Since they were first discovered in 1952, blue stragglers have baffled astronomers. Appearing strangely youthful amid their ageing brethren, many now believe blue stragglers are formed when two smaller stars merge. There is, however, one theory which is rather more... outlandish. In a fascinating paper dating back to 1990, Martin Beech suggests an alternative explanation. What if an advanced civilization, having invested so much time and effort into their home star system, decided to artificially prolong the lifetime of their host star?

There are billions of stars in the Milky Way. Assuming that we're not alone in the Universe and some of those stars also have civilisations around them, statistically at least a handful of civilisations must be facing mortal peril. A fair amount of discussion has been devoted over the years to how an advanced civilisation may survive the demise of their parent star. Stars are, after all, inconstant. After leaving the main sequence and swelling into a red giant the slow inexorable increase in stellar radius and luminosity would steadily fry the inner planets. Most discussion about how to survive this is centred around a civilisation moving their homeworld to a higher orbit (keeping the stellar radiation they receive constant) or simply fleeing to the expanses of space in the hope of finding a new homeworld capable of weathering a few billion years more. The possibility explored by Beech is, at least in concept, simple. Stop your host star leaving the main sequence, and your homeworld could simply stay where it is.

Simply put, it's possible to drastically alter the course of stellar evolution, and if a civilisation were to do so, it's host star would bear a striking resemblance to one of the mysterious blue stragglers. Stars leave the main sequence when their cores run out of hydrogen to fuse, so they start to fuse helium instead. The thing is, the bulk of the star will still be rich in hydrogen. If you could mix it all together, chemically homogenising the star, it could burn for longer. This is astroengineering in its truest sense -- the engineering of a whole star. But that isn't all that's required. A chemically homogenised star would still evolve to have a hugely greater luminosity, and while it's lifespan would be longer, it wouldn't be significantly longer. The key to this problem lies in mass loss. Less mass means less pressure in the core, and less pressure in the core means less helium burning, resulting in a lower output. The end result is that the star becomes much hotter (and therefore appears bluer), while maintaining roughly the same luminosity output and with a minimal variation in its radius.

So by chemically homogenising the star and causing it to lose a sufficient amount of mass (around 60-80% of it's original mass over the course of its life is about ideal), a star's lifespan can be increased to almost 10 times it's natural length. This means that for a star like the Sun, instead of lasting for only 8 billion years on its main sequence, it could last closer to 80 billion years! Even if only one civilisation in the whole universe has achieved this, they wouldn't need to move anywhere for a long, long time to come.

As for how to achieve all of this, there are a few ideas. Mixing the star's interior could be achieved by creating hotspots inside the star, near its core, to cause convection. This could be done with hydrogen bombs, for instance, or with huge targeted laser blasts. Interestingly, this means the human race already has the technology (albeit not the ability) to do this! Alternatively, an increased rotation speed or magnetic field could cause mixing within the star (and many blue stragglers have been seen to have higher rotation speeds than average).

For the mass loss, an increased rotation rate might cause more material to be centrifuged off the star's surface. Another idea, actually considered by Hawking to explain blue stragglers, involves a relatively small black hole centred in the star's core, which would cause the effective mass loss while devouring the star from the inside.

Personally though, I'd like to add the sci fi option. Virtually unimaginable for a race with our current level of technology, "Star lifting" is a concept which might allow an advanced civilisation to mine a star. Constrained to science fiction for us, the Star Forge from the Star Wars universe (pictured to the left) is quite a good illustration. Mining a star this way would give two benefits to such a civilisation, both helping them prolong their star's lifespan, and providing a virtually limitless supply of resources for the duration of that lifespan.

Of course, Beech doesn't intend to make the claim that all blue stragglers are artificially produced (indeed, some may be stealing fuel from their neighbours), admitting that his discussion is, by definition, quite speculative. Simply the possibility that some of these curious stars might be artificially created is a fascinating one. Perhaps he has a good point, and perhaps blue stragglers might be a good place to look for evidence of advanced civilisations and astroengineering activities -- particularly any which lie outside the globular clusters where most of them usually reside. While admittedly, such civilisations would still eventually have to venture out into space to fend for themselves, they may not need to do so for significantly longer than we'd previously thought.

Delaying the demise of your host star... You have to admit, as a survival strategy, it kinda works.


Martin Beech (1990). Blue stragglers as indicators of extraterrestrial civilisations? Earth, Moon and Planets, 49 (2), 177-186 DOI: 10.1007/BF00053979

Everyone knows the Deathstar. By now, it's surely an iconic depiction of power and technological potential. As such, it's a pretty good example of the bizarre field of astroengineering. In other words, designing and engineering objects on an astrophysical scale. Astroengineering is currently of purely academic interest to us humans. Our technology isn't yet capable of even beginning to construct such vast masterpieces of technology. As such it's relegated to fields like "exploratory engineering" and science fiction.

The fact remains though, that an advanced civillisation might be able to create such an object (although hopefully not with the intention of destroying planets). Scientists working at SETI have already discussed the possibility of looking for such giant machines. Indeed, the discovery of an artificial planet or moon around another star could give unequivocal evidence of an extraterrestrial civillisation. This was the reason behind my reading about all of this. With the planned space telescope missions aiming to image terrestrial sized planets around other stars, is there a possibility we could actually image an artificial object? What implications might that have for humanity's further exploration of the Galaxy? Should we try and contact them? Might they already know about us? It opens up a whole saga of interesting thought experiments... The biggest question then is, if we want to find intelligent life with all of these powerful new telescopes - Should we be looking for the Deathstar?

Astroengineered artefacts are a recurring theme in many science fiction plotlines. Larry Niven's "Ringworld" is all about an artificial ring shaped object in orbit around a star. Engineered to make the entire inward facing edge a habitable area, the ringworld is an artificial "planet" with many thousand dimes the surface area of a real planet like Earth. The Halo series of games features a similar ringworld concept (the "Halo" itself). These differ from "space stations" in that they're much much bigger! As an example, a typical sci-fi space station (Babylon 5, for instance) would be seen as little more than a couple of pixels on the scale shown below, to the right.

The Deathstar, as it happens, is maybe not the best example I could've chosen. While such an object would be a massive feat of construction generating stresses and strains which no known engineering materials could stand up to, it's still relatively small even compared to the Earth. The first Deathstar (the one destroyed in Episode IV) is actually a lot smaller than Ceres, currently the smallest recognised dwarf planet. Given the difficulty in imaging objects in orbit around other stars (and the greater difficulty in even finding interstellar objects), even a space station this size might slip under the proverbial radar. Objects this size are difficult to detect, even in the fringes of our own solar system. Not that I'm saying there's a Deathstar lurking in the Kuiper belt. You never know though...

There are, however, much larger astroengineered objects which might be easily recognisable as such. For instance, the famous theoretical physicist Freeman Dyson proposed a concept which science fiction authors have adored ever since. The so-called "Dyson Sphere" is a theoretical way to harvest all (or almost all) of the light from a star. Science fiction stories (including at least one Star Trek episode) tend to depict a solid shell around a star, harvesting all the star's light. In reality, this is the least plausible model, largely because a single solar system probably wouldn't contain enough material to build such an object. What's more, with no means of fixing it in place, the shell would be free to drift with respect to the star, possibly with disastrous consequences.

A more likely scenario is known as the Dyson Swarm - a vast array of smaller satellites in orbit around the star. Layer upon layer of these satellites could be used to capture increasingly lower wavelengths of starlight, harvesting a ridiculous amount of energy. A further concept has been devised, known as a Matrioshka Brain, where all of this energy is used to power mind bogglingly large calculations. With that kind of computing power, a civillisation could conceivably model an entire universe inside their own version of the internet. But this is degenerating into wild and flagrant speculation.

Nonetheless, Dyson Swarms are hypothesised to be detectable by an excess of infrared, and only a small amount of the star's actual light escaping. Thus far, nothing matching that description has been discovered. Though many star systems, including Sirius, show an infrared excess, these can all be attributed to dust.

Who knows... When ESA launches Darwin, perhaps we might discover something unexpected!