Let's Talk Physics

A stochasticly updated blog about interesting topics in Physics & Astronomy

How it’s All Going to End: The Sun (Part 1)

I feel like I need to preface this (and the next few posts) with a warning. These posts are going to talk about how the Universe is out to get us. It’s a pretty grim subject, but I’m going to try and talk about it in a light hearted, semi-jokingly way. After all, the more you know!

So, the Universe is a big and scary place. As the last few posts have shown, the Earth is unimaginably small in comparison to standard objects in the Universe, and yet (at the moment) it’s all we’ve got. So I think it’s best to highlight the ways that the Universe is actively trying to kill us now, so we have plenty of time to prepare. The first of these ways that I want to talk about is what will be an inevitable end for Earth, but hopefully not for us. I’m going to talk about our friendly stellar neighbour, that giant ball of plasma in the sky, the Sun.

The Sun is a main sequence star, meaning it’s burning off all of its hydrogen fuel right now. It’s been doing this for 4.57 billion years, and we think it will continue burning hydrogen for another 4.6 billion. But what will happen after this point? Will the sun simple turn off, and our planet will be covered in ice and darkness eternal? No. No, the Earth’s fate is a lot more bleak than that.

When a star is burning hydrogen, what it really does is smash hydrogen atoms in its core together until 2 of the atoms stick. The fused atoms now become an atom of helium, and some energy is released. It’s this energy that helps support the sun from collapsing, and is also what gives us light and heat. So, as a stars life passes, a lot of helium is generated inside the star. However, there is also an envelope of hydrogen outside of the core that doesn’t undergo fusion – it’s simply a shroud of hot hydrogen plasma that hides the core. Since helium is heavier than hydrogen, the helium sinks into the very core of the star, and sits there, waiting. Finally, when the star doesn’t have enough hydrogen in the core left to create sufficient energy to support the star, the star begins to collapse very rapidly. As the pressure in the core of the star increases (due to all of the hydrogen on the outside of the star collapsing down on the core), the temperature soars. When the temperature of the core finally gets high enough (around 200,000,000 Kelvin) so that the helium in the core can now fuse with other helium atoms, energy is again released from the core that pushes out the envelope of hydrogen that had been pressing down on the core. The energy produced by the helium core now stops the star from collapsing, and actually beings to push the outer layers of the star the opposite direction, expanding the star!

So the over all motion of the star will be that when the hydrogen runs out, the star will collapse a tiny bit, and then it will get hot enough in the core that fusion between helium will happen, and the star will expand. But by how much? Well, the picture below shows our solar system as it currently is, with the furthest orbit shown being that of Mars (and a few large asteroids, a bit further out). The distance here from Earth to the Sun is 149,600,000 km while the radius of the Sun is 695,500 km. Looks like there’ll be plenty of space for the Sun to expand and leave us alone after it’s moved onto burning helium right?

The current state of our Solar System

The current state of our Solar System

Wrong. The picture below shows our solar system in 4.6 billion years. Once the Sun has expanded and is actively burning helium (and will then be called a Red Giant star), the radius of the Sun will increase to 173,900,000 km – which is larger than the distance from Earth to our dear Sun. Which means, sadly, the Earth is going to simply be swallowed up. Yikes

The Solar System in roughly 4.6 billion years

The Solar System in roughly 4.6 billion years


Sadly, for the Earth, nothing. This will more than likely be it for our home. However, hopefully by the time the sun has expanded to this size, we’ll have figured out interstellar travel, and shall be able to find a new Earth next to a star that isn’t quite so big. Some people think heading to Mars would be great, but as you can see in the picture above, Mars is actually quite close to the Sun now, and unlike the cold temperatures it has now, it would be unbearably hot. So, out into the Great Beyond it is!


If you liked this post, please comment, share, ask questions and (of course) fact check me. Also, if you have any suggestions for Sundays article, please post them in the comments section. Otherwise, I’m going to tackle how it all ends part 2: Asteroids.


About irishphysicist

I'm a PhD student with the Departments of Physics in University College Cork, Ireland and University of Notre Dame, Indiana. I want to try and bring astrophysics to the public, and also would like world domination. But that's a story for another day.

One comment on “How it’s All Going to End: The Sun (Part 1)

  1. Pingback: How it’s All Going to End: Andromeda (Part 3) | Let's Talk Physics

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This entry was posted on February 7, 2013 by in Astrophysics and tagged , , , , , , , , , , , , .
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