The sun and Jupiter are pretty close in terms of density, and Jupiter would need to get at least an order of magnitude heavier to start fusion. I think it's just a coincidence that the outward pressure of the sun's fusion makes these numbers roughly line up.
Thirteen Jupiters seems to be a commonly-given lower limit for fusion, so let's go with that. To increase mass by thirteen times while maintaining density (and assuming the whole thing is a perfect sphere, which it obviously isn't), Jupiter needs to increase its radius by a factor of about 2.35. This increases its equatorial radius to about 168,000 km, which does swallow up the three innermost moons, but leaves the four big ones alones
Apollo42@lemmy.world 4 months ago
The moons would almost certainly fall into Jupiter or be thrown from its orbit if itsuddenly gained enough mass to become a star.
SorteKanin@feddit.dk 4 months ago
I assumed in this scenario that orbits would be left unchanged aside from orbital velocity - if we can magic Jupiter much bigger, we can magic the orbits too :P
Apollo42@lemmy.world 4 months ago
I mean if we’re talking magic now, then all bets are off haha
SorteKanin@feddit.dk 4 months ago
Not sure how else you would increase Jupiter’s mass that much :P