Stellar Evolution
2. The Circle of Stellar Life
A star's journey from birth to death is often termed stellar evolution. This journey describes all of the changes a star will undergo throughout its existence. Understanding the different stages a star goes through is crucial to understanding how long they live, and what they become when they die.
Stars begin their lives as swirling clouds of gas and dust called nebulae. Gravity pulls these materials together, compressing them until the core becomes hot enough to ignite nuclear fusion. This is when a protostar becomes a star, ready to shine for billions of years (or millions, depending on its mass). This main sequence phase, where hydrogen fuses into helium, is the longest part of a star's life.
Eventually, the star runs out of hydrogen fuel in its core. What happens next depends on the star's mass. Our Sun, for example, will expand into a red giant, engulfing Mercury and Venus in the process (don't worry, that's billions of years away!). It will then shed its outer layers, forming a planetary nebula, leaving behind a white dwarf — a dense, hot remnant that slowly cools down over trillions of years.
More massive stars have a much more dramatic ending. When they run out of fuel, their cores collapse catastrophically, leading to a supernova explosion. These explosions are so bright that they can outshine entire galaxies for a brief period. What remains after a supernova depends on the mass of the original star. It could be a neutron star (an incredibly dense object) or, if the star was massive enough, a black hole (an object with such strong gravity that nothing, not even light, can escape).
The heavy elements created in the cores of stars and dispersed in supernova explosions are the building blocks of planets and, ultimately, life. So, in a way, we're all made of stardust! It's a beautiful and humbling thought.
