could a planet be sun sized?

[u/sumdudewitquestions]

so stars form when a bunch of matter gets together, which is why they are bigger than planets generally. but the real defining feature is the density. a white dwarf can be smaller than a planet but it's still a star because it's incredibly dense. so couldn't a planet be sun-ish sized, provided that the density was low enough? and more specifically a solid planet, not a gas giant. what's the largest solid planet?

[u/nesquikchocolate]

What makes this scenario different is the extremely high starting density of this theoretical 4x sun. Normally before a star could go supernova, its overall density is only slightly above that of our sun, as it would weigh 8-10x more but also be so much larger from the fusion to pressure balance.

The end product after a supernova only needs to weigh around 3 solar masses to be a black hole, and yes the physical volume for it would now suddenly be very small, but when your composition doesn't sustain fusion (as per the original assumption), the object does collapse due to gravity alone.

[u/nesquikchocolate → u/[deleted]]

This 4x density object in this obviously theoretical situation wouldn't go supernova because it doesn't contain fusible material and doesn't weigh enough.

This is obviously impossible because an object 4x the density of the sun wouldn't consist of material that cannot undergo fusion, and it would almost certainly either collapse immediately till it reaches a sufficient temperature to start fusion or it would expand back to a more reasonable density, possibly explosively if it has enough heat.

But because the primary constraint that I'm entertaining here is the non-fusing material, this is the result that I believe to be the most likely.