Understanding the Spin of Celestial Bodies and Black Holes

The cosmos is a complex ballet of spinning entities. Whether it's asteroids, planets, moons, or even the enigmatic black holes, each has a spin unique to its nature. Let's take Sagittarius A*, the supermassive black hole at our galaxy's heart, spinning nearly at its maximum speed.

Celestial bodies like our Earth have a maximum rotational speed set by surface gravity. Your weight on Earth reflects not just the planet's gravitational tug but also a minuscule "centrifugal" force due to the planet's spin. This force, though small, pushes us outwards, making us weigh marginally less at the equator than at the poles.

Earth's 24-hour day results in a mere 0.3 percent weight difference between the equator and the poles. However, a faster spin like Saturn's 10-hour day results in a staggering 19% difference, so much so that Saturn's equator bulges slightly outward. Now, imagine a planet spinning so rapidly that the difference reaches 100%—the planet's gravity and centrifugal force cancel each other out. If the planet sped up any further, it would disintegrate.

Black holes, however, play by a different set of rules. With no physical surface and composed of nothing that could be thrown off by spinning, their maximum rotational speed is still defined. A black hole's enormous gravity twists space and time, encapsulating everything within its event horizon. Interestingly, a black hole's spin isn't dictated by the rotation of physical mass, but the warping of spacetime surrounding it—an effect called frame dragging. 

This spin has an upper limit due to the intrinsic properties of space and time. In Einstein's General Relativity, black hole rotation is quantified by a value known as 'a', ranging between zero (no spin) and one (maximum spin).

A recent study investigated the rotation of the supermassive black hole in our galaxy. Researchers analyzed radio and X-ray observations of the black hole, which allowed them to estimate its spin. This spin distorted the light spectrum from material close to it. By examining the light intensity at different wavelengths, the team could gauge the amount of spin.

What they found is that the a-value of our black hole is between 0.84 and 0.96, which means it spins extremely fast. In the upper range of the estimated spin, it would rotate almost at maximum speed. This is even higher than the rotation parameter of the black hole in M87, where it is estimated to be between 0.89 and 0.91.

 

Source: Universe Today, Royal Astronomical Society