Black holes are one of the most fascinating entities in the universe. But what are they? How do they form and evolve? What effect do they have on the surroundings and the rest of the universe - and most importantly, why should we bother about them?
Black holes play an important role in the history of universe, in sculpting galaxies that we live in , and possibly in the ultimate faith of the universe. Black holes are a place so much so as a thing. They are a region in spacetime where gravity is so strong that nothing not even particles or electromagnetic radiation like can escape from it.
Scientists can't directly observe black holes but they know of their existence by studying the effects they have on the nearby matter. Black holes, it turns out, are everywhere, in the center of most galaxies and spread throughout them, and they come in different sizes. Depending on the size, black holes are categorized into three types- primordial, stellar and supermassive. Sagittarius A* is the supermassive black hole at the Galactic center of the Milky Way.
It turns out that there are only three quantities that define a black hole, its electric charge, its mass, and its spin. Supermassive black holes, in particular, might play an important role in star formation within galaxies, dictating when production slows or ceases altogether.
Say someone falls into a black hole and there’s an observer that witnesses this. The person who fell into the black hole’s time slows down, relative to the person watching. This is explained by Einstein’s Theory of General Relativity, which states that time is affected by how fast you are going when you’re at extreme speeds close to light. A scary fact - If you fell into a black hole left over when a star died, you would be shredded.
Now the black holes might sound like vicious creatures but surprisingly they aren't. That nearby black hole is no threat to Earth. No known black hole is. If anything, we benefit from their existence. The stellar explosions that produce black holes also spew elements such as carbon, nitrogen, and oxygen into space. The collisions of black holes and neutron stars help spread heavier elements, such as gold and platinum. These elements make up our Earth, and our own selves.
The event horizon, a black hole's "surface", defines the boundary where the velocity needed to escape exceeds the speed of light. Matter and radiation fall in, but can't get out.
Materials, such as gas, dust and other stellar debris that have come close to a black hole but not quite fallen into it, form a flattened band of spinning matter around the event horizon called the accretion disk.
Super-massive black holes in the centers of some active galaxies create powerful jets of radiation and particles travelling close to the speed of light. Attracted by strong gravity, matter falls towards the central black hole as it feeds on the surrounding gas and dust. But instead of falling into the black hole, a small fraction of particles get accelerated to speed almost as great as the speed of light and thrown out in two narrow beams along the axis of rotation of the black hole. These jets are believed to be the sources of the fastest-travelling particles in the Universe -- cosmic rays.