I plan on building a (very strong) sphere of radius 93 million miles around the sun. Of course the inside will be as reflective as possible at all possible wavelengths.
What effect will my sphere have on the sun? It’s planned to turn into a red giant in about 5 billion years, and then morph slowly into a white dwarf, but I can’t wait that long to find out.
WIll my sphere expedite this process, or cause a completely different sequence? Something else?
I’ll leave it to the master to decide if I should leave the earth on the inside or outside of the sphere (if outside, I could always leave a peep-hole to keep us supplied with light.)
Yeah, but the OP takes the idea in a whole new direction. Other writings about the Dyson sphere focus on harnessing the sun’s energy output. I understand the OP to be asking something else: What would happen if the Dyson shell reflected the sun’s energy back upon itself? Would the sun burn up faster, or what?
I realize that the sun is not burning in a chemical sense, and that it is actually undergoing nuclear reactions and such, and that relecting the heat back would be a different sort of energy, but still… it’s an interesting question.
Presumably you’ll also have some method of keeping it in place. Even with the light pressure and particle flux pushing outwards, a solid sphere around a sun is gravitationally unstable, and will rapidly go off-center, get subjected to tidal forces, and crumple. Heck, even without the tidal forces, it’ll crumple under the inward gravitational pull.
I must admit, I have no idea what will happen with the energy reflected in.
Actually, a sphere is of neutral stability. Nothing prevents it from being pushed off course, but the net attraction on one side is balanced by the attraction on the other side. As the radiation and charged particle flux are essentially symmetrical there shouldn’t be any net force. Of course, the Sun does occasionally have coronal mass ejections (CME) where charged particles are spit out in a giant cloud; the degree to which they’ll push the shell off-center depends on what the mass of the shell is.
However, the temperature within the shell is going to increase exponentially, as the thin interplanetary medium absorbs energy and can’t re-radiate it. Eventually, the temperature will exceed the limits of any real world material that the shell is made of and cause it to start to sublimate. If you leave the Earth inside (and the shell is perfectly reflective) the inability of the Earth to radiate away the energy absorbed on the Solar-facing side will rapidly disrupt climate cycles and cause the Earth to be covered in clouds, further exacerbating the problem.
Assuming the sphere is arbitrarily strong, and has very low emissivity: The Sun’s temperature would increase, and I’d expect that the Sun would expand because of that. The core would also expand, lowering its density, reducing the rate of fusion, but not stopping it. This would continue until the Sun is pressing against the sphere, with the temperature continuing to increase as long as some fusion is occurring (this would be a very very long time).
Actually, a complete sphere has neutral stability. It’s a ring that’s unstable.
To the OP, if the Sun stopped losing heat radiatively, its temperature would increase, which would surely hasten its inevitable demise. At a SWAG, I would say that this would destroy the Sun within a few million years, rather than the few billion it’s currently projected to have left.
Stars are held at stable size because the inward force of gravity is balanced by the outward force of heat. The heat is maintained by nuclear reactions in a proper star.
So the increase in heat (assuming an unobtanium sphere of negligible mass, invulnerability to heat, perfect reflective capability an maximum stability) would cause the star to expand earlier than scheduled. With the sphere only 1 AU out, you’d eventually have the star expand into your sphere. At that point, even unobtanium would have to break and the star would finally be able to release its energy and cool back down to main sequence.
I’m no expert on it, but I doubt the sphere would affect the internal nuclear reactions of the star very much. The core is already tens of millions of degrees and very deep, so I’m sure most of the heating will affect the cooler outer layers that are only tens of thousands of degrees.
It would take a very long time before the increase in “ambient” temperature would result in a significant change in the conditions in the core where fusion is occurring (on the order of hundreds of millions if not billions of years) as the pressure contained by the shell is insignificant compared to the pressure in the core and the temperature would have to exceed the 7x10[sup]6[/sup] Kelvin temperature at the core/radiative zone interface for any external heat to actually transfer back into the core. The largest effect on the Sun (and this is after millions or tens of millions of years) might be some disruption of the granular structure in the convective zone as the ambient temperature exceeds the ~5700 K temperature at the top of the convective zone.
Just a WAG, but if you could get to the point of surrounding the Sun with a sphere, the solar wind would probably blow it open as soon as you sealed it.
Which is why it’s important to ensure that you build stabilizing thrusters that don’t need you to sterilize an Earth-sized portion of the ring’s inner surface in order to function.
Depends on how thick it is. The required structural strength (assuming that it isn’t rotating) is negligible; gravitational attraction from the Sun is about half of a percent of 1g. The best way to construct the shell from conventional materials would be a very thin layer of foil tacked to a lightweight hexagonal truss structure which would distribute the very modest bending loads around the circumference. The actual load is like a suspension bridge of infinite length. If you could make some kind of material that would rely upon internal tension to provide rigidity (i.e. expansion on heating on the interior versus the unexpanded exterior to amplify rigidity) might allow you to dispense with the supporting truss structure similar to how thin wall pressurized “balloon” tanks are used to provide rigidity in some liquid engine rockets, but would require two or three layers of material with different thermal mechanical characteristics (a lightweight inside material, a thermal barrier, and a high tensile strength exterior). I don’t know how you would go about putting such a thing together, though; you’d probably need to start with a ring of loose material in orbit being drawn in radially on tethers, and then expanded circumferentially to make a shell.
As long as the mass is symmetric, it will have a negligible effect on the Sun.
Sure. It won’t be containing any gas pressure or radial loads to speak of, so it really just needs to support its own weight (against the 0.006g load mentioned above). Of course, once you exceed the thermal capability for the material, it will start to break down even without load.
I’m not convinced of the last part: if the core of the sun is creating heat (through fusion) and transferring the heat to the outer layers, then adding heat from another source to the outer layers (even if they’re still not as hot as the core) will slow the rate of heat transfer from the core, leaving more heat to remain in the core, and causing the temperature of the core to rise.
After all, wrapping yourself in an electric blanket heated to 80 F will warm you, even though the blanket is not as hot as your core.
