Scientists have discovered the fastest known star, orbiting at enormous speed around the supermassive black hole at the center of our galaxy, and could be the key to the first measurement of the rotation of this celestial giant.
The star, which is about 50 percent more massive than the Sun and five times brighter, is moving at speeds of up to 90 million kilometers per hour as it orbits the black hole at the center of the Milky Way, called Sagittarius A*, or Sgr A*. That's more than eight percent the speed of light. No other star in our galaxy or beyond has ever been recorded traveling at such a speed, Reuters reports.
The star, called S301, moves in a very elongated elliptical orbit around Sgr A*, taking about 8,7 years to complete one orbit. At its closest point to the black hole, it is about 11,5 times farther away than Earth is from the Sun, measured in astronomical units. This means that at its closest approach to the black hole, it is slightly further away than Saturn is from the Sun. However, at its farthest point, S301 is about 1.360 astronomical units away from Sgr A*.
Black holes are extremely dense celestial bodies whose gravity is so strong that not even light can escape them. Sgr A* has a mass about four million times that of the Sun and is located approximately 26.000 light-years from Earth. A light-year is the distance that light travels in one year, or about 9,5 trillion kilometers.
Although Sgr A* is currently in a relatively quiet state, its gravitational pull is enormous, meaning it can swallow stars or other material that gets too close. However, researchers say S301 should remain safe, Reuters reports.
"Its orbit is not going to shrink any time soon. So the orientation of the orbit will change, but the star is not in danger of being swallowed," said astrophysicist Stefan Gillezen of the Max Planck Institute for Extraterrestrial Physics in Germany, one of the leaders of the research published Wednesday in the journal "Nature."
No other star is known to orbit as close to Sgr A* as this one. So how did S301 end up in such an unusual orbit?
Gilezen says that S301 was likely originally part of a two-star system, a so-called binary system, in which it was gravitationally bound to a companion star. However, at some point, as the two stars traveled through space, they came too close to the black hole, and a three-way gravitational interaction completely changed their trajectories.
"One star is ejected from the center of the Milky Way at high speed and even leaves the galaxy. The other star remains trapped forever in orbit near the black hole. That's S301," Gilezen said.
The researchers observed S301 using the European Southern Observatory's Very Large Telescope Interferometer in Chile. The constant influence that Sgr A* exerts on S301 during its orbit could allow scientists to better understand the black hole, including the ability to measure its rotation rate.
"Currently, we know very little about the rotation of Sgr A*," said study co-author Felix Mang, a doctoral candidate at the Max Planck Institute for Extraterrestrial Physics.
Black holes are thought to rotate so strongly that they warp the very fabric of space and time around them. However, confirming this rotation is very difficult because black holes do not emit or reflect light, making them invisible, Reuters reports.
"We can only see their rotation by observing its effect on the space around them," Gilezen said.
The researchers say that the motion of S301 is still very sensitive to the influence of Sgr A*.
"The star is getting so close to Sgr A* during its orbit that it will actually feel its rotation," Gilezen said.
"It's not a wobble, like a back-and-forth motion, but rather the star is pushed into a slightly different orbit each time it passes the black hole. This means that by observing this orbital change over about another ten years, we should be able to directly determine the rotation of the black hole. Such a measurement has not been recorded before," Gilezen added.
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