We are back with a big apocalyptic question! In this episode, we talk about how the Sun will change and evolve and how that will impact the planets, moons, and space rocks of the Solar System

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Chris
Today, I’m back on my apocalyptical bullshit. And we’re going to go on another adventure down a “What if” scenario!

Alfredo
Okay…

Chris
Everything seems quite bleak at the moment to put it, frankly. But it could be worse. And you know how it could be worse.

Alfredo
How?

Chris
the sun could explode.

Alfredo
Not really

Well, I mean, it could get bigger.

Yeah, yeah.

Chris
And when it gets bigger, it could just start eating planets. Because in my limited understanding of what stars do, during or before supernova, they get bigger. And bigger and anything that happens to be in the path of that happening would effectively be destroyed. Talking about our solar system if our sun were to swell up first in line to be eaten would be Mercury. So, a little bit of a two part question just here at the start, how long would it take for the sun to sort of swell up enough to envelop mercury? And what would happen to the rest of the solar system immediately after mercury had been gobbled up?

Alfredo
That’s a very interesting question. A few few corrections. General idea. Yes, stars change their size as they evolve. And yes, often is just before going supernova, and by just before it could be millions of years before they go supernova. They swell up. But it will also happen to the Sun. The sun will increase in size, even though it will never go supernova. The Sun is just not big enough to explode.

Chris
Interesting. I didn’t know our sun could never go supernova

Alfredo
It can never go supernova unless somehow you can find a lot of other material to throw on it.

Chris
It’s interesting, okay!

Alfredo
But what about expansion? Yes, the sun will expand and it will definitely swallow planets, etc. When that happens, the sun will expand when it goes from what it is now: a star in the so called main-sequence, so it’s where stars spend most of their life, to the red giant phase. So what is the red giant phase? The sun like all the other main sequence stars are burning hydrogen. Burning is not the correct term but we use it. They are using hydrogen in their core to generate energy. So there is nuclear fusion happening in the core of the sun. That is what burning hydrogen means. What is the sort of waste product of this fusion? It’s the production of a lot of helium. What you can imagine is that at the center of the sun, there is a big ball of Helium, which it’s extremely hot, but nothing is happening to it because all the hydrogen around this big ball of helium is being turned more helium and generating more energy.

Chris
It’s just feeding into it.

Alfredo
Yeah. But a certain point in about four or 5 billion years, the hydrogen in the core of the sun would have been spent. At that point, the sun will start collapsing on itself and it will suddenly due to this gravitational collapse heat up the core enough to just

Chris
Push all of the helium away?

Alfredo
No, no. So the helium is at the very center. The gravitational collapse on this helium core will heat it up to a level in which the helium start fusing and the release of energy will sort of swell-up the sun.

Chris
Got it. Okay! so it’s the burning of the helium, once all of the hydrogen has gone, which causes this expansion?

Alfredo
Yeah. Okay, so suddenly, the outer layer of the sun will become more spread out. The sun will get cooler after being spent the 5 billion years to get brighter and brighter, and it will easily, not only expand past the orbit of Mercury but also Venus and potentially all the way to the orbit of our planet.

Chris
How long will that take roughly?

Alfredo
I don’t think that we know enough about stores to say. From what I know is there is this thing called the helium flash in which the star is suddenly brighter again, once had lost its ability to burn up hydrogen, and then is just like expanding. And it could be that it’s a relatively slow process. And we know that red giants are constantly changing, because they have Stiller the same mass of the original star, but suddenly they have volume hundreds of times bigger, and so you have a much much lower density. Is it clear?

Chris
Not really

Alfredo
So you have the same mass, but suddenly is something that is hundreds of times

Chris
More spread out,

Alfredo
Yeah, it’s more spread out. So the outer layers of the star are only tenuously gravitationally bound with the rest of the star. So the star is not a profit sphere anymore (as much as any celestial object is a perfect sphere) but it’s being constantly changed by the motion, the electromagnetic fields, the heat, the radiation there’s going on inside the star. But whether it’s a slightly slow process in human lifetimes or fast process, what we know is that there are going to be changes for the rest of the solar system. Up to that point up to the point in which the sun becomes a red giant. The sun has been getting brighter. So there’s definitely going to be a point in which the oceans on Earth had evaporated. If there’s anything left on Earth, not only Mars had gone to a phase in, which is warm again, but also the moons of Jupiter and possibly Saturn are probably melted or something’s going on there, maybe not completely melted because they have a very low atmospheric pressure. So there’s definitely been changes through the solar system.

As always things, when you look at them in cosmic terms, over eons are dynamic: things change. So the whole solar system is already changing the expansion would affect obviously the planets that will be completely destroyed being inside the red giant sun. outer layers will eventually lead them to spiral towards the center. of the sun, but also all the asteroids near Earth would have been completely destroyed. And even though a red giant is much cooler… A red giant star is about 3000/3500 degrees Celsius compared to the current surface temperature of the sun, which is about 5700 degrees. So, although it’s cooler, it will still affect things because it’s much much wider. So the planets, the outer planets will be affected other asteroids and comets in the solar system will be affected. long period comments like for example, the alley comments moves around the Sun every 70 odd the years more or less, or the other comments that are between 50 to 200 years going around the sun that they definitely get quite close to the Sun and will completely destroyed by this change the asteroid some will be pushed away, some will be broken up. And it will affect the atmosphere and what’s going on around the giant planets.

Chris
Okay, well in terms of this I have one final little bit of the question which I was saving until the end. All of this has happened. The hydrogen has run out. The sun is now burning up helium. It’s expanding. It’s slowly or cosmically quickly eating up planets. Would there be a way of stopping that process? Would there be a way of refueling hydrogen into a star?

Alfredo
That’s how you get supernovae

Chris
Oh!

Alfredo
No, no, but it’s a very interesting question like, so let’s…

Chris
So you wouldn’t be able to just put it back to how it was now.

Alfredo
The issue is that, for example, any red giant, it’s not like, oh, the red giant now is completely devoid of hydrogen, there is no hydrogen to found.. No, there’s still a lot of hydrogen in the stars. It’s simply that is not so fusing anymore.

Chris
It’s not like those non left. It’s just it’s, it’s got old and it’s tired.

Alfredo
No, it’s just that is not in the right place to fuse or at the right conditions.

Chris
Oh ok. It’s not just sick of hydrogen.

Alfredo
It’s not just sick of hydrogen. So the hydrogen is not where it is simply because there are less and less deep layers of hydrogen, or there’s less and less hydrogen in the deepest layer where fusion can happen. And that is why it moves on to helium. But you might say, Okay, what if we can just grow more and more helium on to the core of the star was going to happen, that’s going to happen, a supernova and actually, potentially a very specific type of supernova. Once the sun has reached the red giant phase, it will start pushing its outer layers away. So the sun is constantly streaming particles into the solar system that is called the solar wind, but once got to the red giant phase in which the outer layer are, as I mentioned, so loosely gravitationally bound to the sun, this sort of flow of material is going to get more and more dramatic and eventually, the sun will lose all of its outer layer leaving behind just a dense core and that dense core is a white dwarf.

Chris
So it’s going to turn into a white dwarf.

Alfredo
Yes, the sun will be a white dwarf. A white dwarf is a degenerate star so it’s made of degenerate matter. not matter that we are familiar with. And white dwarfs can turn into a supernova, if they can steal enough material from a companion star. So the sun doesn’t have a companion star. So it’s definitely not happening to the sun. But if a white dwarf is in a system with another star, usually a red giant, they can sort of syphon the gas away from its companion. When they reach the limit of about 1.4 times the mass of the Sun, the pressure is just too much and star explodes. And that is a very specific type of supernova, cola type one a supernova. And it’s a special type because it tends to have more or less the same luminosity. And so it can be used as a standard candle. So knowing that it always has the same luminosity. When we look at them, we can measure different brightness because they have different distances. So it’s like picturing just streetlights on our straight line. And you have one that is really close to you is quite bright, and one that is far away is quite dim. But in reality, they all have the same luminosity. But the distance changes how bright or dim they are. More or less the same with this kind of supernovas and we can use them to measure distances to faraway galaxies pretty accurately.

Chris
Awesome. Okay, thank you for answering my question today.

Alfredo
You’re very welcome.