Practical sensibilities aside, if you made a solid sphere of any material, what’s the biggest size you could reach before the stuff at the center starts turning into something else due to pressure, temperature, etc? What would be the choice material for this experiment?
What if you allow for any form of internal structure but still limit it to one uniform material (say a spongy structure with empty space inside)? What would be the material for this?
Order of magnitude is enough, I assume we are at the celestial body scale.
The mass you are probably looking for is about 80x the mass of Jupiter.
The line of Demarcation between a Brown Dwarf and a full fledged Star.
Even with a mass of the Earth, the material at the center is super pressured and has different properties than the same stuff would on the surface, but it’s still the same element.
In a Star we have fusion which is creating Helium from Hydrogen. In bigger stars and during supernova explosions we see heavier elements created in the immense pressures.
Every atom in existence started as Hydrogen immediately after the big bang. Through fusion and the chaos/pressure of supernovae, the other elements were brought into the universe.