Did you know that anything could become a black hole? Even you! It suffices to shrink all of your individual matter into an area 1 sixtilion times the size of a grain of sand. Did you actually know that some black holes are not black at all? And did you also know that black holes have different masses?
But what is a black hole? A black hole, in very simple terms, is an object that packs a lot of mass into a tiny region of space. If you can confine whatever mass to the correct small size then you can make a black hole. The gravitational pull of these objects is so strong that nothing can escape, not even light. And this is why they are called black.
Some black holes are formed during the violent explosions of massive stars, called supernovae. When a star, several times more massive than our Sun, burns all the fuel inside, it cannot anymore fight the strong gravity of its stellar mass. So the core collapses and the star explodes. In one second it blasts out 100 times the energy our own Sun will release in its entire life. The explosion leaves behind a supernova remnant, and at the center a new black hole is born. But those are the small ones, weighing just a few times the mass of our Sun!
Then there are the monstrous ones, the ones that lurk at the center of galaxies, the supermassive black holes. Our own Sun is sitting at the edge of one of these galaxies, the Milky Way. Bright glow in the night sky, it is made of 100 billions of stars and hosts at its center a black hole. Black by nature, how do we know it’s there and how do we know its mass? Using some of the most powerful telescopes in the world, scientists were able to pierce through all the stars and gas and dust that separates us from the Galactic center, and exactly trace the orbits of the stars surrounding the supermassive black hole. Using the universal law of gravity, their motion was enough to tell us that there is a black hole, that it is calmly sitting at the center of our Galaxy, and that it weighs as much as 4 million Suns.
Some of these black holes are not as calm as our own. Instead, they are actively devouring gas from their surroundings, making the galaxy they reside on a not-so-quiet place. Matter is pulled and stretched as it falls closer and closer to the black hole and “screams”. That “scream” is radiation. And the closer to the hole, the faster the coaster goes, so the louder, or more energetic, the “scream”. They are so “loud” that you can hear them up to X-rays!
Here’s a fun fact! These monsters are the most efficient converters of mass into energy that we know of. Take a simple nuclear power plant on Earth. It has an efficiency of around 0.08%. In comparison, for a single black hole like these, this number is 40%. This means that by itself the output of a black hole is the equivalent of that of 500 nuclear power plants! This should give you a sense of how important it is to study them.
These black holes are also capable of launching intense jets , which are cones in which particles get swung up to million light years away from the center. Going almost as fast as the speed of light, these particles make the jets as bright as a hundred trillion Suns, and they produce the most powerful radiation we know of, gamma-rays.
We live in a Universe that is now almost 14 billion years old. Using the most advanced technologies, building telescopes on the ground and launching satellites in space, we are trying to explore it until its very beginning. And as it is our home, it is also home for 100 billion more galaxies, each made of 100 billion stars, each having a supermassive black hole lurking at the center. And it turns out that out of all these galaxies about 1% (so we are talking about 1 billion galaxies) host these powerful jets. Incredibly, many of them are found when the Universe was very young. They are so powerful that their light has traveled for 13 billion years, through all the other galaxies, stars, black holes, and gas to reach us. And we can still see them at the most powerful gamma-ray energies. Imagine living next to one of these breathtaking monsters!
These black holes and their jets have influenced the history of our Universe and the history of our own galaxy. These are the biggest baddest black holes!