Answer:
See explanation
Step-by-step explanation:
Jupiter’s diameter is in fact larger than that of the smallest star, at 140,000 kilometres against 121,000 km for the tiniest star.
However it is mass, not size, that counts. This determines the internal pressure that, if sufficiently high, can overcome the mutual repulsion of hydrogen nuclei and convert these to helium through nuclear fusion. This releases the huge amount of energy that makes stars shine.
If a large cloud of interstellar gas came Jupiter’s way, maybe the planet could gain enough extra mass to start fusion. Fusion would be short lived if it became a brown dwarf, an object midway between star and planet. If it accreted even more mass, just enough to become a true star, it would be a dim red dwarf. Its radiation would barely affect us and it wouldn’t look very different to now. A bigger worry would be Jupiter’s increased mass disrupting the solar system, not to mention the raised temperature of the sun, as a result of it capturing most of the gas cloud.
Exact figures are uncertain, but calculations suggest Jupiter would need to be 80 times as massive as it is to turn into a small red dwarf star. Another possibility, though, is a brown dwarf, which is a kind of half-star. This isn’t massive enough for ordinary hydrogen to fuse into helium as in most stars. Instead it uses the rarer hydrogen isotope deuterium. It is estimated a brown dwarf needs to be about 13 times the mass of Jupiter.