h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

So what happens to the White Dwarf after it's shrugged off all that annoying, nebulaic mass?

Mo

I suppose it could splurge by absorbing a little deuterium.

It cools down... I think eventually becoming a Red Dwarf, then a brown dwarf... and then I guess just a lump of matter. Seems a bit sad, really...

I know how it feels.

Mo.

It does take quite a while to radiate all that heat... you still have a big blob of star-matter with heat trapped in a thousands-of-miles-thick blanket, and I suspect its thermal conductivity is, if you'll pardon the expression, less than stellar.

If our left-over white dwarf is massive enough, it'll eventually mash those electrons and nuclei together into neutronium; no electron shell and no charge on the nucleus means no electrostatic repulsion, means matter with an acute shortage of the empty space of which, say, your common or garden steel girder is made. If you've ever been whacked over the head with an iron bar, you probably said to yourself "Gosh, that felt like an awful lot of empty space."

Neutronium is outrageously dense stuff; a thimbleful would sink a battleship. Unless Larry Niven was pulling numbers out of the air when he wrote "There Is A Tide", the surface gravity of a ten-foot neutronium sphere would be about ten million Earth gravities... the mass of the thing is not humungous in astronomical terms but you can get unfeasibly close to the centre of mass of the object, which gives you a monstrous kickback in terms of the inverse square law.

However, Don't Panic. Despite the enormous *surface* gravity of a neutron star, and the more-or-less complete lack of light output, it's not a big navigation hazard at stellar distances. Nor is a Black Hole, which is what you get if you push this thing to ridiculous extremes...

Our sturdy neutron star is maintaining its structural integrity by the reluctance of its hard-pressed neutrons to be compressed any further. They're not infinitely strong though. Start with an object about two and a bit times the Sun's mass -- the Chandrasekhar Limit -- and you get something which, once there's no longer any radiation to keep it pumped up, will carry right on collapsing past the neutron star stage, curving space just like Einstein told us until it vanishes with Cheshire-cat-like aplomb leaving not even a grin behind.

It then looks mighty like what you have is a lump of distorted space without even the incredibly dense object in the centre that started off the whole sorry affair, but time behaves strangely in a gravitational field of this intensity and, from the point of view of the Universe, it takes the collapsar an infinitely long time to finish disappearing. However, it becomes invisible long before that; once its escape velocity exceeds light speed, any light trying to leave it gets red-shifted all the way to zero.

The notes on Hawking radiation are left as an exercise for the student.

The terms "Red Dwarf" and "Brown Dwarf" are reserved for certain types of objects: a red dwarf is a little cool star, say half solar mass; brown dwarfs are like giant planets, kinda big Jupiters, but unable to start nuclear reactions.
The final fate of a White Dwarf is called Black Dwarf; however, it will take a lot of time until the White Dwarf cools down to less than 3000 K, so it does not exist any Black Dwarf right now.
Astrônomo

No White Dwarf can be transformed into Neutron star without mass accretion (and there are serious doubts it can at all).
A Neutron Star is a hazard to the navigation in space. Neutron stars are also known as pulsars - their huge magnetic fields (a billion times Earth's one) cause a "jet" of radiation to be projected from the magnetic poles. They are powerful enough to be detected on Earth, even at 150000 light years from here. The jets are not aligned with the rotation axis, so they seem like a lighthouse, turning with the pulsar. The majority of pulsars rotate tens of times per second.
It is possible that neutron stars are related to powerful explosion known as Gamma-Ray Bursts. These explosions seems to happen so far away from us, that the energy implied is enormous. One of those explosions released so much energy as the entire Universe (all stars, galaxies and planets) releases on the same time... It was proposed that Gamma-Ray Bursts can be the cause for some life extinction episodes on Earth...