Fastest-known star zooms under Milky Way black hole's influence

WASHINGTON — Scientists have detected the fastest-known star, a stellar speedster that is zooming around the supermassive black hole residing at the center of our galaxy and might hold the key to measuring the spin of this celestial beast for the first time. The star, around 50% more massive and five times brighter than our sun, travels at speeds up to about 56 million miles (90 million km) per hour as it orbits the Milky Way's central black hole—named Sagittarius A*, or Sgr A*. That is more than 8% the speed of light. No other star in our galaxy or elsewhere has ever been clocked at that speed. The star, named S301, is traveling in a highly elliptical path around Sgr A*, taking about 8.7 years to complete an orbit. Its closest approach to the black hole is about 11.5 times Earth's orbital distance from the sun, a metric called an astronomical unit. That means the star's nearest pass is somewhat more than the planet Saturn's orbital distance from the sun. But at its farthest, S301 is about 1,360 astronomical units from Sgr A*. Black holes are extraordinarily dense objects with gravity so strong that not even light can escape. Sgr A* possesses about 4 million times the sun's mass and is located about 26,000 light-years from Earth. A light-year is the distance light travels in a year, 5.9 trillion miles (9.5 trillion km). While Sgr A* presently is in a quiescent state, its gravitational pull is considerable—meaning it could gulp down stars or other material straying too close. But the researchers said S301 should remain safe from the black hole's wrath. "Its orbit will not decay anytime soon. So the orientation of the orbit will change, but it is not in danger of being swallowed," said astrophysicist Stefan Gillessen of the Max Planck Institute for Extraterrestrial Physics in Germany, one of the leaders of the research published on 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 with this crazy orbit? Gillessen said it appears that S301 originally was part of a two-star system called a binary, gravitationally bound to a stellar partner. But the two stars at some point ventured too close to the black hole while traveling through space, and the subsequent three-way gravitational interaction wreaked havoc. "One star shoots out from the center of the Milky Way traveling at high speed and even leaving the galaxy. The other star is stuck forever on an orbit close to the black hole. That is S301," Gillessen said. The researchers observed S301 using the European Southern Observatory's Chile-based Very Large Telescope Interferometer. The continuing influence that Sgr A* exerts on S301 during its orbit may enable scientists to better understand the black hole, including possibly measuring its spin. "Currently, there is very little that we know about the rotation of Sgr A*," said study co-author Felix Mang, a doctoral student at the Max Planck Institute for Extraterrestrial Physics. Black holes are believed to rotate, so much so that they twist the very fabric of space and time around them. But confirming this spin is difficult because black holes do not emit or reflect light, rendering them invisible. "We only can see their rotation by seeing the effect on the space around them," Gillessen said. The researchers said that the motion of S301 remains sensitive to Sgr A*. "The star during its orbit comes so close to Sgr A* that it actually will feel the spin of Sgr A*," Gillessen said. "It is not a wobble—like in back and forth—but rather the star gets pushed onto a slightly different orbit each time it passes the black hole. That means that by observing that change in orbit for another good decade, we should be able to directly determine the spin of the black hole. Such a measurement is unheard of, so far," Gillessen added. — Reuters