A nearly empty cloud wins. Against intuition, interstellar gas so rarefied it would rank as high-grade vacuum on Earth can still surrender to its own gravity and fold inward. The trick lies in scale: stretch that gas across light-years, and the total mass quietly adds up. Even with only a few hundred atoms per cubic centimetre, the integrated pull over such volume becomes significant.
Self-collapse is not automatic. First, the cloud must cool through radiation from molecules and dust grains, shedding thermal energy until gas pressure can no longer resist compression. At that tipping point, the Jeans instability kicks in, the mathematical threshold where gravity overwhelms pressure in a given mass and size. Turbulence and magnetic fields delay the fall, but they do not erase it; they merely sculpt knots and filaments where collapse proceeds fastest.
Star birth is brutally selective. Dense cores inside the cloud shrink, their free-fall time shortening as density climbs and gravitational potential energy converts into heat. When the centre grows opaque, radiative cooling stalls; temperature rises, and nuclear fusion ignites in a new star. That object, now compact and incandescent, concentrates what was once diffuse and invisible into a furnace that can outshine millions of suns.
