We Have New Record For Fastest Star Enlarging Supermassive Black Hole

A newly discovered star is so close to the center of our galaxy’s supermassive black hole that it completes an orbit in just four years.

It is the shortest orbit for any of the stars around Sagittarius A*. This is an oval-shaped journey around a black hole that takes the star to an orbital speed of more than 2.5 percent of the speed of light.

This discovery adds exciting new information about the strange dynamics of the center of the Milky Way.

Although the center of our galaxy is quite quiet compared to other galaxies, the environment around Sgr A* is an extreme place. Black holes are monsters, which have a mass about 4 million times the mass of the Sun. Before astronomers confirmed its existence with live images, scientists deduced its existence and calculated its mass based on the star locked in orbit around Sgr A*.

The star, called S2, is just one of a group of stars known as S-stars, which follow a long, sharp elliptical orbit around Sgr A*, with a black hole at one end of the ellipse. That edge, where the star is closest to the black hole, is the periapse, and the way the star changes its speed as it moves in and out of the periapse is one tool that helps ‘weigh’ the black hole.

But S2 was far from the only star at the party.

A team of astrophysicists led by Florian Peissker at the University of Cologne in Germany have been looking to see what else they might find in this strange high-speed treasure trove.

“The S2 behaves like the big guy sitting across from you in the cinema: It blocks your view of what’s important,” explains Peissker. “The view of the center of our galaxy is therefore often obscured by S2. However, within a short time we can observe the surroundings of the central black hole.”

The orbits of several S stars at the center of the galaxy. (Peissker et al., ApJ, 2022)

The researchers discovered this star, named S4716, thanks to the evolution of observational and analytical techniques. This is evident in the data from five different instruments on their extremely dangerous orbits around Sgr A*.

The team calculated that the periapse is 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 periapse, the star reaches a speed of about 8,000 kilometers (4,970 miles) per second.

It’s not the closest, or fastest, S star at the galactic center. That honor goes to a star named S4714, also discovered by Peissker and his colleagues, which approaches Sgr A* by 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 an orbit of four years, has the shortest average distance to a black hole across its entire orbit of all S stars discovered to date.

“For a star to be in a stable orbit so close and fast around a supermassive black hole is completely unexpected and marks the limit that can be observed with traditional telescopes,” Peissker said.

This discovery smooths out some of the oddities in previous observations attributed to other S stars. However, the S4716 presents something of a new mystery: It’s not entirely clear how it got there. This, the researchers say, may take more work to solve.

“The compact orbit of S4716 in such a short time is quite puzzling,” said astrophysicist Michael Zajaček of Masaryk University in Czechia.

“Stars don’t form easily near black holes. S4716 has to move inward, for example by approaching other stars and objects in the S cluster, causing their orbit to shrink significantly.”

This research has been published in Astrophysics Journal.

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