The fastest star ever detected is moving at about 8% the speed of light. Astronomers have dubbed it S4714.
The fastest known star, S4714, orbits close to the supermassive black hole at the center of the Milky Way. It’s moving at more than 8% of light speed, or 15,000 miles per second (24,000 km/second). That’s faster than any other known star.
At the center of our galaxy lies a supermassive black hole about four million times more massive than our Sun. All galaxies have one of these beasts.
The Milky Way may have a quiet center compared to more active galactic centers, but even the environment around a quiescent supermassive black hole can get pretty wild.
This black hole called Sagittarius A* isn’t alone. A variety of objects, including dozens of massive stars, exist there. These, orbit so close to the black hole that the enormous gravity of the black hole swings them around at enormous velocities.
And because the center of the Milky Way is a crowded region it’s very hard to find stars there.
Most of these stars have large enough orbits that scientists describe their motion by Newtonian gravity and Kepler’s laws of motion. But a few, orbit so closely that their orbits can only be accurately described by Einstein’s theory of general relativity.
S4714 is just one of a group of stars that we have now discovered skimming Sgr A* on closer orbits than any other stars discovered previously.
This discovery suggests there are even more stars on daredevil orbits around our galaxy’s supermassive black hole. But it has also given us the first candidates for a type of star originally proposed nearly 20 years ago – those that orbit so close to a black hole, they are ‘squeezed’ by its tidal forces. Astronomers refer to them as ‘squeezars’.
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Another star orbiting close to Sagittarius A*, called S2, was once considered to be the fastest star. At its closest approach or periapse, its 16-year orbit brought it within around 11 billion miles (18 billion kilometers) of the supermassive black hole, the gravitational kick from this close approach accelerating the star to 3 percent of the speed of light.
But in 2019, a team led by Florian Peissker, an astronomer at the University of Cologne in Germany, detected a new speedy contender, the star S62. At the time, the researchers thought S62 was the closest star speeding around the supermassive black hole. On a 9.9-year orbit, it practically grazes Sagittarius A* at a periapse distance of 2.4 billion kilometers. That’s closer than the average distance between Uranus and the Sun. As it orbits around, it reaches velocities of 12,400 m/s (20,000 kilometers per second) or 6.7 percent of the speed of light.
But last year, the same team has discovered five new “S stars,” or stars that travel in long, elliptical orbits around Sagittarius A*, that are even closer — S4711, S4712, S4713, S4714, and S4715.
S4714 was the fastest of stars just absolutely cannonballing past Sagittarius A* at 8% lightspeed. It takes this star 12 years to orbit around the black hole. Even though it takes it longer than S62 star to complete one revolution around the black hole, its orbit is extremely eccentric.
At periapse, S4714 skims closer than S62, coming within about 1.2 billion miles (1.9 billion kilometers) of Sagittarius A*.
The team has been studying our galaxy’s center and the ultra-fast stars orbiting its supermassive black hole using the ESO’s Very Large Telescope in Chile for the past seven years. They have also used near-infrared data from SINFONI (Spectrograph for INtegral Field Observations in the Near Infrared).
S4714 is so close to Sagittarius A* and so faint that even the world’s biggest infrared telescopes have a hard time picking it out of the crowd of stars on that spot in the sky. But there may be stars even closer still, perhaps just less massive and too faint to spot.
However, there’s a limit to how close they can get without getting shredded to pieces. It’ll be very exciting to find out just how close these stars get. Hopefully, future, more advanced telescopes will have an even better view.
The discovery of these stars could help us understand the interactions between black holes and the stars they (eventually) devour.
But they have also helped astronomers test general relativity. For this, they used S2 star. Both the way the star’s light stretches when it approaches the black hole, and the way its orbit shifts around like a spirograph, confirmed Einstein’s theory in some of the most difficult tests yet.
The research has been published in The Astrophysical Journal.