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White Dwarfs & Planetary Nebulas

Normal Star: Nulcear energy provides pressure to offset gravity / White Dwarf Star: No Nuclear energy, so what keeps it from collapsing? The paradox was not resolved until the quantum theory of matter was developed in the 1920s. This theory showed that matter in so-called "degenerate" states of extremely high density could produce a new type of pressure never observed in a terrestrial laboratory. This is because the quantum theory prohibits more than one electron from occupying the same energy state.

Normal parking lot with plenty of spaces: car is in no hurry / Degenerate parking lot with few spaces: cars race for the spot To see how this works, think about a parking lot. Only one car is allowed per space. When there are many empty spaces, there is very little motion in the parking lot. As an occasional car enters the lot it is quickly parked. When the parking lot is full, however, the picture changes. There is continual motion as cars move from one row to another while drivers search for a space. The pressure builds to get into position whenever a space is opened up.

Similar picture with electrons Extremely dense matter is like a crowded parking lot. All of the low energy "parking spaces" are taken, so electrons are forced into higher energy states, not because they are hot, but because there is nowhere else to go. This creates a "degenerate" electron pressure (degenerate refers, not to the moral character of the electrons, but to the fact that all the low energy states are occupied). This pressure is what prevents white dwarf stars from collapsing under their own weight.

Chandrasekhar Limit
While still in his twenties Subrahmanyan Chandrasekhar, the Chandra X-ray Observatory's namesake, used relativity theory and quantum mechanics to show that degenerate electron pressure can do only so much. If the mass of the white dwarf becomes greater than about 1.4 times the mass of the Sun—called the Chandrasekhar limit—it will collapse. In a binary star system this could happen if a nearby companion star dumps enough material onto a white dwarf to push it over the Chandrasekhar limit. The resulting collapse and explosion of the white dwarf is believed to be responsible for the so-called Type Ia supernovas.

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