(Intro)

If you're at all like me, you enjoy the sun and are a big fan of its work.  We all depend on our nearest star to keep earth at just the right temperature to sustain life, and that just right temperature is a function of the sun's luminosity.

People often use luminosity as a synonym for brightness, but that's not really what it is.  Luminosity actually describes the total amount of energy released by a celestial object every second.

Now it may not surprise you that the sun hasn't always had the same luminosity that it has now, and (I'd rather you hear this from me than anyone else) it definitely won't stay the same in the future.

There, there, I know...

Currently a middle aged 4.5 billion year old star, the sun will spend the next 7.7 billion years getting bigger, hotter, brighter, cooler, changing color, and eventually transforming into a white dwarf, all while doing its best to destroy the earth.

At which it may or may not succeed, we're not really sure.  I mean, as a chunk of rock hurdling through space, Earth might survive all the sun's upcoming shenanigans, but us and everything that's alive on it, not so much.

Let's start with the sun that we know and love today. 

It's currently what's known as a main sequence star, which means that it's fusing hydrogen into helium in its core.  Not all stars do this, and the sun won't do it forever. 

As the sun ages, its core fills up with helium that it can't fuse.  And as the helium builds up the core starts to shrink, which makes it burn even hotter.  As a result the sun starts burning through its hydrogen more quickly.  That extra heat from the fusion going faster trickles out through the outer layers of the sun, called its envelope, and all this rising heat gradually makes out star larger, hotter, and more luminous.

This whole process I'm talking about is going on right now in the sun, but extremely slowly.  Today, the luminosity of the sun is increasing by about 1% every hundred million years.  If you do the math, you'll realize that after 1 billion years, the luminosity of the sun will increase by over 10%. 

It might not sound like much, but trust me, it's enough to put a real hurtin' on the Earth.

A 10% increase in the sun's luminosity will make the Earth so hot, that the oceans will start evaporating, and it won't end there.  Water vapor, like CO₂ is a greenhouse gas, so all that extra water vapor in the atmosphere will set off a runaway greenhouse effect.  As the planet warms, the ocean will evaporate faster, putting even more water vapor into the atmosphere, which will warm the planet faster still.  It's a classic case of positive feedback, and it'll keep going until the oceans boil away completely.

So, not only will there not be any liquid water on future earth, but the molecules of that evaporated water will also find there way to the upper atmosphere.  There, the sun's ultraviolet radiation will break off atoms of hydrogen, which will be permanently lost to space.  With the hydrogen gone, the water molecules will never be able to reform, and the earth will dry out.  Permanently.

So let's get this out of the way right now:

Even if humans manage to stick around by this time, the changes in the sun will render the earth completely uninhabitable in about a billion years.

So, you know, start planning accordingly.

But, the sun will still have a long way to go before it reaches the end of its lifespan, and even though it won't be habitable, earth might still exist. At least for a while.

Because at this point, the sun is beginning to get bigger and more luminous.

It won't be until five and a half billion years from now that the sun will run out of hydrogen in its core and end its main sequence life time.

That doesn't mean it will be totally out of hydrogen. There just won't be any in its core. But there will be a ton of fuel left in the region around the core. 

And that reserve of fuel will keep on fusing. Marking the beginning of the next phase of the sun's evolution.

A shell of fusing hydrogen around the sun's core will start pumping a massive amount of heat into the sun's envelope. And just like a hot air balloon, the hot gas in the envelope will expand. This will make the sun expand very rapidly. And although the core will be hotter than ever, because the sun has expanded so much, its surface will be a lot cooler, around 3000 Kelvin.

Combine the increasing size and cooler surface, which by now has turned from white to red, and you get a giant red star. That experts call a Red Giant Star.

In this phase, the sun's luminosity and radius will continue to grow faster and faster. Until, in about 7.2 billion years, things get crazy fast.

By this time, the sun will have become over 2000 times brighter than it is today and have a radius over a 100 times its current size. That's about as big across as the entire orbit of earth.

The sun will be so spread out, that it'll have only a weak hold on its fluffy outer atmosphere. So soon, it'll start leaking much of its loosely held gas, creating a super charged version of the sun's current solar wind.

Exactly what this means for earth, depends on how the end of this red giant phase plays out.

Some models predict that by 7.5 billion years in the future, the sun will have lost up to 30% of its mass. 

But it's still enormously huge. The ironic sounding fact is: even as a red giant sheds its mass, it keeps getting bigger in radius and even more luminous.

But its loss of mass will have some pretty serious consequences for the metro solar system neighborhood. Because a less massive star will loosen its hold on the planets, causing their orbits to drift outwards.

If the sun loses enough mass, the earth could migrate out fast enough to keep pace with the still growing luminosity of the sun and not get totally incinerated.

If it does, then the earth stays pretty safe through the rest of the sun's evolution.

But other models show that the tidal interactions between the earth and the sun could create another effect, a slight bulge in the sun's atmosphere. If this tidal bulge is big enough, it'll slowly pull the earth back in. And if that happens, our planet could be swallowed up by the atmosphere of the sun and just spiral farther and farther in until it vaporizes... and, game over.

But, in yet another, perhaps more likely, scenario, earth will be able to stay out of the sun's atmosphere but will still become so hot, up to several thousand kelvin, that the entire planet will just melt and turn into a sloshing ball of molten rock.

At this point, the sun still has more changes ahead - although it doesn't have a lot of time. In about 7.6 billion years, it'll undergo another dramatic but nearly immediate switch: it'll stop burning hydrogen in its shell, and begin fusing helium in its core.

This will happen in a near instantaneous event called the helium core flash, when the core temperature finally hits about 100 million kelvin. This burst of energy restructures the entire inside of the sun in a cosmological blink of an eye, allowing the sun to start fusing helium into carbon and oxygen, which it has never done before. 

This helium burning phase will go on for about 100 million years, until it goes through one last crazy growth spurt. By about 7.7 billion years from now, the core will be full of carbon and oxygen, which you can't generate enough heat to fuse. So it'll be forced to start fusing the remaining hydrogen outside its core, into helium again, and then turning that helium into carbon and oxygen - again.

All this fusion will send the sun hurtling back upward in luminosity and radiance, briefly becoming a red giant star one last time. 

But since its core temperatures will never push high enough to fuse the carbon and oxygen in its core, the sun's lifetime is basically over at the end of this second red giant phase, just over 7.7 billion years from now.

The luminosity of the hot carbon and oxygen core will finally push off the small amounts of remaining hydrogen and helium, briefly creating an extended shroud of ionized gas called a planetary nebula. The naked core of the sun will have about half the mass it does now, and pressed down to about the size of earth.

This is what we call a white dwarf; the remnants of a relatively low mass star, like the sun. As for earth, if it managed to survive the red giant phase, in 7.7 billion years it will be a smoldering cinder traveling around that white dwarf, which, because of its tiny size, will be too dim to keep earth warm

Both the sun and any surviving planets will spend the rest of the lifetime of the universe cooling and fading. After their fiery finale, they will dwindle into cold obscurity.

But hey - don't sell off all your stuff, or quit your job, or tell off your boss or anything, because we still have a good billion years ahead us. So let's be sure to make the most of it.

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