Overview of Stellar Evolution Stages
Stellar evolution describes the life cycle of a star, beginning with a protostar and potentially ending in a supernova for massive stars. The process is driven by nuclear fusion and gravitational forces, transforming clouds of gas and dust into luminous stars and eventually dispersing their material back into space. This cycle varies by the star's initial mass, with low-mass stars like our Sun evolving slowly over billions of years, while massive stars burn brightly but briefly.
Key Stages from Protostar to Main Sequence
The journey starts with a protostar, formed when a molecular cloud collapses under gravity, heating up as it contracts. It then enters the pre-main-sequence phase, where fusion of hydrogen begins in the core. Once stable, the star reaches the main sequence, fusing hydrogen into helium for the majority of its life—about 10 billion years for a Sun-like star. During this stable phase, the star's energy output balances gravitational pull.
Post-Main Sequence: Red Giant and Beyond
After exhausting core hydrogen, the star expands into a red giant, with its outer layers swelling as helium fusion ignites in the core. For Sun-like stars, this leads to helium flash and further expansion. Massive stars (over 8 solar masses) undergo successive fusion stages—helium to carbon, then heavier elements—developing layered cores until iron forms, which halts fusion due to energy absorption rather than release.
The Dramatic End: Supernova Explosion
For massive stars, the iron core collapses under gravity, triggering a supernova explosion that outshines entire galaxies briefly. This ejects outer layers into space, enriching the cosmos with heavy elements, while the core may become a neutron star or black hole. Low-mass stars shed outer layers to form planetary nebulae, leaving white dwarfs. Supernovae are crucial for galaxy evolution, seeding new star formation.