The supermassive black hole at the center of our galaxy, known as Sagittarius A* (Sgr A*), is spinning at an incredibly high speed, dragging anything in its vicinity along with it. Physicists have used NASA’s Chandra X-ray Observatory to observe the X-rays and radio waves emitted from the outflows of material surrounding Sgr A* to calculate its rotational speed.
The rotational speed of a black hole is denoted as “a” and ranges from 0 to 1, with 1 representing the maximum rotational speed for a particular black hole, approaching the speed of light. In a study published in the journal Monthly Notices of the Royal Astronomical Society, physicist Ruth Daly from Penn State and her colleagues determined that Sgr A* has a rotational speed between 0.84 and 0.96, approaching the upper limit defined by a black hole’s width.
This discovery has significant implications for our understanding of black hole formation and the astrophysical processes associated with these enigmatic cosmic objects. Xavier Calmet, a theoretical physicist from the University of Sussex who was not involved in the research, explained that Sgr A*’s maximum rotational speed sheds light on how black holes form and the unique phenomena that occur near them.
Black holes, unlike other celestial bodies such as planets and stars, have a different type of spin. While the rotation of planets and stars is determined by the distribution of their mass, a black hole’s rotation is described by its angular momentum. Due to the extreme gravitational forces near a black hole, its rotation causes spacetime to become highly curved and twisted, forming what is known as the ergosphere. This effect, unique to black holes, does not occur with solid bodies like planets or stars.
As a result, when black holes spin, they twist the fabric of space-time itself and drag anything within the ergosphere along with them. This phenomenon, referred to as “frame dragging” or the “Lensing-Thirring effect,” introduces peculiar visual effects around black holes. Light traveling close to a rotating black hole experiences its path being curved or twisted, a phenomenon called gravitational lensing. The frame-dragging effect can create light rings and even the black hole’s shadow, showcasing the gravitational influence of black holes on light.
The maximum speed of a black hole is determined by its feeding habits and growth. As matter falls into a black hole, it increases its spin, but there is a limit to how much angular momentum it can possess. Additionally, the mass of the black hole plays a role, with more massive black holes having a stronger gravitational pull, making it more challenging to increase their spin. The interaction between the black hole and its surroundings, such as accretion disks, can also transfer angular momentum and impact the black hole’s spin.
This may explain why Sgr A*, with a mass equivalent to around 4.5 million suns, has a rotational speed between 0.84 and 0.96. In contrast, the supermassive black hole at the heart of galaxy M87, the first black hole ever to be photographed, has a mass of 6.5 billion suns but spins at a speed between 0.89 and 0.91.
Understanding the rotational speed of black holes provides valuable insights into their formation and the effects they have on their surroundings. The observations made using NASA’s Chandra X-ray Observatory contribute to our knowledge of these fascinating cosmic objects and broaden our understanding of the universe.
