We have found the fastest known star in the Milky Way. In the extreme environment in the center of our galaxy, a newly discovered star called S4714 orbits the supermassive black hole Sagittarius A *.
On its orbit, the S4714 reaches a speed of about 8 percent of the speed of light – an absolute jaw output of 24,000 kilometers per second (15,000 miles per second). But it’s not even the most amazing thing.
S4714 is just one of a group of stars that have now been spotted obscure Sgr A * on closer orbit than any other stars spotted before.
Not only does this discovery suggest that there are even more stars in daring orbits around our galaxy’s supermassive black hole, it̵
The region in the middle of the Milky Way may be quiet compared to more active galactic centers, but even the environment around a quiet supermassive black hole can get quite wild.
Astronomers studying the region have identified a number of stars on long, winding, sharply elliptical orbits around Sgr A * – think of an oval, with the black hole at one end. These are called S-stars, and we can use them to investigate the properties of the gargantuan invisible object they orbit.
For several years, a star named S2 was considered the closest star to the black hole. At its nearest approach, or periapse, its 16-year orbit took it within about 18 billion kilometers of Sgr A *, the gravitational park from this close approach that accelerates the star to 3 percent of the speed of light. Careful work is required to observe and characterize this trajectory.
But last year, a team led by astrophysicist Florian Peissker at the University of Cologne in Germany found a much weaker but also much closer star: S62.
On a 9.9-year-old track, it grazes practically Sgr A * at a periap distance of 2.4 billion kilometers. It is closer than the average distance between Uranus and the sun. As it winds around, it reaches speeds of 20,000 kilometers per second (12,400 mps), or 6.7 percent of the speed of light.
But Peissker and his team were not ready. After several years of work, they have now discovered five new S stars even closer than S2 – S4711, S4712, S4713, S4714 and S4715.
“I’m glad I’ve been able to work with and observe (with the Very Large Telescope) the galactic center for the past seven years,” Peissker told ScienceAlert.
“Since then I have worked with SYMPHONY (almost infrared) data. You need knowledge of data reduction, a good eye, a little luck and time. And of course a good knowledge of low and high pass filters.”
Of the new stars, S4711 and S4714 are the huge awards.
S4711, a B-type blue star about 150 million years old, has an even shorter orbital period than S62; it goes around Sgr A * once every 7.6 years, with a periapse distance of 21.5 billion kilometers.
Although it does not foam so close, its shorter orbital period means that it has the shortest mean distance to the black hole through its entire trajectory that we have yet discovered.
At the same time, the S4714 has a longer orbital period than the S4711 – 12 years – but its orbit is extremely eccentric, which means that the elliptical shape is elongated; about as long as a stable path can actually be. Orbital eccentricity is described in values from 0 to 1, where 0 is a perfect circle and 1 is the flight path. S4714 has an orbital eccentricity of 0.985.
In periapse, it drags closer than S62 and comes within about 1.9 billion kilometers (SGR A *). During this close-up speed, the star accelerates up to 24,000 kilometers per second, and slows down as it turns back out to 250 billion kilometers from the black hole.
These extreme stars, Peissker said, are the first real candidates for squeezars, first theorized back in 2003.
Astrophysicists Tal Alexander and Mark Morris proposed a class of stars on very eccentric orbits around massive black holes. At each pass, the tidal forces convert a fraction of the star’s near orbital energy into heat. This firstly causes the star to shine more light than it normally would; and, secondly, contributes to the descent of the star. In other words, squeezars are dead stars orbiting.
“At least S4711 and S4714 are squeezar candidates,” Peissker said. “I would say I’m sure of the S4711 because the orbital elements are consistent with the predictions of Tal Alexander 2003. In this respect, the S4711 is the very first squeezer ever discovered.”
If confirmed, these stars can help us understand the interplay between black holes and the stars they (eventually) eat. But they also offer other opportunities.
For example, S2 has recently been used to test general relativity. Both how the starlight extends as it approaches the black hole, and how its orbit moves around like a spirograph, confirmed Einstein’s theory in some of the most strenuous tests yet.
“We are actually one size closer to Sgr A * and almost four times faster than S2 during its pericenter passage,” Peissker explained. “With that, we actually find stronger relativistic interactions with the stars S62, S4711 and S4714 as with S2.”
These tests have not yet been performed, and SYMPHONY has since been discontinued, so it may still take some time to obtain observations. But it’s really on the astrophysical radar.
So too is the search for more of these near stars. It is possible that even more extreme speeds and orbits may be hiding in the region around Sgr A * – and with more powerful telescopes to be launched in the coming years, including the extremely large telescope, we should be able to find them.
It’s just a matter of time.
“I’m constantly working on the galactic center and I’m pretty sure this was not our last publication,” Peissker said with a flashing facial emoji. “The high dynamic environment is for researchers like a candy store for children.”
The research has been published in The Astrophysical Journal.