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Nothing lasts forever, whether it's a cold November rain or the world you live in. One day, our sun will go out, and, speaking of the apocalypse, things will get very bad, no matter what part of the world you live in. Luckily for us, that won't happen anytime soon. You can be sure that's someone else's problem.
The thing about apocalypses is that many of them are hypothetical. Nuclear war, climate change, zombies. They're all possible, but none are guaranteed. The death of the Sun? Absolutely guaranteed.
Since we know it's coming, and since you've probably been feeling a little overwhelmed with positive emotions and optimism lately, given how joyful the world is, why don't we take a look at what will happen when the Sun finally goes out, and what the unfortunate inhabitants of Earth can expect at that moment?
When will this happen?
The Sun is approximately 921 TP3 T of hydrogen, which is constantly undergoing a continuous fission reaction and generating energy. At 27 million degrees Fahrenheit, the pressure and temperature in its core are so high that hydrogen fuses into helium through thermonuclear fusion. The Sun is approximately 330,000 times more massive than Earth. The exact mass of the Sun is extremely small. In pounds, it's 4.4 followed by 30 zeros. That's a lot.
The Sun will die once it uses up all its hydrogen. This will happen in about 5 billion years. It's currently about 4.5 billion years old, so it hasn't even reached middle age yet.
At this point, you might be thinking, "Oh no, I better check my calendar in five billion years." Well, don't worry just yet, because that's when the Sun will run out of hydrogen. Once the hydrogen is used up, our star will have another two or three billion years to survive its death throes.
Our Sun is a yellow dwarf, or G-type main sequence star. Basically, any star roughly the size of our Sun that experiences the same type of thermonuclear reaction is a yellow dwarf. But as it consumes its hydrogen, it will turn into a red dwarf, and we'll learn more about that soon. This will wreak havoc not only on the Sun, but on our entire Solar System. And the process isn't over yet. The star must reach the white dwarf stage, and then it officially dies.
How will the Sun go out?
As we've already mentioned, in four to five billion years, the Sun will run out of fuel. Once all the hydrogen is gone, only helium will remain. Our Sun doesn't produce the heat or pressure in its core needed to initiate helium burning.
Currently, the Sun's enormous gravitational force constantly pulls on its core, but the production of helium through nuclear fusion pushes it back out, creating a sort of balance. Helium is essentially a waste product of nuclear fusion. The Sun doesn't need it, and it can't use it. So, like a nuclear reactor here on Earth, it simply produces this waste, and it sits there unused, because what else would it do? At least on Earth, we can bury it in a mountain and pretend we don't have to worry about it. The Sun, however, simply needs to hold it there, and eventually, that will become a problem.
As we run out of hydrogen to burn, gravitational forces will take over, making the core denser and denser. It will contain more helium than hydrogen, and the hydrogen will begin to burn outside the core. This will transform the yellow dwarf into a red giant. If you don't know what a red giant is, you can probably guess from the name that it will be significantly different from what we have now.
Since the core will be essentially dead and incredibly compressed by gravity, the hydrogen burning outside will finally be able to expand rather than remain trapped inside. It is this expansion that defines the giant portion of a red giant. The Sun, no longer bound by gravitational forces, will grow. It could reach a distance comparable to Mars.
What happens next?
Once the red dwarf has formed and expanded to its limit, the remaining Sun has only a few options. It will burn through all its available fuel until the red dwarf collapses, forming a much cooler white dwarf. This is essentially the end state for most small stars. All the fuel has been burned, and the reaction no longer occurs.
For many years, it was believed that this was how our Sun would end up forever—a cold, forgotten mass in space. But computer models suggest that the rest of the Sun's material will be ejected into space. Essentially, this is just gas and dust, and it could make up to half of the star's total mass, all of which will be ejected, forming a nebula in our Solar System.
If you've seen photos of nebulae online, some of them stunning, this is exactly what you're seeing: the interior of a once-bright star, illuminated by the last vestiges of energy from its incredibly hot core. On a cosmic scale, such nebulae are extremely short-lived, and our Sun's nebula may only last 10,000 years.
Could the Sun turn into a black hole?
A massive, dense star can potentially form a black hole upon its death. These enormous stars undergo the same process as our Sun: burning through all their fuel as their cores become denser. Due to their size, the star's mass becomes so large that upon explosion, it collapses into a black hole. This singularity is so powerful that no light can escape. Anything nearby, including planets, can potentially be drawn in and destroyed.
Stars that collapse into black holes are typically 8-10 times larger than our own star. Our Solar System, relatively speaking, is a very small star. Not only is the Sun not large enough to form a black hole, it's not even large enough to explode as a supernova upon its death or transformation into a neutron star. This doesn't mean the fate of our Solar System is better or worse—there's no such thing as a happy ending for the Solar System as a whole—but no, a black hole doesn't form.
Can Earth survive?
Obviously, we don't know exactly what the Sun will look like when it fades and transforms from a yellow dwarf into a red giant. However, based on available data on the Sun's current density, its elemental composition, and so on, computer models suggest that the red giant it becomes will engulf Mercury, Venus, and possibly Earth itself.
Again, if you plan to live a few billion years from now, you might want to prepare for what will happen before the entire planet is engulfed by a red giant. For starters, in a few hundred million years, perhaps a billion years, Earth will likely become uninhabitable. Some projections suggest we have about 500 million years before the Sun's heat becomes so intense that the oceans will evaporate, turning Earth into a vast, inhospitable greenhouse zone.
The Sun has been growing brighter throughout your entire existence. It has been growing hotter and brighter throughout Earth's existence. It's obviously a slow process, but our Sun is brighter than it was during the time of the dinosaurs. In fact, its luminosity has increased by 301 TP3 T since its formation.
The Sun's brightness is closely linked to the functioning of life on Earth. Its luminosity is expected to increase by another 101 TP3 T in a billion years. It increases by 101 TP3 T every billion years. This could lower CO2 levels so dangerously that complex plant life would no longer be able to survive. This would effectively wipe out most life on the planet.
However, some optimistic researchers believe that this cycle regulating CO2 levels isn't as temperature-dependent as we initially thought, and plants may still have another 0.6 billion years before they go extinct. Isn't that remarkable? They believe that at that point, the so-called wet greenhouse transition will kill all plants, as our atmosphere will become so saturated with moisture due to rising temperatures that plants won't be able to survive. This is essentially the same 500-million-year prediction we mentioned earlier, only on a longer timescale.
Whatever the case, the Earth's atmosphere will eventually be unable to withstand the Sun's power and will be destroyed. This will effectively end all life on Earth. Earth will become like Venus: a scorched landscape of unbreathable air, composed primarily of carbon dioxide.
Let's say in a billion years or 1.6 billion, the most complex life on Earth will go extinct. And then, three billion years from now, the red dwarf event will occur. Venus and Mercury will definitely not survive it. Scientists aren't sure whether Earth will be swallowed whole or whether we'll simply find ourselves on the very edge of a new, red sun.
If the Sun engulfs us, it will take less than a day to destroy the entire planet. Perhaps just a few minutes. If the Sun doesn't engulf us, we'll be so close that it will incinerate everything on the planet except the iron core. Everything on the surface—the crust, the mantle, all the rocks, everything beneath the surface—will be dissolved.
As an interesting example, some of the outer planets, like Pluto, may well have habitable environments with liquid water on their surfaces. It's not that life ever arose there or that it had much time to do so, but it's possible.
In short, there is no scenario in which the Earth will outlive the Sun.
Can the end of the world be prevented?
So, let's assume that humanity exists in a billion years. The Sun is growing brighter, hotter, and we, as a species, know the end is near. Is there a way to stop the destruction of ourselves and the Sun itself?
Plot of the film «"Sunlight"» 2007 was essentially like this: astronauts went into space with the goal of saving the Sun by detonating a nuclear device inside it. In real life, that's probably not entirely plausible. But we should remember that we have a billion years to come up with something better. Let's hope technology advances beyond what anyone could have imagined in 2007.
People have pondered the technical difficulties involved in trying to "fix" the Sun so it doesn't "die." Essentially, this means either getting rid of excess helium or introducing more hydrogen. Most of the hydrogen in the Solar System is already in the Sun, so finding a significant supply will be difficult.
The idea of "stirring" the Sun was proposed because it's not the outer layer that burns, but the inner core. Most of the outer hydrogen is lost and would be ejected into space before it can be used by the Sun. However, any technology capable of stirring hydrogen would likely increase the Sun's brightness and thermal output. This would require ejecting 1 to 5 million tons of helium per second.
Other ideas might involve using lasers to literally remove hydrogen in an attempt to create several smaller red dwarf stars that would burn less intensely but much longer.
Injecting hydrogen directly into the core could also cause instability, leading to intense flares and temperature spikes. Creating smaller stars would dramatically reduce their temperature. In other words, even theoretically, attempting to manipulate an entire star is a very risky proposition. But who knows what will happen in a billion years?
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