radiation pressure

Light feels weightless, but it is not pressureless. Every photon carries a tiny bit of momentum, so when light strikes or streams through matter it gives a small shove — the way a stream of ping-pong balls would nudge a wall. On Earth this push is far too feeble to notice. Deep inside a hot, luminous star, where the radiation is millions of times more intense than sunlight, that gentle shove adds up to a real outward force called radiation pressure, and it can help hold the star up alongside the ordinary gas pressure.

Radiation pressure depends on temperature very steeply — it grows as the fourth power of temperature, so doubling the temperature multiplies it sixteenfold. In a star like the Sun, the core is hot but radiation pressure is still a minor partner, contributing well under a percent of the support. In a massive, blazing star ten or twenty times the Sun's mass, the core is far hotter and radiation pressure becomes a major or even dominant share of what counters gravity. This is one reason massive stars are built and behave so differently from the Sun.

Where radiation pressure dominates, the star loses some of the stabilizing thermostat that gas pressure provides, making the most massive stars puffed-up, restless, and prone to shedding their outer layers in powerful winds. Radiation pressure also sets a ceiling on how bright an object can shine before its own light starts blowing its outer material away — the Eddington luminosity — which limits how massive stars can grow and how fast material can fall onto compact objects.