Jupiter did not become a star because it lacks the necessary conditions and mass to sustain nuclear fusion in its core, which is the process that powers stars.
Stars, including our Sun, are primarily composed of hydrogen gas. In their cores, the immense gravitational pressure and temperature cause hydrogen atoms to undergo fusion, converting them into helium and releasing an enormous amount of energy in the process. This energy generation is what makes stars shine.
Jupiter, on the other hand, is a gas giant planet primarily composed of hydrogen and helium, similar in composition to a star. However, it is not massive enough to sustain the sustained fusion reactions required to become a star. The minimum mass necessary for sustained fusion is about 75 to 80 times the mass of Jupiter, which would cause the planet to undergo gravitational collapse and ignite as a star.
Jupiter's mass is approximately 1/1000th that of the Sun, making it significantly smaller. Without the immense gravitational pressure and temperature at its core, the fusion reactions necessary for star formation cannot be initiated.
Instead, Jupiter is considered a gas giant planet, playing an important role in our solar system as the largest planet and exerting gravitational influence on other celestial bodies.