What to do after the Sun dies

A recent posting to the Physics arXiv asks how best to save the Earth from the Sun’s demise. While the Sun is on the Main Sequence, and in the Redwards Traverse, a soletta shade in the Earth-Sun L1 point should suffice. But when the Sun starts puffing up into a Red Giant, peaking at about 2730 times present insolation levels, the authors suggest building an artificial sun, from brown dwarfs, and parking Earth (and other planets) in orbit around it. A mammoth undertaking and, to my mind, not the most efficient use of resources. One possibility is to enshell Earth, and other useful planets, in a variably reflective shell and control surface temperatures by reflectivity. Thus, when insolation has climbed to 2730 times the present day, a reflectivity of 0.999 will be needed – the Sun’s 33.2% mass loss will put Earth 1.5 AU out and so the insolation will be ~1,218 times present day levels.

But that mass-loss will be a problem – a solar wind some 380,000 times present levels. Remember just 1,000 times present day stripped ancient Mars of ~90-95% of its atmosphere. In fact the problem is so bad it might cause the Earth to deccelerate and end up in the Sun – if it doesn’t, tidal forces might well. So what to do? The current solar wind exerts a force of roughly 17.2 MN against the Earth’s magnetic curtain (about 10 times bigger than Earth), so that will rise to 6.54 TN as the Sun blows itself away. A reflective shell deflecting 1,000 times present day sunlight and the same size might add 120 TN to that. Coupled mechanically to the Earth this will exert an outward acceleration of 2.11E-11 m/s^2… more than enough to push it outwards during the 60 million years of solar mass-loss.

When the Red Giant Branch of the Sun’s evolution is over, its core explodes in a runaway fusion reaction heaving the rest of its mass onto the Helium Main Sequence, some 50 times brighter than at present, where it happily burns for 100 million years. Now at 2.25 AU Earth’s reflective shell needs to reflect ~90% of the incident light to remain pleasant for current DNA-based life-forms. At the end of the Helium Main Sequence the core turns into a soon-to-be White Dwarf and about ~0.13 Solar Masses is blown away, while the luminosity jumps again into the thousands. This extreme behaviour lasts mere millions of years and by the end, some 7.7 billion years from now, the Sun is a Carbon/Oxygen white dwarf massing ~54% of its present day mass, shining only by its immense store of heat. Fusion reactions of carbon/oxygen can’t proceed because the core is too light-weight. The core needed to be ~0.9 solar masses to initiate the next stage, but it would do so nearly uncontrollably, proceeding to burn incredibly fast at ~1000 times brighter than today.

But a White Dwarf isn’t merely a dead star. It’s a very dense object with high gravity – given the right encouragement it’s a gigantic fusion reactor waiting to happen. If our descendents feed it (call it the X-Sun) hydrogen/helium from interstellar space, burning at a sedate x1 present levels, then the X-Sun can burn another ~0.36 solar masses up to the carbon/oxygen fusion mass-limit. That much hydrogen/helium fused represents ~40 billion years. If the carbon/oxygen fusion can be controlled, another 0.54 solar masses can be burnt before the Sun hits the Chandrasekhar Limit – the mass at which it can collapse in an implosive runaway into a neutron star/Type Ia supernova. If the fusion can be controlled further, burning carbon/oxygen all the way to Iron/Nickel, then 0.9% of the total mass-energy of the X-Sun can be liberated. At the sedate x1 output that means about 100 billion years total.

Enough for some, but can we do better? Greg Laughlin recently posted a graph on the Galaxy’s Deep Future… That Sunday Afternoon Feeling …which shows the Galaxy staying roughly as bright as it is today for next 800 billion years, due to stars aging and becoming brighter as they become Red Giants. Eventually the stars stop becoming Red Giants (at 0.3 solar masses) and instead brighten slightly and fade out as helium white dwarfs – they can’t hit the Helium Main Sequence. Conceivably this can be remedied by merging stars or feeding the helium to White Dwarf X-Stars of higher masses, but even such titanic efforts will eventually come to naught if we stick with fusion powered stars alone.

So what next? Two options –

  • catalysed proton-decay, via small black-holes, could allow the total mass energy of stars to be used up. In the case of the X-Sun that’d be another 20 trillion years of shining bright before all the mass is gone.
  • Dark-Matter burning. Some Dark-Matter models have self-annihilating particles which produce real, visible energy. These could, theoretically, be channelled in to old stars to power them. Assuming x10 dark-matter to star-matter, this might power a bright Galaxy for ~400 trillion years.
  • By then all that’s left is a slowly expanding cloud of leptons and a swifter bubble of photons shooting away at lightspeed. Just a bit chilly, unless some other energy source can be found. While 400 trillion years sounds like an Eternity, it’s really a speck in a vast sea of Time. Matter itself – protons and neutrons – can persist 10^40 years or so before virtual black-holes cause proton-decay. The ratio of 400 trillion years to 10^40 years is the same as the ratio between the Universe’s current age and the first 0.2 of a micro-second. Freeman Dyson, in 1979, suggested Life needed to scale itself to the natural processes of cosmic evolution – essentially running its clock ever slower as energy sources became more attenuated. Assuming long periods of inactivity (though how do you mark the time?) his equations suggested Life could ‘think’ an infinite number of thoughts with finite energy supplies during Endless Time.

    The ‘Clock problem’ and, more recently, the Waste Heat problem, mean that Dyson’s first version of the argument is disproven. Infinite subjective time will need either infinite energy or will result in endless repetition of thoughts if non-dissipative computing is used, thus effectively being ‘finite time’ anyway. Infinite energy can only become feasible if the Universe is collapsing AND space-time is infinitely divisible. The first is open to falsification, and thus is meaningful scientifically (even if eventually disproven), but the latter is a philosophical issue that’s hard to turn into meaningful physics. Frank Tipler thinks he has done so, and his Omega Point Theory posits one way that Life might have an Infinite Subjective Life, but with the curious result that it all happens in a hot, dense speck growing ever smaller in a finite “objective” time.

    A lot of physicists like infinities – either Singularities, or Infinite Space, Parallel Worlds or whatever. I find them rather mind-boggling, but that doesn’t necessarily make them unreal. But none of them are open to scientific verification – they can only ever be falsified by running up against a finite Limit. And there’s no telling where or when that’ll happen…

    5 thoughts on “What to do after the Sun dies

    1. But that mass-loss will be a problem – a solar wind some 380,000 times present levels.

      Do bear in mind, however, this is mass loss rate, not outflow velocity. While red giants have a whopping mass loss rate, they have a relatively sedentiary wind speed of around 10-30 km/s (as a rule, it’s the same as their surface escape velocity. Main sequence stars like the Sun have a much faster wind speed at circa 200 km/s, abeit with a lower mass loss rate.

      I’m not entirely sure of the effect this would have — but I suspect a red giant wind would significantly alter a planetary atmosphere, chemically. Being less energetic though, it might not quite so readily strip away the atmosphere itself…

      Just my 2c. :)

    2. Hi InvaderXan

      What drives the wind? Some sort of MHD process? Plasma physics is largely unknown territory for me – the maths tends to scare me. Tends to scare even the physicists from what I’ve read ;-)

    3. To be honest, plasma physics isn’t exactly my forte either… Though thankfully, the processes behind stellar winds are fairly easy to grasp.

      In red giants, the stellar wind is believed to be driven by radiation pressure (literally, the pressure exerted by light!). That high mass loss causes lots of collisions. As atoms collide, they form into molecules which eventually aggregate into dust. Radiation pressure from the star then pushes against that condensing dust and forces it outwards.

      For Sun-like stars, stellar winds are driven by the star’s hot corona. At several million kelvins, atoms are accelerated away from the central star. Of course, some are also accelerated back towards the star (it’s an entropy thing). Because the corona is hot ionised plasma it’s guided by magnetic field lines, giving it’s characteristic appearance.

      I forget what the mechanism is for massive stars, but I think it’s largely radiation pressure again. Around blue hypergiants, for instance, the radiation pressure is so powerful (combined with the temperature of the surface) that it can accelerate the stellar wind to around 2000 km/s!

      Hope that makes sense! This is all from memory, and I’m sure the reality is a little more complicated, but I think that’s a fairly good explanation (and without any scary maths!). ;)

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