The supermassive black hole at the heart of our galaxy isn’t just spinning — it’s doing so at nearly maximum speed, dragging anything near it along for the ride.
Physicists have calculated the rotational speed of the Milky Way’s supermassive black hole, Sagittarius A*, using NASA’s Chandra X-ray Observatory to observe the X-rays and radio waves emanating from outflows of material.
The spin speed of a black hole is defined as “a” and given a value from 0 to 1, with 1 being the maximum rotational speed for a particular black hole, a significant fraction of the speed of light. Physicists at Penn State, led by Ruth A. Daly, found that the rotational speed of Sgr A* is between 0.84 and 0.96, close to the upper limit defined by a black hole’s width. The research team described Sgr A*’s rapid speed in a study published in the journal Monthly Notices of the Royal Astronomical Society.
“Discovering that Sgr A* is rotating at its maximum speed has far-reaching implications for our understanding of black hole formation and the astrophysical processes associated with these fascinating cosmic objects,” said Xavier Calmet, a theoretical physicist at the University of Sussex, who was not involved in the research, in an email to Live Science.
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Black holes are such a drag
A black hole’s spin is different from those of other cosmic objects. While planets, stars, and asteroids are solid bodies with physical surfaces, black holes are actually regions of space-time bounded by an outer non-physical surface called the event horizon, beyond which no light can escape.
“While the rotation of a planet or star is governed by the distribution of its mass, the rotation of a black hole is described by its angular momentum,” Calmet said. “Due to the extreme gravitational forces near a black hole, the rotation causes spacetime to become highly curved and twisted, forming what is known as the ergosphere. This effect is unique to black holes and does not occur with solid bodies like planets or stars.”
That means that when they spin, black holes literally twist up the very fabric of space-time and drag anything within the ergosphere along.
This phenomenon, called “frame dragging” or the “Lensing-Thirring effect,” means that to understand the way space around a black hole behaves, researchers need to know its spin. This frame dragging also gives rise to weird visual effects around black holes.
“As light travels close to a rotating black hole, the rotation of spacetime causes the light’s path to be curved or twisted,” Calmet said. “This results in a phenomenon called gravitational lensing, where the light’s trajectory is bent due to the gravitational influence of the rotating black hole. The frame-dragging effect can lead to the formation of light rings and even the creation of the black hole’s shadow. These are manifestations of the gravitational influence of black holes on light.”
The theoretical top speed of a black hole is determined by how it feeds on matter and thus how it grows.
“As matter falls into a black hole, it increases the black hole’s spin, but there’s a limit to how much angular momentum it can possess,” Calmet said. “Another factor is the mass of the black hole. More massive black holes have a higher gravitational pull, making it more challenging to increase their spin.”
“Additionally, the interaction between the black hole and its surroundings, such as accretion disks, can transfer angular momentum and affect the black hole’s spin,” he added.
This could explain why Sgr A*, with its mass equivalent to around 4.5 million suns, has a spin speed between 0.84 and 0.96 but the supermassive black hole at the heart of galaxy M87 is spinning at a rate of 0.89 to 0.91, despite having the mass of 6.5 billion suns.
