Life Cycle of Stars

Hydrogen Burning: This is the longest stage of a star's life, where it fuses hydrogen into helium in its core, maintaining a stable size and temperature.

Stability Phase: During this period, the star is in a state of equilibrium; the energy released by fusion prevents the star from collapsing under its own weight.

Lifespan Variation: High-mass stars burn through their fuel much faster than low-mass stars due to higher core temperatures and pressures, leading to significantly shorter lifespans.

Low-Mass Evolution: When hydrogen runs out, the core collapses and heats up, allowing helium fusion. The outer layers expand and cool, forming a Red Giant, eventually shedding these layers as a Planetary Nebula.

High-Mass Evolution: These stars expand into Red Supergiants and can fuse elements up to iron. Once iron is formed, fusion stops because it consumes rather than releases energy, leading to a catastrophic core collapse.

Supernova: The sudden collapse of a high-mass star's core results in a massive explosion that creates and disperses heavy elements throughout the universe.

White Dwarf: The remaining hot, dense core of a low-mass star after its outer layers are ejected; it no longer undergoes fusion and slowly cools over time.

Neutron Star: An incredibly dense remnant of a high-mass star, composed almost entirely of neutrons, formed when the core collapse is halted by neutron degeneracy pressure.

Black Hole: The final state of the most massive stars, where gravity is so intense that the core collapses into a singularity from which not even light can escape.

Identify the Divergence: Always check the initial mass of the star in the question; this determines whether you follow the 'Red Giant' or 'Red Supergiant' path.

Equilibrium Check: If a question asks why a star is stable, always mention the balance between inward gravity and outward radiation pressure.

Common Misconception: Students often think all stars become black holes. Remember, only the most massive stars ($> 20$ solar masses) end this way; most become white dwarfs.

Sequence Order: Memorize the specific order: Nebula $\rightarrow$ Protostar $\rightarrow$ Main Sequence $\rightarrow$ Red (Super)Giant $\rightarrow$ Remnant.