How can I make a black dwarf star?

Question

In response to a question about an asteroid filled with degenerate matter, Ender Look suggested that using matter from a black dwarf would be better than using matter from a white dwarf since black dwarfs are cooler. A typical white dwarf's temperature might be $\sim$10,000 K, which is way too high.

The problem is, it takes a long time to form a black dwarf. The distinction between a white dwarf and a black dwarf is somewhat arbitrary, partly because "black dwarf" is not a technical term. Some simply say that black dwarfs are white dwarfs that have cooled to the point where they no longer emit a substantial amount of visible light. According to that definition, we may already have some black dwarfs (e.g. the companion star to PSR J2222-0137). However, this corresponds to a temperature of about 2800 to 3000 K, which is still too high for the purposes of the original question about degenerate matter. Therefore, I need to define a black dwarf as a white dwarf that has cooled to about 1000 K or lower.

The problem is, this is going to take a really long time, and there may not be any black dwarfs this cool in the galaxy. This means that I'm willing to use artificial means to turn a regular white dwarf ($T\sim$10,000 K) into a black dwarf ($T\sim$1000 K). The civilization doing the transformation ranks as a Type II civilization on the Kardashev scale and has all the powers and technologies you would expect them to have (key exception: no FTL travel).

How can this Type II civilization turn a white dwarf into a black dwarf, within a reasonable timeframe (say, 100 to 1000 years)?

Other specifications:

• We'll assume a mass of about 0.25 solar masses, and a radius of about 0.0038 solar radii (radius derived from a mass-radius relation, with Sirius B as a reference).
• The white dwarf in question is alone, with no binary companion or planets.
• There might be a handy interstellar cloud of gas in the region if needed.
• Large-scale mass removal or addition is likely undesirable.
• The object shouldn't merely appear black, so you can't paint it black, as it were.

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I should add that What could be done to make a red dwarf become a blue dwarf, then a white dwarf and then finally a black dwarf in a short amount of time? does talk about this, but it seems like they're using a black dwarf that's extremely cool (more like 10 or 100 K). There are also some other differences:

• Their civilization is a Type III civilization; I have a puny Type II one.
• They're fine with FTL travel; I am certainly not.
• They want to start with a red dwarf, meaning they have more control over the object. I'm starting from just a white dwarf.
• I'm looking for a bit of a hotter black dwarf than they are, it seems.

Answer 1

Basically, you're asking how to suck the heat out of a while dwarf without sucking all the mass out. That's really hard.

The classic three methods of heat transport are radiation, convection, and conduction. A white dwarf already emits essentially all of the radiation it can from its surface, so there's not much more you can do with radiation, other than waiting a hundred billion years or so.

Convection carries away heat by the movement of hot matter. That would work, but if you don't return the matter you wind up with nothing left of the star. If you do return the matter, you then need some enormous source of energy to lift the hot matter from the star, let it radiate its heat away, and then lower it back without it turning the (huge) gravitational potential energy it has away from the WD into heat and re-heating the WD.

Conduction doesn't appear to be relevant since the WD is far hotter than any solid matter.

Given near-magical technology, the one thing I can think of is somehow stirring the interior of the WD to bring the vastly hotter material inside to the surface where the good old Stefan-Boltzman T⁴ dependence would radiate heat away much more quickly. It's still a long, slow process, but much quicker than just sitting there watching the WD cool on its own. Note that this stirring should be done very carefully so as to not mix too much unfused material into the hotter regions and cause re-ignition or even a nova. See this article for some information on WD structures. Some WDs might be stable against stirring; others will go bang or re-ignite.

Any technology beyond that looks like entirely magical technology and, of course, with that, you can do anything.

Note: The material from a black dwarf would indeed be very, very dense, but it would not be stable. In the star, its stability is maintained by the immense pressure from the star's immense gravity. Once a sample is lifted out of the star's center (the surface of a WD or a black dwarf is ordinary matter because there's not enough pressure there to make it degenerate) it would explode quite violently.

Answer 2

Disassembling and reassembling the star

The surface of a star is where all the energy gets out by radiation. In a regular sphere that is $4 \pi r²$, but it's volume $\frac{4}{3} \pi r³$ implying a surface-volume-ratio of $\frac{3}{r}$. With $r=0.0038 R_{solar} = 2643.66 km$ you'd have a ratio of $1.13479 \times 10^{-6} \frac{m²}{m³}$.

If you were to change the shape or "split" the star into smaller objects you'd improve that ratio significantly in favor of surface area and thus radiation output.

Type II Civilisation

Wikipedia states about Type II Civilisations:

Star lifting is a process where an advanced civilization could remove a substantial portion of a star's matter in a controlled manner for other uses.

Why not disassemble the star into smaller parts of which each has a significantly stronger energy output per volume?

Assuming a civilization that can "remove substantial portions of a star's matter" to repurpose it could also stabilize the removed portions, this should work decently to cool it (at least a lot quicker than it would cool on its own). They might even cool the smaller portions with some additional process.

The only thing you'd need after that is the reassembling of all the gathered star matter, but I do not see this being an issue for a civilization that can take a star apart and repurpose the smaller fractions. Basically, you'd just need to put it together - and since it is not an IKEA wardrobe the order in which you put it together does not matter either.

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Please note: I am not an astronomer, just a very physics-interested engineer. I am not certain about what would happen if you were to split a star beside the obvious.

Answer 3

With lasers.

I am serious. Laser cooling is a thing. And it works best on gas, which is nice in this context.

Now, it might take too long, and too many lasers, to get what you want if you just shoot at the star. You may wish to bring in another star, much heavier than your mark. It will start stealing matter from the smaller one, like this:

Source for image

Shoot your cooling lasers at the matter being transfered between the stars to cool it down. Then collect it before it falls into the heavier star. Store it somewhere else. If you stick to the goal of the linked question, which is to have pieces of star and not a whole one, you may process the matter you are getting from this process into asteroid sized packets.

Being a Kardashev II civilization, your people may even harvest some energy from the heavier star to power the lasers and whatever collecting machinery is involved in the process.

Answer 4

Turn heat energy into matter via iron fusion.

1: Start with a specific type of white dwarf amenable to this maneuver: https://en.wikipedia.org/wiki/White_dwarf#Type_Iax_supernovae

Type Iax supernova, that involve helium accretion by a white dwarf, have been proposed to be a channel for transformation of this type of stellar remnant. In this scenario, the carbon detonation produced in a Type Ia supernova is too weak to destroy the white dwarf, expelling just a small part of its mass as ejecta, but produces an asymmetric explosion that kicks the star, often known as a zombie star, to high speeds of a hypervelocity star. The matter processed in the failed detonation is re-accreted by the white dwarf with the heaviest elements such as iron falling to its core where it accumulates. These iron-core white dwarfs would be smaller than the carbon-oxygen kind of similar mass and would cool and crystallize faster than those.

The zombie star is good for this application anyway because it cools faster. And we will use that iron core to cool it extra fast with

2: Iron fusion.

https://en.wikipedia.org/wiki/Iron_peak

For elements lighter than iron on the periodic table, nuclear fusion releases energy while fission consumes it. For iron, and for all of the heavier elements, nuclear fusion consumes energy, but nuclear fission releases it. Chemical elements up to the iron peak are produced in ordinary stellar nucleosynthesis. Heavier elements are produced only during supernova nucleosynthesis. This is why we have more iron peak elements than in its neighbourhood.

The fusion of iron is endothermic and so energetically unfavorable. If there is a boatload of energy available it might push it in that direction. Under any energy conditions, fusion of iron (and heavier elements) would consume (heat) energy from the surroundings which would go towards the creation of new matter.

Like other endothermically unfavorable reactions, a catalyst would help move this along. We will use

3: Muon catalyzed fusion. https://en.wikipedia.org/wiki/Muon-catalyzed_fusion

Muon-catalyzed fusion (μCF) is a process allowing nuclear fusion to take place at temperatures significantly lower than the temperatures required for thermonuclear fusion, even at room temperature or lower. It is one of the few known ways of catalyzing nuclear fusion.

Fortunately your Type 2 civilization is master of the muon, and they routinely use muon catalyzed fusion to meet all energy needs. But here we turn those persuasive muons on recalcitrant iron, persuading the partners to meet and merge. A tight beam of muons penetrating the oxygen / carbon outer layer might work, or perhaps the muons will need to be generated in situ (if we can find machinery robust enough to withstand the pressure).

Additional spatial dimensions could help with this if available but probably that would be downvoted as pure fantasy.

Answer 5

Compression. Then expansion. Via artificial gravity.

When matter is squeezed down to occupy less space, it heats up. Conversely if it is expanded, it cools down.

https://physics.stackexchange.com/questions/17948/why-does-a-gas-get-hot-when-suddenly-compressed-what-is-happening-at-the-molecu

If this civilization can simulate gravity, perhaps they have ways of curving space - simulating the effect of mass on space (to produce gravity) without having to use the mass. Exactly how the civilization accomplishes this is left as an exercise for the reader.

So: curve the space within the black dwarf more strongly, effectively increasing its gravity. As it is pulled inward, it will heat up. Be careful! If you compress it too much you will get it to the point where it can fuse its oxygen and carbon, and that will make a bunch of new unwanted heat! Stop short of that. Squeeze it to a cozy blackbody glow and let it get back to rapidly radiating heat out into space like it did in its white dwarf days.

Then turn off the juice and let space relax. As it expands, the black dwarf will cool. You might still need oven mitts.