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
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?
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.
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.