Why Do Red Giants Expand?

Article Updated: 23 Feb , 2017
by

We know that the Sun will last another 5 billion years and then expand us a red giant. What will actually make this process happen?


One of the handy things about the Universe, apart from the fact that it exists, is that it lets us see crazy different configurations of everything, including planets, stars and galaxies.

We see stars like our Sun and dramatically unlike our Sun. Tiny, cool red dwarf stars with a fraction of the mass of our own, sipping away at their hydrogen juice boxes for billions and even trillions of years. Stars with way more mass than our own, blasting out enormous amounts of radiation, only lasting a few million years before they detonate as supernovae.

There are ones younger than the Sun; just now clearing out the gas and dust in their solar nebula with intense ultraviolet radiation. Stars much older than ours, bloated up into enormous sizes, nearing the end of their lives before they fade into their golden years as white dwarfs.

The Sun is a main sequence star, converting hydrogen into helium at its core, like it’s been doing for more than 4.5 billion years, and will continue to do so for another 5 or so. At the end of its life, it’s going to bloat up as a red giant, so large that it consumes Mercury and Venus, and maybe even Earth.

What’s the process going on inside the Sun that makes this happen? Let’s peel away the Sun and take a look at the core. After we’re done screaming about the burning burning hands, we’ll see that the Sun is this enormous sphere of hydrogen and helium, 1.4 million kilometers across, the actual business of fusion is happening down in the core, a region that’s a delicious bubblegum center a tiny 280,000 kilometers across.

The core is less than one percent of the entire volume, but because the density of hydrogen in the chewy center is 150 times more than liquid water, it accounts for a freakishly huge 35% of its mass.

It’s thanks to the mass of the entire star, 2 x 10^30 kg, bearing down on the core thanks to gravity. Down here in the core, temperatures are more than 15 million degrees Celsius. It’s the perfect spot for nuclear fusion picnic.

There are a few paths fusion can take, but the main one is where hydrogen atoms are mushed into helium. This process releases enough gamma radiation to make you a planet full of Hulks.

Proton-proton fusion in a sun-like star. Credit: Borb

Proton-proton fusion in a sun-like star. Credit: Borb

While the Sun has been performing hydrogen fusion, all this helium has been piling up at its core, like nuclear waste. Terrifyingly, it’s still fuel, but our little Sun just doesn’t have the temperature or pressure at its core to be able to use it.

Eventually, the fusion at the core of the Sun shuts down, choked off by all this helium and in a last gasp of high pitched mickey mouse voice terror the helium core begins to contract and heat up. At this point, an amazing thing happens. It’s now hot enough for a layer of hydrogen just around the core to heat up and begin fusion again. The Sun now gets a second chance at life.

As this outer layer contains a bigger volume than the original core of the Sun, it heats up significantly, releasing far more energy. This increase in light pressure from the core pushes much harder against gravity, and expands the volume of the Sun.

Even this isn’t the end of the star’s life. Dammit, Harkness, just stay down. Helium continues to build up, and even this extra shell around the core isn’t hot and dense enough to support fusion. So the core dies again. The star begins to contract, the gravitational energy heats up again, allowing another shell of hydrogen to have the pressure and temperature for fusion, and then we’re back in business!

Red giant. Credit:NASA/ Walt Feimer

Red giant. Credit:NASA/ Walt Feimer

Our Sun will likely go through this process multiple times, each phase taking a few years to complete as it expands and contracts, heats and cools. Our Sun becomes a variable star.

Eventually, we run out of usable hydrogen, but fortunately, it’s able to switch over to using helium as fuel, generating carbon and oxygen as byproducts. This doesn’t last long, and when it’s gone, the Sun gets swollen to hundreds of times its size, releasing thousands of times more energy.

This is when the Sun becomes that familiar red giant, gobbling up the tasty planets, including, quite possibly the Earth.The remaining atmosphere puffs out from the Sun, and drifts off into space creating a beautiful planetary nebula that future alien astronomers will enjoy for thousands of years. What’s left is a carbon oxygen core, a white dwarf.

The Sun is completely out of tricks to make fusion happen any more, and it’ll now cool down to the background temperature of the Universe. Our Sun will die in a dramatic way, billions of years from now when it bloats up 500 times its original volume.

What do you think future alien astronomers will call the planetary nebula left behind by the Sun? Give it a name in the comments below.

, , , , , , , , , , ,



7 Responses

  1. MarsFKA says:

    Well…that was badly written…

    • Bo Zo says:

      Well, presumably you’ve written many much better articles. It would be very generous of you, and no doubt edifying, if you were to share a few links.

      I for one would especially like to learn more about maturity and politeness.

      • mark_scheuer says:

        Hey, Bozo – since when do you have to be a writer to have and express an opinion? Can it, clown. MarsFKA is well within his rights to comment. Being a reader is sufficient.

        I actually enjoyed the article for its information. Some of the quips seemed a bit out of place but that’s Fraser’s style so that’s okay with me. Trying to stifle free speech in the Good Old USA, however, is totally uncool.

      • Bo Zo says:

        Hey what?
        I’m not sure what you’re saying.

        If MarsFKA is free to casually insult the author, then I reckon I am free to casually defend the author.

        Stifle free speech? Moi? But, no! I only encouraged MarsFKA to regale us with even more of his/her speech, knowing it would surely be much better written than Fraser’s. In fact, I’m probably the ONLY person that says he wants to hear more like that from MarsFKA.

        Possibly I am just dense, but I remain confused as to why you said what you said.

  2. JukKluk says:

    Terrence, I think that those future alien astronomers will call our sun’s planetary nebula Terrence.

  3. Bo Zo says:

    “What do you think future alien astronomers will call the planetary nebula left behind by the Sun?”

    Hopefully?
    “Where we came from.”

    Or else Terrence.

  4. TedH says:

    Il Silenzio, I guess… the alien astronomers will call this planetary nebula “Il Silenzio”, because it’s now quiet in that spot; no more brabbling about, 😉

Comments are closed.