal.