What Separates The Novas From The White Dwarves?

The Chandrasekhar Limit lets astronomers know which stars will end with a bang, and which stars will end with a whimper.

If there's one thing I've learned from tabloids and internet email forwards about psychological studies done in the seventies, it is that inside all of us lies a monster - a remorseless, rapacious, out-of-control psychopath who wants to see the entire world burn, even if we burn along with it.


Well, too bad. It isn't going to happen. Our sun is comfortably tucked underneath the Chandrasekhar Limit, which means that, barring something unforeseen, it will end its days as a slowly cooling white dwarf.

Stars begin life as gigantic clouds of hydrogen floating in space. Gravity pulls the hydrogen into a denser and denser bundle at the center of the cloud. Eventually, the pressure becomes great enough that the hydrogen at the center of the star fuses, and becomes helium. This releases a large quantity of energy – read ‘boom' – and puffs out the star, which decreases the pressure on the core until the energy dissipates and the hydrogen particles fall inward again.

Eventually there isn't much hydrogen left at the center of a star. Everything has been fused into helium. The star collapses further. That is, until the helium starts fusing.

Heavier sets of elements need more pressure to fuse, and so the cycle goes on, the star collapsing and the material at its core climbing the periodic table of elements. Eventually the elements at the center of the star have fused until they just can't fuse no more. At that point, the star rests on electron degeneracy pressure.


And have no doubt about it, electrons are prone to degeneracy. They are the rebels of the particle world; relentlessly negative, always running from one place to another and breaking their proton's hearts. They need their space, man, so they can find themselves, and live life, and smoke behind the bleachers between classes. Electron degeneracy pressure is basically the pressure exerted by electrons trying not to be squished into the same place by The Man.


Stars above the Chandrasekhar Limit, 1.44 solar masses, are authoritarian stars, hostile to electron counter-culture and stopping at nothing to crush it. What that gets them is one moment of high-pressure electron degeneracy-smashing collapse, followed by a T-bird-style rumble that lights up the sky. There is so much pressure on the center of the star that the electrons there are squished into the protons to form neutrons. When the outer layers of the star hit the star's core they rest hit neutron degeneracy pressure – it's always the seemingly-neutral ones who make trouble – and bounce off so hard that they blow themselves into space.


Stars under the Chandrasekhar Limit, like our sun, know that the wisest course of action is to let the electrons have their way. Sure, they may look tough now, but if the electron degeneracy pressure is able to balance out the gravitational force on the star, the star remains in equilibrium. It sits back, cools its heels, and soon its goatee is shot with white, and all it wants to do on weekends is sit in recliner, drinking beer and listening to "Glory Days." White dwarves are like old soldiers. They don't die. They just fade away.

Via: Hyperphysics and NASA.


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