The Fastest Star on Record Has Been Found Hurtling Past Our Galaxy's Black Hole : ScienceAlert

The heart of the Milky Way galaxy is an extreme environment where phenomena can become quite unpredictable. At the center, a supermassive black hole named Sagittarius A* resides, weighing in at approximately 4.3 million times the mass of the Sun. Despite being relatively modest for a supermassive black hole, its intense gravitational field causes extraordinary behaviors in nearby stars.

Astronomers have now identified an object that stands out in this chaotic region. Detailed in a new Nature paper, S301 follows an 8.7-year orbit around Sagittarius A* and reaches a peak speed of over 25,000 kilometers (15,534 miles) per second at its closest point to the black hole, a distance comparable to that between Saturn and the Sun. This speed equals 8.5 percent of the speed of light, enabling crucial measurements that could uncover Sagittarius A*’s properties, such as its spin and its compliance with the theory that black holes have no ‘hair’.

“The Galactic Center is like a remote laboratory, and the stars are the measuring probes,” explained Felix Mang from the Max Planck Institute for Extraterrestrial Physics in Germany, part of the GRAVITY+ collaboration that examined the star. “Our measurements improve as the star approaches Sagittarius A* more closely. Having a probe to measure the spinning spacetime curvature around this massive black hole is just really cool.”

Studying the black hole at the Milky Way’s center is challenging since black holes do not emit detectable light. The best method to understand them involves observing the behavior of surrounding matter. Sagittarius A* is surrounded by stars on elliptical orbits, which scientists have used to assess the black hole’s properties and test general relativity in a highly extreme environment.

One particularly useful star, S2, has provided valuable data. Its 16-year orbit, which in 2018 brought it within 17 light-hours of the galactic center, allowed astronomers to measure gravitational redshift and Schwarzschild precession, phenomena caused by the immense mass of Sagittarius A*.

S301, however, offers an even more thrilling journey. Its elongated orbit takes it about ten times closer to Sagittarius A* than S2, diving deeper into the black hole’s gravitational pull. This results in an acceleration to its top speed before it exits again, much like a rollercoaster.

At its closest point, or pericenter, S301 travels through spacetime distorted by the black hole’s spin, akin to swirling spaghetti with a fork. This effect, known as frame-dragging or the Lense-Thirring effect, should slightly alter S301’s orbit. By comparing the predicted and actual orbits, scientists can calculate Sagittarius A*’s spin.

This analysis could help test the no-hair theorem, which posits that an uncharged black hole’s gravitational field can be fully described by its mass and spin. “Our group has already measured the mass of Sagittarius A* to a sub-percent level,” Mang noted. “Determining the spin of such a massive black hole would be unprecedented.”

While measuring spin alone isn’t sufficient, observing S301’s orbit could reveal another aspect of Sagittarius A*’s gravitational field called its quadrupole moment. General relativity predicts this measurement based on the black hole’s mass and spin. If observed data matches predictions, it would validate the no-hair theorem.

S301’s orbit suggests it may have been part of a binary system that came too close to Sagittarius A*. The black hole could have separated the pair, capturing one star in a tight orbit while ejecting the other at high speed, a process known as the Hills mechanism. Discovering its former companion would bolster this theory.

There isn’t long to wait, as S301 will reach pericenter again toward the end of 2031, when the effect of the spin on its orbit will be most pronounced.

The GRAVITY+ collaboration will continue to monitor S301, and the upcoming MICADO instrument on the European Southern Observatory’s Extremely Large Telescope will aid in reconstructing its motion in three dimensions. Together, these observations could determine Sagittarius A*’s spin within the next decade.

“We expect more stars like S301 to exist in the Galactic Center, and if we discover a similar one, it would facilitate a spin measurement even more,” Mang said. “The strength of the signal would no longer depend only on the relative orientation of the black hole’s spin axis and the orbit of S301.”

The findings are published in Nature.

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