The Stellar Furnace
Deep within every main-sequence star, temperatures and densities are extreme enough to overcome the electrostatic repulsion between protons, enabling nuclear fusion. In the Sun's core — 15.7 million Kelvin and 150 times the density of water — hydrogen nuclei fuse into helium through the proton-proton chain, releasing 3.8 × 10²⁶ watts. This simulation lets you explore how mass and core conditions control the fusion process.
Two Paths to Helium
Nature provides two hydrogen-burning pathways. The pp-chain, dominant in stars below ~1.3 M☉, fuses protons directly through deuterium and helium-3 intermediates. The CNO cycle, dominant in more massive stars, uses pre-existing carbon, nitrogen, and oxygen as catalysts in a cyclic reaction sequence. The CNO cycle's extreme temperature sensitivity (ε ∝ T¹⁶) produces convective cores in massive stars, thoroughly mixing the nuclear fuel.
Mass Controls Everything
The mass-luminosity relation L ∝ M^3.5 is the most consequential relationship in stellar astrophysics. It means that doubling a star's mass increases its luminosity eleven-fold but only doubles its fuel supply, cutting its lifetime by a factor of six. The most massive O-stars exhaust their hydrogen in a few million years, while the smallest M-dwarfs can shine for trillions — outlasting the current age of the universe many times over.
The Hydrogen Clock
As hydrogen depletes in the core, the mean molecular weight increases, reducing pressure support. The core contracts slightly and heats up, paradoxically increasing the fusion rate and luminosity. The Sun today is about 30% more luminous than when it formed 4.6 billion years ago. When core hydrogen fraction drops below ~1%, the star leaves the main sequence — a transition this simulation tracks through the core hydrogen fraction parameter.