Gas ignites explosively. Orion nebula 2. B. Protostar stage. Star expands, balanced by gravity, enters main sequence. C. Main-Sequence Stage Balance gravity and fusion for billions of years.
Hot stars burn quick, few million years.
Small red stars last hundreds of billions of years. D. Red Giant Stage Star uses H going outward, leaves He core. Can make variable stars. Helix nebula 3. Stellar Remnants Consume all nuclear fuel, gravity wins, collapse to small, very dense objects 1.