What powers the stars, and how do we read their history from their light?
Stars are fusion reactors held together by gravity. In their cores, temperatures exceed 10 million Kelvin — high enough for hydrogen nuclei (protons) to overcome electrostatic repulsion and fuse into helium, releasing energy because helium-4 has greater binding energy per nucleon than the hydrogen nuclei. Our Sun converts about 600 million tonnes of hydrogen to helium every second; the "missing mass" emerges as the sunlight that sustains all life on Earth.
The Hertzsprung-Russell (HR) diagram is the central tool of stellar physics. Plot luminosity against surface temperature (or spectral class) and most stars fall on the main sequence — a diagonal band where stars spend the bulk of their lives fusing hydrogen. The position on the main sequence is determined by mass: massive stars are hotter, bluer, more luminous, and short-lived (millions of years); low-mass stars are cooler, redder, dimmer, and can last trillions of years. When hydrogen in the core is exhausted, a star leaves the main sequence: it expands into a red giant (or red supergiant), then ends as a white dwarf (low mass), neutron star, or black hole (high mass). The path on the HR diagram tells the entire life story.