Just watched Chernobyl (A HBO Mini-Series) and wondered what would happen if other materials were made of Uranium(U-235)? In this article, I will be discussing the possibilities that would happen if half of the Sun’s mass were made of uranium-235
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So you create a ball of hot gas with a radius of about 695 thousand kilometers, containing about 1030 kilograms of U-235 and the same quantity of hydrogen, preheated to several thousand degrees near the surface, a lot hotter in the interior?
And what?
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That would be a mighty big kaboom.
For starters, even though you took away half the hydrogen, nuclear fusion would not stop in the interior. It will proceed at a reduced rate. At the same time, the decay heat of the U-235 is also considerable, amounting to about one third the average thermal power of the fusion reaction taking place inside the Sun. I do not have the means to calculate the fusion rate, but I would not be surprised if the overall thermal output (fusion plus radioactive decay) exceeded the thermal output of the Sun at present.
But wait… U-235 and hydrogen? Hydrogen is a pretty darn good neutron moderator (although especially in the solar interior, it won’t do much good as the temperature is too high, to begin with). But even without a moderator… while the U-235 gas at this density is rather tenuous, there is so much of it, it is practically impossible for a neutron not to hit a U-235 atom.
In short, this will be the biggest fission bomb the universe has ever seen, with a significant fraction of that octillion metric ton of that U-235 undergoing fission within a matter of seconds. And it would be a pretty efficient bomb, too, on account of its sheer size, as it just cannot disintegrate fast enough for the fission chain reaction to stop.
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So my best guess is that the result would be akin to a supernova explosion. The Sun itself would be obliterated, along with most of the solar system. And alien astronomers thousands of light-years from here, observing the explosion, would forever wonder about the strange, one-of-a-kind temporal behavior and spectral signature of this unexplained supernova in the sky.