Roughly speaking, Jupiter is 300 times the mass of the Earth. The sun is 300,000 times the mass of the Earth, or about 1,000 times the mass of Jupiter.
As best I recall, stars don't form unless the mass is at least 0.1 solar masses (and maybe even bigger), so a minimum stellar mass would be about 30,000 times the mass of the earth, which would be 10 times larger than a planet with a mass of ten Jupiters.....
That makes Jupiter too small to be a star by a factor of about 100....
Objects that are slightly too small to initiate fusion reactions and become full fledged stars are called "brown Dwarfs"... a google search will probably tell much more than I can about them....
SR