|MadSci Network: Physics|
Often called the 'fourth state of matter', plasma is extremely common in the universe, but rather rare here on Earth.
Everyone's familiar with heating up a solid to get a liquid, and heating up a liquid to get a gas, but it's possible to add still more heat and get to a "fourth" state of matter -- a plasma. When you add enough heat to a gas to ionize the atoms, you're left with a charged gas -- composed of negatively charged electrons and positively charged ions. This is called a plasma, and isn't like a gas at all. (In fact, in some ways, it acts more like a liquid!)
So what's the hottest thing you can think of? The sun comes to mind. Sure enough, the sun is a plasma, as are all the stars in the universe. Because stars make up the vast majority of the universe (that we know about), the universe is mostly plasma!
There's another way to make a plasma without being as hot as the sun; instead you can make cool plasmas so long as you have very low pressures. The interstellar gas between stars is an example of a very cool plasma, at a very low pressure. And there are plasmas at intermediate temperatures and pressures; some common examples are fluorescent lights, flames, neon signs, the northern lights, and lightning. Click on the above link to see how all these different types of plasmas are organized on a temperature/density graph.
As it happens, though, the Earth is rather cool and rather dense, which is a regime where plasmas can't easily exist. But even though you don't encounter too many plasmas in your daily life on Earth, plasmas seem to be just about everywhere else in the universe.
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