What would make a star good for star lifting?

My civilization is planning to being starlifting, mining a star by heating up portions of its surface and using a powerful magnetic field to channel the matter away from the star and into storage units. This is obviously not an easy task, and so these beings are looking for any advantage they can get. This means they're willing to pick and choose stars that will make the endeavor as easy as possible, and because they're only interested in hydrogen and helium, they have their pick of the galaxy.

They want stars that are intrinsically favorable to star lifting because of their properties - temperature, mass, etc. Unfortunately, I don't know much about star lifting, and therefore I don't know what properties my civilization would want in a star. I have some basic ideas:

• A low surface gravity, so it's easier for material to escape the surface
• Perhaps a low magnetic field, so there's little interference with the artificial magnetic field
• A cool surface temperature, to make it less dangerous to handle the hot plasma

I don't know just how important these factors are, however. What characteristics should a star have to make starlifting easy? By "star", I mean a body fusing hydrogen or heavier elements. It doesn't have to be on the main sequence, of course, but it also shouldn't be a brown dwarf or a compact stellar remnant like a white dwarf or a neutron star.

• Any civilization ready to start starlifting is surely not going to phased by problem like dealing with extremely hot plasma or generating huge magnetic fields on a vast scale or storing the resulting material. These are minimum requirements for even considering starlifting. Could you clarify what you mean by "star" here : brown dwarf to red giant is quite a range, so what's their target size ? May 14, 2020 at 14:12
• If you set up Dyson swarm mirrors to collect the light, according to an Isaac Arthur video the larger stars will provide enough energy to strip them faster, an advanced enough civilization could change the size of stars by controlling their output and metal rich stars might be popular for lifting their resources.
– user69935
May 14, 2020 at 14:16
• @RandySavage In an effort to keep the question as narrow as can be, I'd like to keep the method I've got, though I will keep that in mind for the future. May 14, 2020 at 14:18
• Wait... you’re asking us a question about astrophysics? I.. I’m not sure I can handle this kind of pressure! May 14, 2020 at 16:46
• I am curious why parties interested in hydrogen and helium would not obtain them from a cool gas giant. May 14, 2020 at 21:40

Previous Concepts

I did a quick literature review for this on google scholar, and unfortunately it seems like there isn't much research on the topic. I did however find this short article, which has a useful, if extremely simplified model (n.b. for whatever reason, it seems like the values they got in this article for specific energy required to remove mass from the photosphere are off by exactly two orders of magnitude, so keep that in mind if you want to do your own calculations with this).

Basically, it just assumes that you have some kind of laser array rotating around the star dumping energy into a patch of the photosphere (whose depth is determined by photon mean free path), and that the energy imparted is enough to get every atom within the patch to escape velocity. This model is, of course, very simplified-- it doesn't take into account the fact that there is going to be energy transport from the heated patch to the rest of the star that reduces escaped particle flux, or that as the star's mass changes, the mean free path and escape velocity will change, amongst other things. I'm not going to probe to deeply into that article's model, but I'll leave it there for reference about some improvements we can make to it's model.

One worthwhile thing we can note about the model is that the closer the typical thermal speed of atoms in the photosphere are to escape velocity, the less energy we have to apply to them. Unfortunately, I don't know enough about stellar equations of state to determine how this relates to other properties of the star like mass, but it might be a good direction to look in if you're well versed in that area.

Now, the other thing to note is that this model is a pretty extreme way of going about the task of star lifting-- depending on the timescales you want to go about this on, the required power of your array is somewhere on the scale from a sizable percentage of to many times the output of the sun. A more feasible method would be to instead take advantage of the processes in the sun that are already responsible for ejecting material into space.

An Alternate Approach

Specifically, taking advantage of magnetic reconnection would likely significantly reduce the required energy. You see, in plasmas with zero resistivity, the plasma completely follows the magnetic field-- this is called the frozen in flux condition and a consequence is that the mangetic field topology can never change. So, in this limit (known as ideal MHD), when two different ropes of magnetic fields collide, they can get tangled and bounce off each other but they stay two separate ropes. However, when there is a non-zero resistivity, the magnetic field can change topology and so the two colliding ropes can snap into a lower energy configuration of two loops where the magnetic tension carried in the initial fields is converted to kinetic energy of the bulk plasma fluid. If the two initial ropes are part of magnetic field loops wandering around the surface of the sun, then this can result one of the high kinetic energy loops being ejected out into space-- this is precisely what a coronal mass ejection is. My thoughts are that a sufficiently advanced civilization might be able to drive the surface of the sun in such a manner that accelerates and seeds this process of re-connection. If you then have a strong guide field set up, you can direct this ejection into some area where you can store it.

There are a few advantages to using this process. The main one is that you are using the sun's energy itself in the form of its magnetic field to push your plasma into space, which should decrease the energy requirements significantly. The second is that the plasma that makes it into space has a far lower temperature than in the other scheme, which makes it easier to contain. This isn't actually as trivial as it sounds-- a naive calculation of diffusion in fully ionized plasma actually suggests that higher temperature plasma is easier to contain magnetically. To get the full picture you have to take into account turbulent processes and collective affects, after which it is generally found that diffusion across magnetic field lines is indeed more pronounced in higher temperature plasmas.

As for downsides, the main one is that you are beholden to the internal magnetic field of the star to do the bulk of the work for you, so you might not be able to move as quickly or capture as large a percentage of the sun's mass.

Now, reconnection is a notoriously finicky process and tomes have been written describing it in various regimes. One of the simplest models is the sweet parker model, which I won't really delve into here, but despite it's many inaccuracies it has some useful results. One of them is that the outflow velocity of the plasma bubble is approximately the alfven velocity:

$$v_A = \frac{B}{\sqrt{\mu_0 \rho}}$$

Where $$\rho$$ is the density of the plasma in the magnetic flux ropes and $$B$$ is the magnetic field strength of these ropes. In general, if your civilization were to use this method they would want to find a star where $$\rho$$ and $$B$$ were such that the Alfven velocity was comparable to the star's escape velocity. They would also want to find a star with a very active magnetic field and lots of surface flux ropes to facilitate harvesting energy from it. Another thing they might look for is one where the re-connection rate $$R = v_{in}/v_A$$ is high, since that means they can pull plasma off more quickly. In the Sweet-Parker model,

$$R = S^{-1/2} = \sqrt{\frac{\eta}{L v_A}}$$

Where $$\eta$$ is plasma resistivity, $$L$$ is the length of the reconnecting strip, and $$S$$ is a dimensionless parameter called the Lundquist number. However, the Sweet-Parker model differs from observations by several orders of magnitude thanks to a whole host of instabilities and turbulent effects. In reality we tend to see reconnection ocurring at a fairly quick rate that depends very weakly on $$S$$, so it probably wouldn't be a high priority parameter for your civilzation. I just figured I'd include it for posterity.

TL;DR: If the star-lifting model you want to use is hitting the star with enough radiation that the hot spot particles are above escape velocity, then it's pretty hard to avoid ridiculous energy requirements and your best bet is to try to find a star whose photo sphere particles are as close as possible to escape velocity already.

If instead your approach is to try to seed coronal mass ejections, finding a star with a very magnetically active surface and alfven velocity close to escape velocity would be good zeroth order metrics for star suitability. If you want more in depth answers then I recommend looking towards the literature on magnetic re-connection and turbulent plasma dynamics for some more precise criteria.

blue supergiant stars would be ideal, but hypergiant stars is what you'd want. these die quite young because their awesome size means they fuse elements such as iron. and such stars would have a lot of them in quantity equal to dozens of times more mass than the entire solar system.

it is speculated that machine life would favor these stars more than main sequence stars like ours. and would be a one stop shop for all of their needs. Provided you could milk it fast enough.

• The problem with hypergiants though is that they have a pretty deep gravity well. May 19, 2020 at 9:56
• @Philipp The gravity well is not the problem as those stars also have insanely strong stellar winds. There is actually a max. size a star can have before it simply blasts any additional matter away. The problem is the immense heat. A blue color means a really high surface temperature, and a much, much more intensive radiation. The bluer the star, the further out you need to be to avoid melting yourself. May 19, 2020 at 11:28
• @Philipp They don't have high surface gravities; while they're more massive than the majority of stars, they also have extremely large radii, often hundreds of times larger than the Sun, leading to very low surface gravities. May 19, 2020 at 14:14

It would seem that the easiest approach might be to look for a binary or tertiary system where the masses and attraction between the stars creates a "matter-swapping" phenomenon, allowing your civilization to harvest the H/He at further distances from the star surface.

An example would be: https://www.eso.org/public/news/eso2002/ regarding the HD 101584 system. (a lighter read explanation of that specific example: https://www.syfy.com/syfywire/hd-101584-a-binary-star-casts-out-a-bizarre-hourglass-of-gas )

for fiction purposes if you don't have to stick to very specific real-life examples, maybe go for a combination of Red Giant / Blue SuperGiant and a White Dwarf? Could be a basis for some cool problems trying to implement a stable system for lifting the matter from the specific point/area in space where the gas streams converge or something alike :)

It took some Googling to find this answer so here you go:

TL;DR at the bottom

Brown Dwarfs.

I found a few quotes on Quora about these

CFBDSIR 1458 10b, the star is what's called a brown dwarf. These oddball objects are often called failed stars because they have starlike heat and chemical properties but don't have enough mass for the crush of gravity to ignite nuclear fusion at their cores.

With surface temperatures hovering around 206 degrees F (97 degrees C), the newfound star is the coldest brown dwarf seen to date.

and

WISE 0855−0714 is the coolest star discovered2—it is a sub-brown dwarf star 7.2 light-years away from Earth, and its temperature ranges from 225–260 Kelvin.

The first quote provides a very comprehensive definition of brown dwarfs, basically these stars have the hydrogen necessary for fusion, but they don't have the mass, and therefore the heat and gravity for fusion to take place. Certain gas giants like Jupiter also have an abundance of hydrogen gas. The advantage here is that, the surface temperature is just very close to the boiling temperature of water, which means that heat shouldn't be an issue. However, you will not be able to obtain helium from these stars.

Just one quick sidenote, hydrogen gas doesn't necessarily need to be obtained from stars or gas giants, it is available in abundance in space, it would be far easier to collect it from there.

Star Color-Temperature Chart

As you can see the redder the cooler. It's not visible there, but the neutron star, due to its high density, has a surface temperature of a whopping 599726.85ºC. So avoid them. Therefore, you're better off going with orange stars if you want both hydrogen and helium.

One Thing Though.

Honestly, star-mining is a wasteful and unnecessary endeavor (obviously I don't your book's plot so I don't know if its a key lot element) but still, here's a tip. If the society just sent collector probes out in every direction, or if they mined from the brown dwarfs and/or gas giants I mentioned earlier, they would obtain an abundance of hydrogen, which they could then put in a fusion reactor to get helium, along with some free energy in the process.

TL;DR: Redder stars are cooler; Collect hydrogen from brown dwarfs, then instead of mining stars, make your own in a fusion reactor.