A newly discovered star is so close to our galaxy’s central supermassive black hole that it completes an orbit in just four years.
This is the shortest orbit to date for any of the stars looping around Sagittarius A*. It’s an oval-shaped trip around the black hole that takes the star to orbital speeds greater than 2.5% 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 quite quiet compared to other galaxies, the environment around Sgr A* is an extreme place. The black hole is a monster, clocking in at about 4 million times the mass of the Sun. Before astronomers confirmed its existence with a direct image, scientists inferred its presence and calculated its mass based on a star stuck in orbit around Sgr A*.
This star, called S2, is one of a group of stars known as S stars, which follow long, highly elliptical orbits around Sgr A*, with the black hole at one end of the ellipse. That end, where the star is closest to the black hole, is the periapsis, and the way stars change speed 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 from the University of Cologne in Germany set out to see what else they might find in this strange high-speed treasure trove.
“S2 behaves like a tall person sitting in front of you in a movie theater: it blocks your view of what’s important,” Peissker explained. “The view towards 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 researchers discovered this star, named S4716, thanks to the evolution of observation and analysis techniques. It was clearly seen in data from five different instruments in its dizzying orbit around Sgr A*.
The team calculated that its periapsis 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 periapsis, the star reaches a speed of about 8,000 kilometers (4,970 miles) per second.
It is not the closest nor fastest S star to the galactic center. That honor goes to a star named S4714, also discovered by Peissker and his colleagues, which is approaching Sgr A* at 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 over its entire orbit of any S star discovered to date.
“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 said.
The discovery fixes several quirks 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, according to the researchers, may take even more work to resolve.
“S4716’s compact, short-period orbit is quite puzzling,” said astrophysicist Michael Zajaček of Masaryk University in Czechia.
“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 .”
The research has been published in The Astrophysical Journal.