I stand corrected. Tanks.

Still, I would love to see a neutron star in close orbit of a BH. We stand to learn a great deal about degenerate states. IF neutronium could remain degenerate in the absence of that high G field, there might be a bit of it floating around the galaxy. What a great construction material that could be.

Gary 7

Don’t confuse white dwarfs with neutron stars . A white dwarf is made up of “normal” atoms — or maybe I should say “normal” ions. The WD is stable due to the degeneracy of the electron gas. A NS is stabilized by the degeneracy of neutrons and sometimes the matter of the neutron star is called neutronium.

This doesn’t change your argument a lot, I just wanted to make this clear.

Tidal forces are, indeed, very important here. Sooner or later the WD will become “spaghettified” (such a nice word ). But for this to happen the WD must be rather close to the BH (although not as close as if it would be a NS). So, we still face the problem of angular momentum, which is, as I think, the most important factor why it takes “so long” for the BH to swallow the WD.

“Given that a white dwarf is a dead star with around the mass of our Sun crushed into a sphere about size of the Earth”

This refers to the point I was attempting to make. If the WD is orbiting close enough to the BHs event horizon for the BH to rip pure neutronium from its surface the other side of the WD is 11,000 km away from the event horizon, with its consequent decrease in Gravity tides. Thus, as the WDs mass reduces on the event horizon side, the opposite side of the WD should(logically) revert to normal matter, with electrons ejected from the neutrons, protons left behind, and the whole WD should begin outgassing,,,but that’s assuming that ONLY the WDs gravity has been the force holding everything “down”. Like a hugh spring with the weight compressing it removed, the WD should rebound(on the side away from the event horizon). Unfortunately, we have no idea how neutronium really behaves in the absence of its G field. It may well be that once neutronium is formed, it would be a stable super nucleon bound by its strong nuclear force.

GAry 7

Well, my understanding may be wrong, but I was under the impression that while the overall field strengths of a 10 solar mass star and black hole are the same, the field intensities would be very different. The gravitational field of the BH originates with the singularity, while in the original star the field originates with the star matter, which is spread out over a very substantial 3-D volume. And since gravity obeys the inverse square law it drops off rapidly with distance from the source (maybe this isn’t relevant here–not sure).

23. Grimbold,

I don’t think that’s the right way of thinking about this. The White Dwarf’s gravity is being overcome by an even stronger gravitational source, which is of course the BH. Yeah, the WD is probably getting ripped to shreds but it’s only because an even bigger dog is taking over. Therefore the opportunity to decompress is not going to arise–the superior gravity of the BH simply takes over and incorporates the WD into itself.

The way to think of this is as an additive process, with the larger object entirely in control of the addition. I’m supposed to say a merger, but as a practical matter, when a 1,000 mass BH swallows a WD, the BH isn’t affected that much. The WD on the other hand exits normal space and leaves only it’s gravity, spin and charge behind as a metaphoric echo of it’s existence.

The degeneracy prevented an implosion, not an explosion. So, when the WD loses enough mass, it should become a gaseous thing again that balances gravity with the gas pressure, but it should not explode.

Given that a white dwarf is a dead star with around the mass of our Sun crushed into a sphere about size of the Earth [..] then I’m also surprised it takes that long for the black hole to devour its stellar victim.

One should not forget that the WD has a strong gravity itself. I guess the black hole must pull rather strong on the WD to get anything at all. The matter is bound rather strongly to the WD and is not some loosely gas.

The Lagrange point between the BH and the companion are quite important here. Only matter of the companion that is “over” the Lagrange point can be sucked in at all, otherwise the gravity of the companion still pulls stronger on the particle than the BH.
Also the BH must form an accretion disk in order to pull down the matter. The material must lose its angular momentum, a process that needs some time and is not done in a few moments.
These are just two examples why the BH has to do some hard work to swallow the WD, and why it also takes some time.

“I’d imagine the material would “depressurise” & explode.”

Just think of what must then occur to the white dwarf, as its mass drops below the level required to maintain that degenerate matter state.

Poof! (ok, a really BIG poof)

GAry 7

Black Hole Monster: “STAAAAAAAAR! om nom nom nom nom nom nom nom”

Any relation of the Moon walker James Irwin – the 8th person to walk on the Moon with David Scott on Apollo 15? See : http://en.wikipedia.org/wiki/James_Irwin

@ 19. Grimbold Says:

I wonder what happens to the degenerate material the white dwarf is made of when the white dwarf is ripped to shreds. It will no longer be compacted by the great weight of matter above it, because that is being stripped away by the black hole; so what happens to it?

I too wonder about this. As I understand it, & I could be mistaken here of course, the “degenerate matter” making up a white dwarf can exist only because it is compressed immensely by the force of the white dwarfs gravity so once released from the star’s gravitational pressure I’d imagine the material would “depressurise” & explode.

Given that a white dwarf is a dead star with around the mass of our Sun crushed into a sphere about size of the Earth (depending on exact mass – do we know? Maximum mass of 1.4 solar a.k.a. the Chandrasekar Limit.) then I’m also surprised it takes that long for the black hole to devour its stellar victim.

@ 21 21. Dr Cy Coe in NL Says:

(can edit, but can’t delete my own post?) Yup, that’s right.