We found the fastest known star in the Milky Way. In the extreme environment at the center of our galaxy is a newly discovered star named S4714 around the supermassive black hole Sagittarius A *.
On its orbital journey, S4714 reaches a speed of about 8 percent of the speed of light – an absolutely key-dripping 24,000 kilometers per second (15,000 miles per second). But that is not even the most amazing thing.
S4714 is just one of a group of stars now discovered Sgr A * in closer orbit than other stars previously discovered.
Not only does this discovery suggest that there are even more stars on dangerous orbits around the supermassive black hole of our galaxy, it has given us the first candidates for a type of star that was originally proposed nearly 20 years ago – those that ‘ t come so close to a black hole, they are ‘pressed’ by its tidal forces. They are also known as ‘squeezars’.
The region in the center of the Milky Way may be quiet compared to more active galactic centers, but even the environment around a quiet supermassive black hole can become quite wild.
Astronomers studying the region have identified a number of stars on long, running, sharp 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 giant invisible object that they orbit.
(N. Vogt / NMSU)
For years, a star called S2 was considered the closest star to the black hole. At its closest approach, as a periapse, its 16-year orbit brought it within about 18 billion kilometers of Sgr A *, the gravitational stop of this narrow approach that accelerates the star to 3 percent of the speed of light. It has been worked more closely to observe and characterize this job.
But last year, a team led by astrophysicist Florian Peissker of the University of Cologne in Germany found a much louder but also much closer star: S62.
On a 9.9-year-old orbit, the Sgr A * practically grazes at a peripheral distance of 2.4 billion kilometers. That is closer to the average distance between Uranus and the Sun. As it orbits, 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 done. After 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 had the opportunity to work and observe with the Very Large Telescope (the Very Large Telescope) for the last 7 years,” Peissker told ScienceAlert.
“Since then I have been working on the SINFONI (near-infrared) data. You need data reduction skills, a good eye, some luck and time needed. And of course a good knowledge of low and high pass filter.”
Of the new stars, S4711 and S4714 are the enormous standouts.
S4711, a blue star of type B about 150 million years old, has an even shorter orbital period than S62; it orbits Sgr A * once every 7.6 years, with a peripheral distance of 21.5 billion kilometers.
Although it does not shimmer so closely, its shorter orbital period means that it has the shortest average distance to the black hole through its entire orbit that we have yet to discover.
Meanwhile, S4714 has a longer orbital period than S4711 – 12 years – but its orbit is extremely eccentric, which means that the elliptical shape is long; about as long as a stable job can get, actually. Orbital eccentricity is described in values from 0 to 1, where 0 is a perfect circle, and 1 is an escape path. S4714 has an orbital eccentricity of 0.985.
At periapse it runs closer than S62, and reaches within about 1.9 billion kilometers (Sred A *). During this close approach, the star accelerates to 24,000 kilometers per second, slower as it swings back 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 forces of time convert a fraction of the star’s close orbital energy into heat. This first of all makes the star brighter than normal; and, second, contributes to the orbital decay of the star. In other words, squeezars are dead stars that surround.
“At least S4711 and S4714 are squeezar candidates,” Peissker said. “I would say, I’m sure about S4711 since the orbital elements match Tal Alexander’s predictions in 2003. In that sense, S4711 is the oldest squeezar ever discovered.”
As confirmed, these stars could help us to understand the interactions between black holes and the stars they (eventually) wear out. But they also offer other opportunities.
S2, for example, has recently been used to test general relativity. Both the way the star’s light strikes as it approaches the black hole, and the way its orbit turns around like a spirographer, confirmed Einstein’s theory in some of the most impressive tests.
“We are actually one big 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 well as with S2.”
These tests have yet to be performed, and SYMPHONY has since been discontinued, so obtaining observations may still take some time. But it’s definitely on the astrophysical radar.
That is also the search for more of these closed stars. It is possible that even more extreme speeds and orbits in the region around Sgr A * could be obscured – and with more powerful telescopes launching in the coming years, including the Extremely Large Telescope, we could find them.
It’s all 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 waving face emoji. “The highly dynamic environment is for scientists like a candy store for children.”
The study was published in The Astrophysical Journal.
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