|MadSci Network: Astronomy|
Gravitational collapse in astronomy is the sudden inward fall of a body under the influence of the force of gravity. It occurs when all other forces fail to supply enough pressure to counterbalance gravity. Think about it like a account balance sheet. On one side you have gravity, which is determined by how much stuff the body contains (what scientists call "mass") and how densely/loosely packed the stuff if (what scientists call "density"). The more massive and/or denser an object is, the stronger its force of gravity. On the other side of the balance sheet you have forces that work to keep a body from collapsing - magnetic forces, thermal pressure, etc.. As long as both sides of the balance sheet are equal, the body is stable and won't collapse, but if we change the balance sheet so that the gravity side is more than the other side, the body will start to collapse. There are many reasons this could happen; the body might cool off, reducing its thermal pressure, or a nearby exploding star might send a shock wave through the body, compressing it and increasing its density (and thereby increasing its gravity). Gravitational collapse is a common process in the Universe. The gravitational collapse of enormous clouds of gas and dust in space gives birth to stars (and planets!). In most bodies, as the object collapses, the balance sheet comes back into balance. In stars, it is when the core of the star has become hot and dense enough from the collapse that nuclear fusion begin in its core, providing a new source of pressure to balance gravity. In some bodies, they are so massive/dense that the sheet can't be balanced. These objects collapse into what we call "black holes". The gravity near these objects is so strong that nothing, not even light can escape. The objects 100 to 1000 times brighter than our galaxy that you read about are called quasars. They are the distant objects that are being powered by material falling into supermassive black holes, hence the connection to gravitational collapse. As the material falls towards the black hole, it is compressed and heated to millions of degrees, causing it to radiate tremendous amounts of energy. Einstein taught us to think about gravity as a warping of space. Let's do a thought experiment: Imagine a giant rubber sheet. Now, place a bowling ball on the sheet; the bowling ball warps the rubber sheet around it, making a depression in the sheet. Objects in the Universe do this to space; the amount of warping depends on an object's mass and density. Now, roll a marble towards the bowling ball on the sheet; the path of the marble curves when it nears the bowling ball. This is a mimic for how gravity works. Interestingly, even light follows the warps of space caused by massive objects.
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