Not sure how accurate this is (cue Blob or KennyR to set the record straight), but I think I read somewhere that if Jupiter was sixty times more massive then it would be a star.
Obviously that's a substantial difference in size.
I did read an interesting theory about what lies at the core of Jupiter. There may be a core of hydrogen under such intense pressure that it displays all the properties of a metal, but wilder theories have speculated that the pressures and temperatures within the lower reaches of the Jovian atmosphere are more than capable of compressing the carbon there into a massive diamond.
A huge diamond floating in space has already been observed (IIRC comprised of carbon present in the core of a dead star), so now might be a good time to ditch those shares in De Beers...
Back on topic... If Jupiter were sufficiently massive to become a star, I wonder if the Jovian moons would harbour life. I like to think that Europa possesses geothermal vents under the ice surface, as the constant massaging by Jupiter's gravity is already sufficient to power massive volcanic activity on Io.
And during those months when Jupiter is visible from Earth, we'd be bathed in weak sunlight during night time which would play havoc with our body clocks!
