Stellar Remnants

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All that is left of the star after the outer layers are ejected to space is the core remnant. The core's gas is super-compressed by gravity to form a strange type of gas made of ``degenerate matter''. It is important to remember that what happens to the core depends on the mass of the core, rather than the original mass of the main sequence star from which it came, because the only thing left for gravity to really compress is the core.

Degenerate matter

When gas become super-compressed, particles bump right up against each other to produce a kind of gas, called a degenerate gas, that behaves more like a solid. Normal gas exerts higher pressure when it is heated and expands, but the pressure in a degenerate gas does not depend on the temperature. The laws of quantum mechanics must be used for gases of ultra-high densities.

The first rule is that only certain energies are permitted in a closely confined space. The particles are arranged in energy levels like rungs of an energy ladder. In ordinary gas, most of the energy levels are unfilled and the particles are free to move about. But in a degenerate gas, all of the lower energy levels are filled. The second rule is that only two particles can share the same energy level in a given volume at one time. For white dwarfs the degenerate particles are the electrons. For neutron stars the degenerate particles are neutrons. The third rule is that how close particles can be spaced depends inversely on their masses. Electrons are spaced further apart in a degenerate electron gas than the neutrons in a degenerate neutron gas because electrons are much less massive than neutrons.

Let's see how these rules affect the core remnant.

  1. Degenerate gases strongly resist compression. The degenerate particles (electrons or neutrons) are locked into place because all of the lower energy shells are filled up. The only way they can move is to absorb enough energy to get to the upper energy shells. This is hard to do! Compressing a degenerate gas requires a change in the motions of the degenerate particle. But that requires A LOT of energy. Degenerate particles have no ``elbow room'' and their jostling against each other strongly resists compression. The degenerate gas is like hardened steel!

    energy levels of degenerate matter

  2. The pressure in a degenerate gas depends only on the speed of the degenerate particles NOT the temperature of the gas. But to change the speed of degenerate particles requires A LOT of energy because they are locked into place against each other. Adding heat only causes the non-degenerate particles to move faster, but the degenerate ones supplying the pressure are unaffected.

  3. Increasing the mass of the stellar core increases the compression of the core. The degenerate particles are forced closer together, but not much closer together because there is no room left. A more massive stellar core remnant will be smaller than a lighter core remnant. This is the opposite behavior of regular materials: usually adding mass to something makes it bigger!

adding mass SHRINKS size

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last updated: 25 May 2001

Is this page a copy of Strobel's Astronomy Notes?

Author of original content: Nick Strobel