Why clouds form near black holes
EarthSky Voices in SPACE | May 19, 2020
Clouds in the greater universe are clumpy areas of greater density than their surroundings. Space telescopes have observed these cosmic clouds in the vicinity of supermassive black holes.
EarthSky Voices in SPACE | May 19, 2020
Clouds in the greater universe are clumpy areas of greater density than their surroundings. Space telescopes have observed these cosmic clouds in the vicinity of supermassive black holes.
This artist’s concept depicts a quasar, a type of active galactic nucleus, surrounded by a dusty donut shape (torus) and clumps called “clouds.” These clouds start small but can expand to be more than 1 parsec (3.3 light-years) wide. In this diagram, the clouds are at least 1 parsec from the torus. Image via Nima Abkenar/ NASA. |
Space telescopes have observed these cosmic clouds in the vicinity of supermassive black holes, those mysterious dense objects from which no light can escape, with masses equivalent to more than 100,000 suns. There is a supermassive black hole in the center of nearly every galaxy, and it is called an active galactic nucleus (AGN) if it is gobbling up a lot of gas and dust from its surroundings. The brightest kind of AGN is called a quasar. While the black hole itself cannot be seen, its vicinity shines extremely brightly as matter gets torn apart close to its event horizon, its point of no return.
But black holes aren’t truly like vacuum cleaners; they don’t just suck up everything that gets too close. While some material around a black hole will fall directly in, never to be seen again, some of the nearby gas will be flung outward, creating a shell that expands over thousands of years. That’s because the area near the event horizon – the threshold around a black hole where the escape velocity surpasses the speed of light – is extremely energetic; the high-energy radiation from fast-moving particles around the black hole can eject a significant amount of gas into the vastness of space.
Scientists would expect that this outflow of gas would be smooth. Instead, it is clumpy, extending well beyond 1 parsec (3.3 light-years) from the black hole. Each cloud starts out small, but can expand to be more than 1 parsec wide – and could even cover the distance between Earth and the nearest star beyond the sun, Proxima Centauri.
Astrophysicist Daniel Proga at the University of Nevada, Las Vegas, likens these clumps to groups of cars waiting at a highway onramp with stoplights designed to regulate the influx of new traffic. He said:
Every now and then you have a bunch of cars.