Stellar Birth: From Nebulae to Protostars
Stars begin their lives as vast clouds of gas and dust known as nebulae. Gravity causes these clouds to condense, forming dense clumps that heat up and begin to glow, known as protostars.
Main Sequence: The Star's Longest Phase
Once the core of a protostar becomes hot and dense enough for nuclear fusion to ignite (converting hydrogen into helium), it officially becomes a star and enters the main sequence. This stable phase constitutes the majority of a star's life, as it balances gravitational collapse with outward pressure from fusion.
Post-Main Sequence Evolution: Red Giants and Supernovae
After a star exhausts its core hydrogen fuel, it begins to evolve off the main sequence. Smaller stars, like our Sun, expand into red giants, while much more massive stars become red supergiants. This stage can eventually lead to planetary nebulae for smaller stars or dramatic supernova explosions for larger ones.
Stellar Remnants: White Dwarfs, Neutron Stars, and Black Holes
The final remnant of a star's life depends critically on its initial mass. Less massive stars shed their outer layers to form white dwarfs. More massive stars, after a supernova, collapse into incredibly dense neutron stars. The most massive stars, however, collapse entirely under their own gravity to form black holes.