What would make a star good for star lifting?

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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.

posted over 4 years ago