Life Cycle of a Star

• Birth: Stars form in molecular clouds (cold, massive clouds of gas and dust ranging from 1,000 to 10 million solar masses and spanning hundreds of light-years).
• Molecular clouds are cold, allowing gas to clump into high-density regions that grow through collisions and accretion. As gravity intensifies, these clumps collapse and heat up, forming a protostar. 
• Groups of newly formed stars are called stellar clusters, and such regions are known as stellar nurseries.
• Life: A protostar initially shines from heat released by gravitational collapse. After millions of years, extreme pressure and temperature in its core trigger nuclear fusion, converting hydrogen into helium and releasing energy that balances gravity.
• Stars stably undergoing this process are called main sequence stars, the longest phase of stellar life, during which luminosity, size, and temperature change slowly. 
• A star’s mass controls its lifespan – low-mass stars live much longer, while massive stars burn fuel quickly and die young.
• Death: When a star’s core runs out of hydrogen, fusion pressure drops and the core begins to collapse, causing the star to expand and heat up. 
• In low-mass stars, helium fuses into carbon as the star becomes a giant, eventually shedding its outer layers to form a planetary nebula.
• After shedding its outer layers, a low-mass star leaves behind a dense white dwarf that cools slowly over billions of years. 
• In high-mass stars, fusion continues to form heavier elements up to iron. Once iron forms, energy production stops and the star rapidly runs out of fuel.
• When a star’s iron core collapses and rebounds, it triggers a massive supernova explosion. The core remains as a neutron star or black hole, while ejected material enriches future molecular clouds, helping form new stars.