If Jupiter suddenly gained enough mass to ignite into a star, its heat wouldn't kill us. Instead, its immense new gravity would fling millions of deadly asteroids directly at Earth.
Jupiter is composed of hydrogen and helium, the same elements that fuel our Sun. To actually become a star—specifically, a low-mass red dwarf—the gas giant would need to acquire roughly 80 times its current mass. This influx of material would generate enough internal pressure to ignite sustained nuclear fusion.
If 79 extra Jupiter masses suddenly materialized 5.2 astronomical units away, the new star's radiation would be an afterthought. A red dwarf at that distance would shine in Earth's sky hundreds of times brighter than the full moon, casting distinct shadows and glowing a deep crimson. It would provide, however, less than one percent of the heat Earth receives from the Sun. Earth's global temperatures would barely register the new heat source.
The true catastrophe would be orbital. The sudden introduction of roughly 0.075 solar masses at Jupiter's location would instantly convert the solar system into a binary star system. Earth and the other inner planets would be caught in a gravitational tug-of-war. The immediate gravitational shockwave would tear through the asteroid belt situated between Mars and Jupiter. As those millions of displaced rocks plunged through the inner solar system, the resulting bombardment would extinguish any surface life long before other climate disasters took hold.
Over a slightly longer timescale, Earth's orbit would warp into an extreme ellipse or destabilize entirely. Depending on exactly where the planets were when Jupiter gained its mass, Earth might be pulled too close to the Sun, boiling the oceans. Conversely, it could be slung outward, wandering the freezing depths of space as a rogue planet long after the asteroid strikes had leveled its surface.