White Dwarf mining?

This is a follow-up to Neutron Star mining. Assuming that problem can be solved, it would eventually degenerate to something similar to this case. And if it can't, white dwarfs seem like the next best source of stellar-mass quantities of elements heavier than helium. (And maybe thinking up solutions to this slightly easier problem will help illuminate potential solutions to the harder version.)

So, suppose you want to incrementally mine material from, and eventually completely disassemble, a white dwarf star. ("Incrementally" meaning "let's not just trigger a supernova and pick up the pieces afterward".)

What would be the easiest, most plausible means of accomplishing this using foreseeable technology? (I.e., something that does not require new physics, just potentially ludicrous amounts of time / resources, for which we can do back-of-the-envelope physical calculations.) It is permissible to assume any necessary conditions on the particular star's inherent characteristics or environment, such as that it must be part of a binary system or have pre-existing planets or somesuch, but fewer and weaker preconditions are of course better.

• So why do you need so much carbon dioxide? Sep 10 '17 at 3:23
• @user6760 Why you need it isn't really relevant, but presumably for building large quantities of solid structures for which hydrogen doesn't suffice. Sep 10 '17 at 3:26
• I have a plan for mining white dwarf but every steps involve dying, so do we have a deal? Sep 10 '17 at 3:31
• You have the same problem as with neutron stars. Nothing known to science won't simply be crushed out of existence. Sep 11 '17 at 13:22
• @LorenPechtel I disagree. Anybody can say "well, you just an anti-grav device!" (inventing new sci-fi physics) or "I bet spallation with relativistic impactors would work" (making sci-fi-plausible guesses about real physics), without actually backing it up. The hard-science tag is practically the whole point. If there really is no feasible way to do it, regardless of circumstance, resources, or power availability, that anyone can think of, then fine--there will simply be no good answers. But that doesn't mean its not a question worth asking. Sep 11 '17 at 18:54

Let's look at the energetics of removing material from a white dwarf. Very, very approximately, the energy required to remove 1 kg of material from the surface of a massive object out into space is of order its potential energy: $-G M / R$, where $M$ is the mass (in kg) of the object and $R$ is its diameter in meters. To remove 1 kg from the surface of the Earth requires about 60 megajoules. To remove 1 kg from the surface of a 1-solar-mass white dwarf (radius about 0.01 times that of the Sun) requires about two hundred terajoules. That's about 50 kilotons TNT equivalent.

So on one level, it's feasible. (Leaving aside the engineering problems.) It's also an insanely expensive way to gather relatively common elements.

Now, there is a way of letting nature do it for you. Classical novae are binary systems with a white dwarf and a low-mass main sequence star which is orbiting so close to the WD that it overfills its Roche lobe, leading to accretion of material from the outer part of the main sequence star onto the surface of the WD. Over time, this material builds up until a runaway thermonuclear reaction takes place, which blasts a mixture of accreted material and original WD material out into space with velocities of several thousands of kilometers per second. (This may prove somewhat inconvenient to collect.)

So, provided you're willing to wait several thousand or tens of thousands of years between classical nova outbursts, you can extract material from a white dwarf this way, although it will be mixed with accreted material from the companion star and fresh thermonuclear byproducts (probably mostly helium) from the runaway burning of the nova outburst.

If you really want to collect "stellar-mass" quantities of matter, it would be much better to scoop them out of the surface of a giant or supergiant star, where you don't have to fight gravity so damn much.

Throw stuff at it really hard.

Problems with the white dwarf

1. Gravity is high so stuff won't come loose easily.

2. Stuff down there is dense and might not be knocked loose easily.

To knock something off of the white dwarf you have to hit it with something

1. Going really fast, so imparted energy to knocked off bits can achieve escape velocity.

2. Really solid, so it does not fragment into gas or plasma before it makes impact.

3. Something you do not need to get back.

These are the same principles as those leading to Martian meterorites: something hits Mars very hard, and ejecta from the impact winds up on the earth,.

I was thinking about things hitting things very hard in the context of this question: Feasibility of area-denial weapons in space.

The good thing about space is

1: There is a lot of dense stuff available that you can press into service as your impactor.

2: You can add kinetic energy to your impactor gradually, and you can borrow kinetic energy from large masses (they will not miss it) via a gravitational slingshot maneuver.

The only problem with this scheme is collecting whatever bits you are lucky enough to knock loose. A fair number will probably sally out and then drop back under the gravitational attraction of the neutron star. Some sort of robot observer in a geostationary orbit above the star could observe these pieces on the way out and calculate their trajectories, then intercept and collect them.

Not sure this would work, material strength concerns etc... but you might get away with firing something like a bulldozer to skim the surface of the dwarf and scoop away chunks of the stellar matter. This will only work for older stars below about 4000K using a Hafnium, Nitro, Carbide like this one to build the impact miner. The reason I'm not sure it would work is that there are no hard figures for the alloy's other physical characteristics so I'm not sure it's strong enough to cut pieces off a White Dwarf given the densities that are theorised. I'm ignoring radiation effects etc... because using "fire and forget" will be more efficient than any kind of crewed or guided vehicle (obviously you don't actually forget them, you pick them up out the other end of the skim to unload the scoop).

One thing, the radiation emitted by anything you break off a white dwarf is going to degrade any equipment you use for the mining effort so maintenance turn over will be high.