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Gravity governs the cosmos. It keeps planets in orbit, shapes the birth of stars and galaxies from vast clouds of gas, and ultimately governs the death of massive stars. When a star’s mass is sufficient—set during its formation—its own gravity can collapse it into a black hole.
Nebulae—vast interstellar clouds of gas and dust—pervade the universe. Within them, density variations allow cold gas (just above absolute zero) to coalesce into clumps. When a clump forms inside a dense region known as a molecular cloud, it attracts surrounding material. As the mass accumulates, gravitational compression raises the core temperature: particles collide more often and with greater kinetic energy, setting the stage for star formation.
Star formation takes roughly 10 million years from an interstellar clump. As the core heats, the nascent protostar emits infrared radiation. When it becomes dense enough that radiation is trapped, the core temperature climbs until it reaches about 10 million K (≈18 million°F). At this point hydrogen fusion ignites, producing outward pressure that balances gravity. The star settles into the main sequence, a stable phase lasting from hundreds of millions to over a trillion years, during which its radius and surface temperature remain largely constant.
Stars with masses 25×solar or greater are classified as blue giants. Their enormous core pressure drives fusion at much higher temperatures, giving them bluish luminosity and surface temperatures around 20,000 K (≈35,450°F), compared to the Sun's 6,000 K (≈10,340°F). The accelerated fusion rates cause these stars to exhaust their hydrogen supply in a small fraction of the Sun’s main‑sequence lifetime.
Once a blue giant depletes its hydrogen, the core contracts and ignites helium fusion. As heavier elements fuse, the core grows increasingly dense. When nuclear fuel is exhausted, gravity overwhelms all internal pressure, triggering a core-collapse supernova that ejects the outer layers. If the remnant mass exceeds about three solar masses, no known force can counteract gravity, and the core collapses into a singularity—a black hole.
