(This is Part 4 of a series on what would happen if the Sun stopped. Read Part 1, Part 2, and Part 3 first.)

And the answer is: exactly nowhere. We switch off fusion (magically, of course, don't ask me how this would actually unfold) and the Sun just sits there, being the Sun. Same mass. Same gravity. Same blend of hydrogen and helium. Still ferociously hot in the center, cooler at the surface. Just no fusion.

What happens? Nothing. That's the answer. Nothing happens. You go about your day. You go about the next day. You live your entire life completely, totally, 100 percent, I am not kidding you, unchanged. Tides, equinoxes, plants drinking in the sunlight, beach vacations, all exactly the same.

Temperature? Same. Luminosity? Same. Mass? Same. Spectrum? Same. Size? Same.

Your children's lives, unchanged. So are their children's, and theirs, and theirs, and theirs. Written history runs back what, 5,000 years? For the first 10,000 years after fusion shuts off, twice that entire span, essentially nothing changes.

The Sun is full of hot stuff, the hot stuff is really, truly hot, and there is an awful lot of it, so it can go right on being hot stuff for a very long time.

But after 10,000 years or so (I'm being loose with the numbers here, this is built on models of the solar interior that carry real uncertainty, and besides, the whole scenario is impossible, so please don't lose any sleep over the decimal places) things do start to shift. Remember that photons take roughly 100,000 years on average to escape the core, but that's only an average. Some get out faster, on lucky breaks. Now that no new photons are being created down there, around the 10,000-year mark the Sun begins to look just slightly...thinner. A few fewer photons coming out than there should be.

This is the first moment the evidence becomes clear that something is wrong, the first time the surface finally responds to what happened in the core. This is when the world wakes up to the unfolding catastrophe, and then promptly goes back to business as usual, because there is still plenty of time before anything genuinely interesting happens.

That's because stars, being giant balls of self-gravitating gas, are weird. The core has sputtered out. It's gone cold, or at least cooler, since it still holds a vast store of internal heat, just with the nuclear faucet shut off. But without new heat and new photons welling up, it can no longer hold itself up against gravity. So it collapses. And as the core collapses, like a house built on sand, the rest of the house follows, the rest of the house being the entire Sun.

Over the course of a hundred thousand years or so, the last core-generated photons finally leave. There are still photons, of course, the Sun is still hot, but they're no longer fusion-born photons. They're just ordinary leftover heat. The Sun begins drifting out of hydrostatic equilibrium, because if gravity and pressure are two kids balanced on a teeter-totter, one of them just hopped off and wandered away.

Which means it's time for our two favorite aristocrats, Kelvin and Helmholtz, to take a bow. The Sun is shrinking now, with nothing left to hold it up. You'd expect it to cool. We did, after all, just switch off fusion. But because it's shrinking, it actually HEATS UP. In fact, depending on exactly how this plays out, it doesn't just get smaller and hotter. It might actually get brighter.

That's right. For a few million years, a no-fusion Sun is actually smaller, hotter, and brighter than the fusion-powered Sun it replaced.

I did tell you stars are weird.

This carries on for a few tens of millions of years. We can go ahead and call it Kelvin-Helmholtz coasting, because that sounds reassuringly gentle. The Sun becomes a giant warm thing generating fresh heat purely by squeezing itself smaller.

And then it dies. Slowly. Over those tens of millions of years it does finally begin to cool, its luminosity dwindling. It won't be an overnight apocalypse. It will be a gradual shifting of Earth's climate. Thirty million years is a long time. Thirty million years ago, whales were just getting into the whaling business and grasses were only beginning to spread across the continents.

That's an evolutionary timescale. Weather patterns drift, glacial patterns advance and retreat, and life on Earth has weathered far worse catastrophes over far shorter spans (a giant impact, anyone?). Life would have a real chance to adapt to a slowly cooling Sun. It would not be pretty, and it certainly would not be warm, but at least it would be slow.

After a hundred million years the Sun becomes, well, we don't actually have a name for it, because this is a made-up scenario.

And if that timeline sounds frighteningly short, keep in mind that if fusion stays on, the Sun steadily brightens as it ages, and in another 300 million years or so it gets hot enough to boil our oceans away regardless. Hmm. That probably doesn't help much.

Oh, right. The neutrinos. Those sneaky little devils give the whole game away from the very first instant. Nuclear fusion produces neutrinos (that's the weak force doing its work, flipping protons into neutrons), and unlike the photons, neutrinos stream straight out of the Sun without getting tangled up in anything. Which means that just 8 minutes after the core falls silent, neutrino observatories all over the Earth would suddenly stop detecting anything coming from the Sun at all.

They always spoil the surprise, don't they?