A few years ago, I walked into my department’s weekly Astro Coffee journal club and immediately had an existential crisis about the future of the Earth.
To be clear, the discussion was not about the future of the Earth, per se. We were talking about a newly published research paper about some interesting features in the spectrum of the light from a very distant star – technically a ‘stellar remnant’, or dead star, called a white dwarf.
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This white dwarf couldn’t possibly have any effect on our planet, and nothing in its spectrum was particularly threatening. But what that paper did show us was a glimpse of the future of our Sun and, in a particularly gruesome way, ourselves.
Let me start by reassuring you that the Sun is not going to explode. This may come as a surprise – one of the most common astronomy misconceptions I encounter is the idea that our Sun is fated to go supernova someday, ending its life in a spectacular explosion that will incinerate our Solar System once and for all.
But from what we know of stellar evolution, that’s not at all what the Sun’s future looks like.
There are two main ways that a star can go supernova. One, called a core-collapse supernova, is when a very massive star burns through all its fusion fuel and collapses, rebounding into an extraordinarily intense explosion.
The other is when a stellar remnant, such as a white dwarf, has some unfortunate interaction with a companion star that obliterates them both. Our Sun is not massive enough for a core collapse, and it doesn’t have a stellar companion, so as far as we know, it's safe from either of those two outcomes.
That doesn’t, unfortunately, mean it’s immortal.
Today, the Sun is essentially a giant fusion reactor, transforming hydrogen to helium in its core and releasing a huge amount of energy in the process. Some of that energy escapes into the Universe as light, but some goes inwards, making the plasma inside the Sun bounce around at high speed.
This bouncing plasma is what causes the pressure that keeps the Sun from collapsing under its own weight, in the same way the air pressure inside a balloon stretches the rubber and keeps the balloon round. For the next five or so billion years, the Sun will happily carry on like this, but eventually, it’s going to start running out of its hydrogen supply.
At this point, things will rapidly start going wrong. As fusion slows down, the pressure support will drop, and the core will start to compress. As a result, a bit of the helium inside the core, now much hotter and denser, will begin to fuse into heavier elements, releasing energy even faster. So the Sun will brighten and swell.
By this time, the Sun will have already long since become bright enough to boil off the all of the oceans of Earth, pretty likely ending all terrestrial life. But things will get even worse for Mercury and Venus: as the Sun continues to evolve, it will puff up to hundreds of times its current size, engulfing the orbits of the two inner planets and vaporising them completely.
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What will happen to Earth at this point is a bit unclear. Will it also be engulfed, meeting the same fiery doom? Will it get pushed farther out? Honestly, things don’t look great for the future Earth either way.
At some point in what’s known as the red giant phase, the Sun will no longer be able to maintain fusion of any kind. Its core will compress even further, and its puffy outer layers will slough off into space. On the bright side, this is likely to make a very pretty planetary nebula.
Meanwhile, the Sun’s core will compress and form an extremely dense white dwarf star, which is held up not by fusion but by a weirdness of quantum mechanics – this says that if you try to pile up too many electrons, some of them start moving extremely fast, increasing the pressure enough to stop the collapse. All stars like the Sun seem fated to end their lives as dense white dwarfs, slowly cooling and fading forever.
Which brings us back to that journal club. The researchers in question had been looking at white dwarf spectral lines – the pattern in the light that tells us which elements are present – and noticed a bit of what they called ‘pollution’.
Where they expected only a few light elements, they found heavier elements such as calcium, potassium, and sodium. These elements were not produced by the stars. The researchers concluded they were debris from rocky planets the stars had recently devoured, showing up as bright and distinct as blood on the jaws of a predator.
While still reeling from this existential horror, I discovered that astronomers have been studying polluted white dwarf stars for decades, and as far as I know, maintaining emotional composure. And, sure, one could argue that a supernova would be worse. But to me, there was something particularly visceral about looking at those spectral lines and wondering about the poor lost planet that produced them.
Maybe, someday, billions of years from now, some alien astronomer will look our way. Perhaps they’ll see a smudge of dirt in the light of a lonely white dwarf star, surrounded by the glow of a nebula, and spare a thought for the beautiful world we once were.
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