Sunday, September 14, 2008

How Do Black Holes Form?

The universe is full of mysterious objects called black holes-objects that distort spacetime so much that not even light can escape from their gravitational grip. Many people want to know:where do these mysterious objects come from? Can the sun become a black hole?

There are basically two known types of black holes. The essential difference between them is their size. At the centers of most if not all galaxies we find the supermassive black holes, black holes so big that they contain the mass of literally millions of suns. These black holes play a significant role in the evolution and behavior of galaxies, but we aren't going to talk about them in this post. We're going to talk about how most black holes form. These are smaller mass black holes, on the order of a few solar masses (8-10 solar masses). These small mass black holes are the remnants of stars, formed by complete gravitational collapse at the end of a supernova.

To understand how this happens we need to take a step back and learn a little about what a star is and how it operates. A star is basically just a big ball of gas. The center of the star is so hot and the pressures are so high that nuclear fusion, a reaction that combines two light atoms into a single heavier atom-takes place at the center of the star. Nuclear fusion has a couple of byproducts that are beneficial for things like life on earth: it gives off heat and light. Nuclear fusion is the power source of the star and is what makes stars shine.

Nuclear fusion is also the support structure of the star. Gravity wants to pull those outer layers of gas down towards the center of the star, and would do so if it were not for nuclear fusion. The heat and pressure produced by fusion basically prop up the outer layers of the star. A star lives its life in a delicate balancing act: gravity pulling inward and nuclear fusion pushing outward. When nuclear fusion stops, there is nothing to prevent gravity from taking over.

Just like your car runs out of gas, a star runs out of fuel. Its got a finite amount of hydrogen in its core. The first fusion reaction that takes place, which is the one happening in our sun, is hydrogen atoms are fused together to make helium nuclei. Eventually, there will be no more hydrogen to fuse. When this happens, fusion stops and there is no more heat and pressure propping up the star. The outer layers begin to collapse inward. Basic thermodynamics takes over, and the heat and pressure in the center of the star increases. This is good because to fuse heavier atoms, you need more heat and pressure. The star will collapse and gravity will squeeze the atoms of the core together until it becomes hot enough for a new round of fusion to begin: helium begins to fuse together to make carbon and oxygen.

Large stars run through their fuel much faster than smaller stars. Its kind of like the guy in the red sports car driving fast, he runs out of gas first. Hugely massive stars live fast and die young. This is because the higher mass makes conditions in the core hotter and generates higher pressure, so fusion reactions take place more rapidly and they run out of fuel faster. They are also able to fuse heavier elements because higher temperatures and pressures will be reached. The sun will only be able to make carbon and some oxygen, heavier stars will continue the fusion process toward heavier elements.

The sun is a mid-sized star, with an expected lifetime of around 10 billion years. There are smaller stars, so small and dim the sun makes them seem downright puny. They may be small, but they will be long-lived, they can have lifespans of a trillion years or more. In contrast, a large star destined to end its life as a neutron star will probably live a hundred million years. And a super-massive star destined to become a black hole will burn through its fuel in a mere million years.

At each point in the nuclear fusion cycle, a star burns up its fuel until fusion can no longer proceed. So a hydrogen core is fused until its entirely helium, then fusion stops. Gravity takes over and pushes the core closer together raising the temperature and pressure enough so that helium can fuse into carbon and oxygen. This continues until the core is entirely made of carbon and oxygen, then fusion stops again. Gravity takes over, crushing the core even more, raising the temperature enough so that a new round of fusion can begin. Now carbon atoms are fused together to make magnesium. The process keeps repeating itself. Heavier elements continue to be made: sulfur, silicon, and nickel. At each step, the temperature of the core soars to higher values, reachign 3 billion degrees. And the lifetime of the star shortens. When the core is made of magnesium and neon, there may only be a thousand years until the star dies. Eventually fusion proceeds until the core is a ball of iron, and fusion stops once again. But this is the end of the line. A reaction will proceed spontaneously if it liberates energy. Up to the fusion of iron, nuclear fusion liberates energy. But you would need to add energy to fuse two iron atoms together. In fact its something that just doesn't happen. So when the core of the star is iron, the days of nuclear fusion for the star have ended.

Now fusion stops and there is no more heat and pressure available to prop up the outer layers of the star. When the center of the star is a ball of iron, it may last another few days. Then gravity takes over and the outer layers of the star begin to collapse inward. There is one force left that can impact the outer layers of the star: the nuclear force. When the outer layers of gas strike the core the nuclear force leads to a rebound effect and creates a massive shock wave. This is a supernova: a cosmic explosion with so much energy that the neutrons and protons in the star are forged into the heavy elements so familiar on earth: copper, gold, and uranium are among them. The shockwave carries these elements out into the interstellar gases of the galaxy, where they are incorporated into later generations of stars and planets. Every element you're familiar with on earth was made this way, cooked in a supermassive star that lived long ago before our sun.

The fate of the iron core depends on how massive it is, and hence on how massive the original star was. If it wasn't that massive, it will collapse down until nuclear repulsion prevents the protons and neutrons from squeezing together any further, and nuclear reactions will convert the whole mess into a neutron star. But if the star was big enough, the force of gravity will be so strong that not even the nuclear force can stop it. The core will collapse down to a single point in space-time, producing a dimple from which not even light can escape. This is how small black holes are made: from the ashes of giant stars that once lived in the galaxy.

In future posts we'll talk about related topics from this article in more detail: neutron stars, supermassive black holes, and nuclear fusion.

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