Black holes are some of the most mysterious and fascinating objects in the universe. Their gravitational pull is so strong that nothing, not even light, can escape. Despite their darkness, some black holes are visible due to the glowing discs of gas and stars that swirl around them before being pulled into the hole.
These discs, known as accretion discs, can shine brighter than entire galaxies. But, intriguingly, these discs don’t shine with a steady glow; they appear to twinkle. This twinkling effect, where the brightness of the discs fluctuates over time, has puzzled scientists for years, but recent research has shed light on the cause.
Turbulence and Magnetic Fields The twinkling of black holes is not due to some far-off cosmic light switch. Instead, it’s caused by the turbulent motion within the accretion disc. The extreme conditions within these discs, including intense gravitational forces and magnetic fields, create a chaotic environment. As gas and other matter spiral toward the black hole, friction and these intense fields create turbulence, causing fluctuations in the amount of light emitted. This is similar to how the Earth’s atmosphere can cause stars to twinkle in the night sky, but on a far grander scale.
Supermassive Star-Eaters The black holes we observe twinkling are usually supermassive black holes located at the centres of galaxies. These giants can have masses millions or even billions of times that of our Sun. Our own Milky Way galaxy has one such black hole at its centre, with a mass of about four million suns. While many stars happily orbit these black holes, others get pulled too close. These unfortunate stars are torn apart and devoured, a process that contributes to the growth of these massive entities. In addition to devouring stars, black holes also feed on clouds of gas from old stars known as red giants.
Observing the Flickering To study the twinkling of black holes, scientists have used telescopes like NASA’s ATLAS, which scans the sky nightly. By monitoring the glow of thousands of black holes over five years, researchers have been able to create “movies” showing how their brightness changes over time. These observations revealed patterns in the flickering that provided clues about the behaviour of accretion discs. Astrophysicists had previously theorised that magnetic fields within the discs cause turbulence, leading to flickering. The theory of “magneto-rotational instabilities” suggests that the speed of the orbits of the discs affects the twinkling rate. Larger discs with slower orbits twinkle more slowly, and smaller discs with faster orbits twinkle more rapidly.
Universal Patterns By analysing the flickering patterns of 5,000 black holes, scientists found that when they accounted for the different orbital speeds of the accretion discs, all the patterns looked similar. This discovery confirms the theory of magneto-rotational instabilities and demonstrates a universal behaviour in these extreme environments. It also suggests that the remaining differences in flickering patterns may be due to the orientation from which we observe them. Future studies will focus on these subtle differences to learn more about black hole orientation and their accretion discs.
In summary, black holes “twinkle” because of the turbulent activity in their accretion discs, driven by intense gravitational and magnetic fields. This flickering is not random but follows predictable patterns influenced by the disc’s orbital speed, offering insights into the dynamics of these fascinating cosmic objects.
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