Some people say that Jupiter is a failed star -- that it could have been a star if it were just a bit more massive. So how massive does something have to be to become a star??
The definition of a star is an object made of gas that fuses hydrogen into helium at its core. Nuclear fusion can only happen in environments with extremely high pressure and temperature, and this determines the lower limit for the mass of a star.
In ideal, simplified scenarios, the temperature of a gas increases with the cube root of the density. So if the density increases by a factor of 1000, the temperature increases by a factor of 10.
Given this information, astronomers have determined that the lower limit for the mass of a star is about 8% the mass of the Sun, or about 80 times more massive than Jupiter. Anything less massive than that won't achieve the pressure necessary to fuse hydrogen in its core.
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