Stellar Life Cycle
The stellar life cycle is the sequence of stages a star passes through, from a nebula to main sequence to a giant and a final remnant.
Definition
Stars are born, shine for a long time, and eventually die: a star begins in a nebula, burns steadily for millions or billions of years, swells near the end, and leaves behind a small remnant. The full sequence is nebula, protostar, main-sequence star, and then either red giant and white dwarf for low-mass stars or red supergiant, supernova, and neutron star or black hole for high-mass stars, with mass deciding the path and the timing. Physically, evolution is driven by the sequence of nuclear fuels a star can ignite as its core contracts and heats. Stars below about $8$ solar masses fuse hydrogen and then helium, shed their envelopes as planetary nebulae, and leave carbon-oxygen white dwarfs supported by electron degeneracy pressure; stars above that limit proceed through carbon, neon, oxygen, and silicon burning to an iron core, which cannot release energy by fusion, so the core collapses in a Type II supernova.
Example
The Sun formed from a nebula about $4.6$ billion years ago, will stay on the main sequence about $10$ billion years total, then swell into a red giant and shrink into a white dwarf. A star $20$ times the Sun's mass lives only about $10$ million years before exploding. Betelgeuse, a red supergiant of roughly $15$ to $20$ solar masses, has already left the main sequence and is expected to explode as a core-collapse supernova sometime within the next $100{,}000$ years, leaving a neutron star.
Key Insight
How a star dies depends on how heavy it is: small stars fade quietly and very heavy stars explode. Heavier stars also live shorter lives, because they burn their larger fuel supply so much faster. The stellar life cycle is the reason chemistry exists beyond hydrogen and helium; fusion builds elements up to iron, and supernovae both forge heavier elements and scatter everything into space for future stars and planets.