Dyson Spheres, SETI and Old Stars

…are a fun, but widely misunderstood, concept.

Centauri Dreams has an article on them that caught my eye earlier.

They tend to be imagined as big solid balls, rather like a hollow ball bearing which just happens to have a sun in the middle. Unfortunately, there are lots of engineering reasons why this probably wouldn’t work. The internal stresses of such an object would probably break it apart. Also, if it were even slightly off-centre from the star, tidal forces could cause problems – and it would have to be off-centre. If you wanted to simulate any reasonable ‘gravity’ on the inner surface, you’d have to spin the thing. Only the problem then is that it’d bulge at the equator, just like the Earth does. It’d also be squeezed along the polar axis. The overall shape would be basically be a squashed sphere – so different bits of it would end up different distances from the star by definition.

Dyson’s actual idea was a sort of cloud of asteroid-sized habitats surrounding the star, arranged so that all of the star’s light can be collected and used for industrial purposes. This is more workable then the giant ballbearing idea – but it still has some issues. For instance, an ablation cascade seems inevitable for a configuration this dense. Also, maintaining the orbits would be an absolute nightmare – this is well beyond the territory of the three-body problem. No analytical solution to the system will be possible. All you could hope to do would be try and keep the millions of habitats stable for a finite time – but sooner or later, chaos will ensue.

Eventually, all your habitats have ground each other to dust and you’ll be left with nothing, which is obviously a bit of a waste. (And, interestingly, the one actual search for Dyson Spheres that I’m aware of didn’t find any.)

No, it just doesn’t seem like an economically-sensible project. As Centauri Dreams points out, a more sensible project for such an old and capable society would be to try and keep their existing sun/suns going for longer. And, just for once in SETI, this leads to something that’s actually observable(!). Consider this very badly-drawn colour-magnitude diagram:

(Note, incidentally, that I haven’t tried to calibrate this – this image is here only for illustrative purposes, not quantitative ones.)

Briefly a ‘colour’ represents the ratio between light received in two different regions of an objects spectrum. So here B-V is the ratio between the B and V magnitudes. B and V mean ‘blue’ and ‘visual’, essentially. So, an object that’s hotter (emits more light in the B band) will have a ‘bluer’ B-V than an object that’s cooler (emits less light in the B band). I’ve noted the relative directions on the image.

Mv represents the absolute magnitude in the V band. This is useful as absolute magnitude is directly related to an object’s actual luminosity, while visual magnitude is sort of not. Also, the magnitude scale is a bit confusing – it’s a logarithmic scale, and brighter objects return smaller numbers. (Sirius – very bright – has a V magnitude of -1.4. Proxima Centauri – very faint – is something like +15.) So the faintest things on this plot are at the top.

Now, if you looked at a hypothetical sample of presumed ‘old’ stars (perhaps isolated by their kinematics), you’d expect to see two particular blobs. One for red giants (the bottom blob) and one for red dwarfs (the top blob). (I’ve not tried to show it here, but the red dwarf blob will be much bigger/denser as there will be far more objects in it.) But, suppose you did this and found a few bluish things with relatively-bright absolute magnitudes – say G-dwarf bright – you’d see them lying between the red dwarfs and red giants (greenish dots). Thus they’d really stand out on a diagram like this. And, I suspect, there’d be spectroscopic evidence as well. If someone was keeping a star alive long beyond its time, then it’s going to have a higher than normal helium-to-hydrogen ratio, simply because it’s been burning longer.

So … we’d be looking for a population of kinematically-old stars with unusually blue colours and odd spectra. It sounds like the kind of thing that the Sloan Digital Sky Survey would be suited to. And, unlike many things in SETI, it actually sounds like a practicable research project.

And, oddly enough, some globular clusters are known to contain blue stragglers,. Not that I’m suggesting anything, of course.

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