Death of the Sun - Script (Patreon)

The year is 5.4 billion. Earth is lifeless. What was once a pale blue dot is now a desolate brown rock. The Sun has inflated to around one and a half times its original size, and shines brighter than ever before. But something new is about to happen. In just a moment, our star will fuse the last of the hydrogen in its core, triggering a cataclysmic sequence of events. What’s about to happen? Grab your popcorn and pull up a seat. It’s time to watch the death of our sun.

Like living things, stars have a life cycle. They’re born inside of a cloud of gas and dust, they live their lives fusing hydrogen into helium in their cores, shining bright and stable for as long as that hydrogen lasts. Then they die, each going out in its own beautiful way. How a star dies depends on its mass. The bigger they are, the shorter their lives and the more sublime their deaths. The most massive stars only live for hundreds of thousands to millions of years, and die in spectacular explosions called supernovae. That is not the fate of our sun.

The sun is now 4.6 billion years old. It’s middle aged. Based on its mass, it’s expected to have a lifespan of around ten billion years. And here by “life” I mean the period of core-hydrogen fusion, when the steady flow of energy from the core provides consistent support against gravitational collapse. Well, relatively consistent. Over the course of its life the sun’s core is heating, causing the sun to grow and brighten with disastrous consequences for the habitability of the Earth. In fact we looked at that particular catastrophe recently. It’s a cheery episode. Check it out.

But what are the prospects for our little planet after the sun runs out of fuel? As the last traces of hydrogen are slammed together into helium-4 nuclei, fusion will cease. That helium is useless as fuel – at least for now – because helium fusion requires temperatures of around 100 million Kelvin. At the core’s last gasp it’ll be less that 20 million K. So with no outward flow of energy to fight against gravity, that core begins to collapse, dragging the surrounding layer of hydrogen with it. Soon, a thin shell of fresh hydrogen fuel around the dead core becomes hot enough to ignite. Hydrogen shell burning begins. This is not the new lease on life that we might hope for. In fact it’s catastrophic.

That burning shell is so thin that radiation escapes outwards without providing pressure to resist collapse. Instead, the helium produced in the shell is deposited on the dead core, causing everything to collapse further. Density rises and shell fusion spikes. That incredible outward flow of energy DOES counteract gravity for the outer layers of the sun, and those layers expand. Over the next half billion years the Sun doubles in size and luminosity as a sub-giant. In the following half-billion years the process accelerates and the Sun inflates to around the size of Venus’s orbit, shining with a few thousand times it’s current luminosity. But that energy output is spread over an increasing surface area, so the surface cools down. It shifts from its current yellow-green-hot through orange and finally deep red. The sun is now a red giant.

While the star’s outer layers of hydrogen are expanding and cooling, the core continues to collapse until it hits a quantum mechanical limit. The electrons are packed so close together that they run up against the Pauli exclusion principle; the rule that says fermions like electrons can’t occupy the same quantum state. Be grateful for Pauli exclusion– it’ll save the Sun from collapsing into a black hole, and it’s also currently stopping you from falling through the floor.

The core shrinks slowly even as it gains mass from the shell. It heats up to that critical hundred million kelvin temperature. Then, in an instant, it ignites in a helium flash. The once inert helium fuses into carbon via the triple alpha process. The core is resurrected, with a vast new supply of fuel. It expands again, and the corresponding lower density allows helium fusion to slow down. The entire star dims and shrinks to around 10 times the current radius. But this new lease on life is short – the helium burning will convert that core to carbon and oxygen in only 100 million years or so, after which the final dance of death begins.

Surrounding an inert carbon-oxygen core are two fusing shells – helium fusion inside hydrogen fusion. The energy output is insane, and the sun swells to a red giant once more. This time its outer layers will reach Earth’s orbit, and probably then some. So is that it? Earth gets gobbled? Actually, opinion differs on this. See, the red giant sun is so large and luminous that it has only a tenuous hold on its outer laters. Its been blasting its own mass into the solar system in great winds through both red giant phases. As it loses mass, its gravitational hold weakens and so Earth’s orbit expands… perhaps eventually taking it as far out as Mars’s orbit if the Sun doesn’t over take it first.

But it’s a race. The red-giant sun fills most of the sky and is getting larger. Earth is enveloped in the searing stellar winds, and drag from these may slow its retreat. A tidal bulge in the sun towards the Earth may increase its gravitational hold, slowing our escape. Or the Sun may just expand too quickly and too far for us to escape. Theoretical models are still not entirely settled, but Earth’s prospects look bleak. It may well end up in orbit inside the sun. There, it will revolve through the tenuous, searing plasma as the crust melts and then the entire mantle begins to vaporize.

The final red giant phase is brief – only a few tens of millions of years. Explosions of runaway fusion in the double shell blast away the outer layers, perhaps creating a beautiful planetary nebula. The naked core of degenerate carbon and oxygen

And the Earth? Perhaps it just managed to escape the expanding sun and a lonely, desolate world orbits out in the current location of the asteroid belt. Or perhaps it didn’t quite make it. In that case the entire planet may have been vaporized. It’s also possible that the iron core survives, and continues to orbit the now-white-dwarf sun.

And us? It’s wildly speculative and optimistic to even imagine that our species or its evolutionary … or silicon descendants still exist through all of this. But if we ... if THEY do, then let’s hope that we’ve escaped the solar system by then. If we’re still Sol-bound the outer solar system may harbor us for a little while. The habitable zone – the region with the right solar flux for liquid water – will expand out beyond Neptune’s orbit in the first red giant phase. That’s Neptune’s current orbit. All the planets orbits will have expanded, and so the moons of the outer gas giants may provide brief refuge. During the helium-burning phase the habitable zone contracts again. For a hundred million years, Jupiter’s moons may be a final, temperate home as we wait for the Sun’s inevitable death, and look to the galaxy for a new home, across far reaches of space time.