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What is the smallest sun that can exist in nature? Could a civilization develop an artificial sun that will fit inside the planet?

grldsndrs
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    We are currently building a very small one at Cadarache in southern France, about 65 km north-northeast of Marseilles (100 km by road). – AlexP Jan 15 '23 at 21:31
  • for the first answer here indictes that you can't... what say you – grldsndrs Jan 16 '23 at 00:24
  • The answer refers to a nuclear fusion reactor using gravitational confinement. The one we are building in France uses magnetic confinement. According to the current planning the deuterium-tritium fusion reaction will be first ignited in 2035. Here is a short video showing the current status of the construction. Absolutely worth watching. – AlexP Jan 16 '23 at 00:44
  • so are you saying that it could be done? – grldsndrs Jan 16 '23 at 01:15
  • Regarding feasibility of having objects inside other objects, read about Dyson spheres. – user1079505 Jan 16 '23 at 06:07
  • Possibly worth looking at this? https://worldbuilding.stackexchange.com/questions/237946/could-a-moon-made-of-fissile-uranium-produce-enough-heat-to-replace-the-sun –  Jan 16 '23 at 07:02

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8.7% the diameter of our sun

According to fancy-pants astronomer Todd Henry:

a star can be no smaller than 8.7 percent the diameter of our Sun to sustain nuclear fusion. It must also have an average brightness no less than 0.00125 percent that of the Sun. In addition, the surface brightness of this slim star should be at least 1,727 degrees Celsius (3,140 Fahrenheit). -- Futurism

This would never fit inside a planet.

According to the same article, that size is actually slightly smaller than Jupiter. The problem is that it's not going to work to have a planet crust that's 1 inch thick, with an actual burning star beneath it. The tiny crust will just collapse into the star.

Tom
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    Downvote because you forgot to consider a really strong crust. Extremely crusty. And it would probably be 2 inches thick. – Willk Jan 15 '23 at 22:55
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    @Willk 2 inches?! That's barely enough for any cheese, not to mention herbs and spices! People may be able to survive on such a hellscape, but I wouldn't call it living. – Tom Jan 15 '23 at 23:39