Things can become truly bizarre in the extreme conditions at the center of the Milky Way galaxy. And by bizarre, we mean phenomena that are almost impossible to comprehend.
At the heart of the galaxy lies a supermassive black hole known as Sagittarius A*, with a mass roughly 4.3 million times greater than that of our Sun.
Although Sagittarius A* is relatively small compared with other supermassive black holes, its intense gravitational pull creates extreme conditions that can cause nearby stars to behave in extraordinary ways.
The Fastest Star Ever Seen Orbiting a Black Hole
Astronomers have now identified an object that surpasses all the others in speed. According to a new paper published in Nature, S301 follows an intense 8.7-year orbit around Sagittarius A*, reaching a staggering speed of over 25,000 kilometers (15,534 miles) per second at its closest point to the black hole—roughly the same distance as Saturn’s orbit around the Sun.
That astonishing velocity corresponds to about 8.5 percent of the speed of light. Because S301 passes so close to Sagittarius A*, astronomers can now make highly precise observations that could uncover some of the black hole’s most difficult-to-measure characteristics, including its spin and, eventually, whether it follows the famous “no-hair” theorem.
“The Galactic Center is essentially a distant laboratory, with the stars serving as our measurement tools,” astrophysicist Felix Mang of Germany’s Max Planck Institute for Extraterrestrial Physics, who is part of the GRAVITY+ collaboration that studied S301, told ScienceAlert.
“Our observations can only become as precise as the star that comes closest to Sgr A*. Now that we effectively have a natural probe allowing us to study the way this massive black hole’s spin warps spacetime, we have an incredibly exciting tool for investigating its environment.”
Why Sagittarius A* Is So Difficult to Study
Studying the black hole at the center of the Milky Way is far from straightforward. Since black holes themselves produce no detectable light, astronomers have to rely on how nearby matter behaves to learn about them.
And the region surrounding Sgr A* offers plenty to study. A dense group of stars orbits the black hole along elongated, elliptical paths, giving scientists valuable opportunities to determine its properties and test Einstein’s theory of general relativity under some of the most extreme conditions observable in the Universe.
One star in particular, S2, has proven to be an exceptionally valuable tool for astronomers. Its 16-year orbit brought it within just 17 light-hours of the galactic center in 2018, allowing researchers to measure gravitational redshift and Schwarzschild precession—effects caused by the immense mass of Sgr A*.
S301 Plunges Deep Into Sgr A*’s Extreme Gravity
But S301 takes this extreme cosmic journey to an entirely different level. Its highly elongated orbit brings it roughly 10 times closer to Sgr A* than S2, sending the star deep into the black hole’s powerful gravitational field. Like a roller coaster speeding into a steep drop, S301 reaches its incredible maximum velocity as it sweeps through this region before rapidly moving away again.
And, much like a roller coaster, S301’s journey is fascinating not simply because of its incredible speed, but because of what it encounters along the way. At its closest point, known as pericenter, the star passes near enough to Sgr A* to move through spacetime that is itself being twisted by the black hole’s rotation—similar to stirring a bowl of spaghetti with a fork.
This phenomenon is known as frame-dragging, or the Lense–Thirring effect.
This twisting of spacetime should cause a very small change in S301’s orbit. Astronomers can accurately model the star’s trajectory as it would appear without the influence of the Lense–Thirring effect. By comparing that theoretical orbit with the star’s actual path, they can determine how strongly Sgr A* is spinning.
S301 Could Put the No-Hair Theorem to the Test
This could eventually allow scientists to test the no-hair theorem, which proposes that for a virtually uncharged black hole, knowing its mass and spin should be enough to predict every other characteristic of its gravitational field.
“Our team has already determined the mass of Sgr A* with an accuracy better than one percent,” Mang told ScienceAlert. “The next major step toward testing the no-hair theorem is measuring its spin, particularly for a black hole of this enormous size. That would be something we have never achieved before.”
However, determining the black hole’s spin by itself would not be enough to confirm the theorem. With continued observations, S301’s orbit could eventually help scientists measure another important characteristic of Sgr A*’s gravitational field: its quadrupole moment.
General Relativity Sets a Precise Test for Sgr A*
According to general relativity, the black hole’s mass and spin determine a precise value for its quadrupole moment. If astronomers’ observations agree with that theoretical prediction, Sgr A* would successfully pass a rigorous test of the no-hair theorem.
S301 may also have another extraordinary story hidden in its past. Its highly elongated orbit suggests that the star could once have belonged to a binary system that passed dangerously close to Sgr A*. Through a process known as the Hills mechanism, the black hole can rip a binary pair apart, trapping one star in a tight orbit while ejecting its companion at enormous speed.
This scenario could explain how S301 acquired its extreme current orbit, although the star’s trajectory alone cannot prove that this happened. Discovering its missing companion speeding away from the Galactic Center would offer much stronger evidence for the theory.
Fortunately, S301’s relatively short orbital period means astronomers will not have to wait decades for another opportunity. Astronomers expect the star to reach pericenter again in late 2031, when Sgr A*’s spin should exert its strongest influence on the star’s orbit.
The GRAVITY+ collaboration plans to keep monitoring S301, while the forthcoming MICADO instrument aboard the European Southern Observatory’s Extremely Large Telescope is expected to give researchers a much clearer three-dimensional picture of the star’s trajectory.
Read the original article on: Sciencealert
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