A question about Dyson Spheres

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George O
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A question about Dyson Spheres

Postby George O » Tue Dec 08, 2015 11:53 pm UTC

Assume you have an unbreakable Dyson Sphere (or any sphere, it just has to be wrapped around a star) and the star inside eventually goes through a change (shrinking, expanding, exploding, etc.) where it starts coming into contact with the sphere but is unable to push past it. What would happen?

I'm not well versed in astronomy, physics, or chemistry but I was just interested in seeing this question get answered so I came here and made an account. Thanks in advance.

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Re: A question about Dyson Spheres

Postby Xanthir » Wed Dec 09, 2015 12:59 am UTC

It depends on what the Dyson Sphere is made of. Since it's unbreakable, it certainly wouldn't break, but that's about all we can tell. Whoever's living on the inside probably wouldn't be very happy, tho.
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George O
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Re: A question about Dyson Spheres

Postby George O » Wed Dec 09, 2015 1:06 am UTC

The sphere is made of an unbreakable material that does not interact in any way, shape or form with the star besides blocking everything (atoms, light, radiation, etc.). What I want to know is what the star material would do.

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Re: A question about Dyson Spheres

Postby thoughtfully » Wed Dec 09, 2015 2:36 am UTC

The inability to expand beyond the Dyson sphere would be a bit like considering the star as the core of a much bigger star with a heavy envelope of matter holding it down. Pressure and heat goes up, possibly igniting new sorts of fusion reactions. A lot is going to depend on how heat is conducted through the Dyson sphere to the external environment.
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Re: A question about Dyson Spheres

Postby elasto » Wed Dec 09, 2015 9:03 am UTC

Welcome to these forums! :D

As a small point, given the unrealistic nature of this Dyson sphere, this thread should probably have been posted to the Fictional Science subforum.

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sevenperforce
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Re: A question about Dyson Spheres

Postby sevenperforce » Wed Dec 09, 2015 3:11 pm UTC

That's an intriguing question! I guess the issue is less about Dyson spheres and more about stellar evolution under certain constraints.

If I understand the question right, though, the Dyson sphere is blocking everything: matter, heat, light, and so forth. If that's the case, whatever star is inside it will run into problems right away.

Stars are in hydrostatic equilibrium; the gravity that tries to make the star collapse is balanced by the outward pressure of light and heat. It's a fairly resilient arrangement. If the star gains more matter, becoming heavier, gravity will pull it together into a smaller sphere until it again matches the outward flow of heat and light. If the core undergoes a change and begins to produce more heat, the increased radiation pressure will cause the star to grow in size until the balance is reached again.

However, this balance has its limits. As a star grows in mass, it burns hotter and hotter, causing it to grow in size. But as the star's size grows, gravity's hold on its surface becomes more tenuous thanks to the way gravitational force drops off with radial distance. Very massive stars will eventually become so hot and so large that the force of radiation pressure at their surface is stronger than their surface gravity, and they will begin blowing off large portions of their outer layers.

What does this have to do with our Dyson sphere thought experiment? Well, with ordinary stars, the radiation pressure that balances inward gravitational force is carried away in the form of light...but if our sphere is reflecting that light back toward the star, then all that energy has nowhere to go. The star will re-absorb its energy flux and begin heating up very quickly, ballooning in size until it fills the Dyson sphere completely.

Then things get messy. The core will begin to grow in size as the bounded star becomes hotter and hotter, with more and more of the star's interior now energetic enough to fuse hydrogen. Ordinarily, the radiation pressure is all coming out of the core so it balances gravity...but as fusion spreads throughout the star, the radiation pressure coming in toward the center (and being reflected from the Dyson sphere at the edge) becomes greater than the outward radiation pressure. With the star no longer able to sustain itself against its own weight, gravity will cause the core to begin collapsing.

In planets, gravitational collapse is prevented by a combination of electrostatic Coulomb pressure and electron degeneracy pressure. White dwarf stars are supported by electron degeneracy pressure alone, which can support up to 1.4 solar masses. But this collapse is too energetic. The density and heat are so high that radioactive processes like beta decay start to run in reverse, with electrons and protons merging to form neutrons and neutrinos. The neutrinos fly outward, carrying energy to the outer edge of the boiling star, while the neutrons fall inward under the force of gravity. The core continues to collapse until it reaches the approximate density of an atomic nucleus, at which point neutron degeneracy pressure becomes strong enough to balance gravity on its own.

Ordinarily, this would be a Type II supernova: the outer layers of the star would rebound off the neutron star core and explode outward at a substantial fraction of the speed of light. But that doesn't happen here. Instead, the outer layers of the star remain trapped inside the Dyson sphere. The star is now turned "inside-out"; the hottest region with the most fusion is the outer surface, beaming radiation inward toward the neutron star core. The neutron star core slowly grows as more and more of the star's outer layers fall onto its surface and undergo inverse beta decay.

Here's where things get less predictable. We can model core-collapse supernova well enough to say that the process won't be able to produce a neutron star with more than about two solar masses; stars heavy enough to leave a larger remnant collapse so violently that they invariably produce black holes. But we don't have the right state equations to say how large a neutron star could grow in situ. Some research suggests that neutron/quark degeneracy could support a neutron star up to three solar masses before the gravity becomes too great and a black hole is formed, but the threshold may be lower. In any case, it's going to depend on the progenitor star inside your sphere: if it was more than 3 or 4 times the mass of the sun, it will definitely end up collapsing into a black hole; if it was smaller, it will probably remain a neutron star indefinitely.

EDIT: typo.
Last edited by sevenperforce on Fri Dec 11, 2015 3:12 pm UTC, edited 1 time in total.

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Re: A question about Dyson Spheres

Postby Beavertails » Wed Dec 09, 2015 8:40 pm UTC

sevenperforce wrote:if it was smaller, it will probably remain a neutron star indefinitely.


This will make the Pointer Sisters happy. They'll have something to dance to.

Cool response.

I think we might need to propose an xkcd What If subforum where we can group great questions and responses like this.
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Re: A question about Dyson Spheres

Postby sevenperforce » Wed Dec 09, 2015 8:43 pm UTC

Beavertails wrote:
sevenperforce wrote:if it was smaller, it will probably remain a neutron star indefinitely.


This will make the Pointer Sisters happy. They'll have something to dance to.

Cool response.

Thanks!

I think we might need to propose an xkcd What If subforum where we can group great questions and responses like this.

That's...basically what the Science and Fictional Science forums seem to be, haha!

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Re: A question about Dyson Spheres

Postby Beavertails » Wed Dec 09, 2015 8:45 pm UTC

Beavertails wrote:
sevenperforce wrote:
I think we might need to propose an xkcd What If subforum where we can group great questions and responses like this.

That's...basically what the Science and Fictional Science forums seem to be, haha!


So... I shouldn't have posted something in the What If rules thread already? :mrgreen:
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Re: A question about Dyson Spheres

Postby drachefly » Thu Dec 10, 2015 5:27 pm UTC

as fusion spreads throughout the star, the radiation pressure coming in toward the center (and being reflected from the Dyson sphere at the edge) becomes greater than the outward radiation pressure. With the star no longer able to sustain itself against its own weight, gravity will cause the core to begin collapsing.


I think this can't be right. As the temperature rises on the outer layers, they will expand. This brings them to regions of lower gravitational field, which in turn lowers their weight. This lowers the pressure of the regions underneath, which lets them expand as well.

Basically, the star will boil off to fill the space inside the sphere.

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sevenperforce
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Re: A question about Dyson Spheres

Postby sevenperforce » Thu Dec 10, 2015 5:42 pm UTC

drachefly wrote:
as fusion spreads throughout the star, the radiation pressure coming in toward the center (and being reflected from the Dyson sphere at the edge) becomes greater than the outward radiation pressure. With the star no longer able to sustain itself against its own weight, gravity will cause the core to begin collapsing.


I think this can't be right. As the temperature rises on the outer layers, they will expand. This brings them to regions of lower gravitational field, which in turn lowers their weight. This lowers the pressure of the regions underneath, which lets them expand as well.

Basically, the star will boil off to fill the space inside the sphere.

This stage occurs after the star has already boiled off to completely fill the sphere -- "if our sphere is reflecting that light back toward the star...the star will re-absorb its energy flux and begin heating up very quickly, ballooning in size until it fills the Dyson sphere completely." Once the sphere is completely filled with very hot starstuff, the outer layer will begin to heat up more quickly than the inside (thanks to geometry) until fusion is happening throughout.

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Re: A question about Dyson Spheres

Postby Beavertails » Thu Dec 10, 2015 6:15 pm UTC

sevenperforce wrote:This stage occurs after the star has already boiled off to completely fill the sphere -- "if our sphere is reflecting that light back toward the star...the star will re-absorb its energy flux and begin heating up very quickly, ballooning in size until it fills the Dyson sphere completely." Once the sphere is completely filled with very hot starstuff, the outer layer will begin to heat up more quickly than the inside (thanks to geometry) until fusion is happening throughout.


So any ballpark estimates on the lifespan of this new star-like-thing? I know you said if it turned into a neutron star, it'd last indefinitely, but...
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Re: A question about Dyson Spheres

Postby sevenperforce » Thu Dec 10, 2015 7:19 pm UTC

Beavertails wrote:
sevenperforce wrote:This stage occurs after the star has already boiled off to completely fill the sphere -- "if our sphere is reflecting that light back toward the star...the star will re-absorb its energy flux and begin heating up very quickly, ballooning in size until it fills the Dyson sphere completely." Once the sphere is completely filled with very hot starstuff, the outer layer will begin to heat up more quickly than the inside (thanks to geometry) until fusion is happening throughout.


So any ballpark estimates on the lifespan of this new star-like-thing? I know you said if it turned into a neutron star, it'd last indefinitely, but...

Well, if it turns into a neutron star, it will last until neutron decay slowly transforms it into nothing but photon gas. At that point, it may collapse into a black hole or it may not...again, depending on total mass. If it turns into a black hole, then it will remain a black hole forever.

I suppose that if the Dyson sphere was extremely large and the star was extremely small, then it wouldn't ever get hot enough to collapse into a neutron star in the first place...but for any Dyson sphere with insolation on the same order as the insolation at Earth, it'll collapse.

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Re: A question about Dyson Spheres

Postby Nicias » Thu Dec 10, 2015 9:57 pm UTC

Just wanted to point out that if this thing does collapse to a neutron star, it will be a very hot neutron star. All of the energy obtained by basically fusing the entire mass of the star is still bouncing around inside the sphere in the form of light and neutrinos

If you ever cracked this thing open, it would be an instant supernova.


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