What Will Happen When the Sun Dies? | Unveiled

What Will Happen When the Sun Dies?

When humans only live for around eighty years on average, it can be hard to quantify something with a lifespan as long as the sun ever disappearing. Ancient and constant, the sun has been watching over our planet since Earth’s creation. But even it can’t last forever.

This is Unveiled, and today we’re answering the extraordinary question; what will happen when the sun dies?

The Sun (as it is) has so far been shining for around 5 billion years and it’s thought that it is halfway through a 10-billion-year lifespan. Despite how constant and immovable it seems, it will, like all stars, eventually run out of fuel and die. Humans will most likely have disappeared too by this point - having either died out ourselves or relocated to another star system - but while we don’t know exactly what will get rid of us for good, the Sun’s steady increase in brightness could be a factor. Right now, it’s increasing by about 10% every one billion years, which means that long before the sun itself expires, it will have totally transformed Earth. Over time, our oceans will boil and evaporate, and our once-habitable planet will transform into a desolate, definitely inhospitable fireball similar to Venus. It’s a fairly bleak outlook from our vantage point!

The reason the sun is getting brighter is the same reason that it will eventually die: the fusion of hydrogen into helium that’s happening inside it. The process is ongoing and taking place all the time, but our star is very gradually “running out of” hydrogen, all while the amount of helium inside it builds up… As a result, the pressure is steadily increasing in the sun’s core, for now making it brighter. But eventually, with no hydrogen left, the fusion will stop - and the sun’s cosmological status will then change forever, too.

A supernova is one of the most spectacular ways a star can meet its end, but this only happens to the most massive stars in the universe which – for better or worse – doesn’t include our sun. After a star goes supernova, it can go one of two ways: neutron star or black hole. But, because the sun doesn’t have anywhere near enough mass, neither of those things are going to happen to it, either. That’s not at all to say that its death will be a quiet affair, though; quite the opposite!

As the sun runs out of fuel (in around five billion years’ time) its core will get hotter and hotter, but also smaller. Meanwhile, temperatures will actually drop in the outer layers, but those layers will also be expanding. The sun as a whole will become a much, much larger prospect than it is now… And despite the lower surface temperatures, it would still be deadly hot, at upwards of 3,000 degrees Kelvin… which is bad news for the inner planets. In time, the sun will become so big that it’ll completely engulf Mercury, Venus, and possibly even Earth… That’s two, perhaps three, solar system worlds up in flames, as the sun officially becomes a red giant!

Were you to be standing on one of those planets about now, the sun would appear a vast and red ball of doom slowly absorbing the entire sky… and even if you weren’t on one of the unfortunate worlds to be totally consumed by it, there’d be wholesale changes in terms of climate, atmosphere and general cosmological balance all across the solar system! But a different picture is painted when you imagine looking at it from a greater distance. The material being ejected by the expanding red giant is called its “envelope”, and as it escapes into space the burning-hot core of the sun, still heating up because of compression, is also luminous enough so that all of the debris is visible for thousands of lightyears around. This fascinating phenomenon is otherwise known as a planetary nebula, and our night sky right now is full of other distant nebulae just like it. They’re brightly coloured and beautiful clouds of gas and dust, which could eventually disperse to form brand-new stars in the future. So, even if, by the time of the sun’s death, the human race had long moved out of the solar system to some other distant place, we would be well aware of our original sun’s fiery fate!

But this still isn’t quite the end, as the sun has one last stage to go through - and it’s the longest one of all. Once the entire nebula is gone, the burning-hot core at its heart is all that’s left behind… and it’s now, in the very far future, that our sun will turn into a white dwarf. This white dwarf will be many times hotter than the sun ever was, at about 20,000 degrees - but perhaps up to 100,000 degrees - Kelvin! Much too hot for life on the closest planets around to ever hope to recover.

The solar system will once again look very different, as well. Physically, the white dwarf sun will only be about the size that Earth is now, but it’d also be one of the densest objects in the universe. Because white dwarfs are so small but so hot, they can continue emitting heat and light for as long as 1 quadrillion years after forming - that’s one followed by fifteen zeroes! So, it’s an almost inconceivable length of time before all trace of the sun disappears completely. In fact, white dwarfs are so long-lived that we’ve yet to observe one anywhere that’s died! It’s thought, though, that when a white dwarf does run out of energy it will dim and ultimately become a black dwarf - but the universe hasn’t been around long enough for even one of these to form!

From Earth’s point of view, the sun dying is no good thing. But from humanity’s perspective, if we make it far enough to even be around to witness it, it isn’t necessarily all scorching, burning doom and gloom… If we were still living in and around the solar system at the time, it’d be more a matter of adaptation; the ultimate test for whatever futuristic technologies we’d have developed by then. Who knows… the white dwarf sun could still hold a “habitable zone” at a new, optimum distance around it… so we’d only need to work out where that was, and how to get inside it. As long as we were able to travel away from the sun faster than how quickly it expands during its red giant phase, we’d at least have a little hope. But hey, let’s cross that bridge when we come to it - right?

Regardless of what we do, the sun’s future is set. After running out of hydrogen, it’ll expand before shedding its outer layers in a glorious planetary nebula. Finally, it’ll transform into a near-immortal white dwarf, quietly burning away for the rest of its days. And that’s what will happen when the sun dies.