Radiation pressure is the physical force exerted when electromagnetic radiation—photons carrying momentum—interacts with matter. Though photons are massless, they possess momentum (p = E/c), and when absorbed, reflected, or scattered by material, they transfer this momentum, creating measurable pressure.</p><p>This phenomenon becomes astronomically significant in extreme environments. In massive stars exceeding 20-30 solar masses, radiation pressure from the intense nuclear furnace core can overcome gravitational collapse, contributing to stellar stability. During stellar evolution, radiation pressure helps drive powerful stellar winds, with some luminous blue variables ejecting material at speeds exceeding 1,000 km/s.</p><p>Radiation pressure also shapes cosmic architecture through "radiation-driven feedback." Young, hot stars can halt star formation in nearby gas clouds by literally pushing material away. In active galactic nuclei, radiation pressure from supermassive black holes creates spectacular outflows spanning thousands of light-years.</p><p>The concept emerged from James Clerk Maxwell's electromagnetic theory (1860s) and was first measured in laboratory conditions by Pyotr Lebedev in 1900. Though minuscule on Earth—sunlight exerts only about 4.6 micropascals of pressure—radiation pressure becomes dominant in high-luminosity astrophysical environments where photon energy density rivals or exceeds gas pressure, fundamentally influencing stellar evolution, galaxy formation, and large-scale cosmic structure.