From Dust to Diamonds in the Sky
Stellar evolution describes the process by which a star changes over the course of its lifetime. A star's life path is determined almost entirely by its initial mass.
Birth of a Star: From Nebula to Protostar
Stars are born within vast, cold clouds of gas and dust called nebulae.
1.A dense region within a nebula begins to collapse under its own gravity. This can be triggered by a nearby supernova explosion or other gravitational disturbances.
2.As the cloud collapses, it heats up and begins to glow, forming a protostar. This is not yet a true star because it is not powered by nuclear fusion.
3.The protostar continues to accrete mass from the surrounding cloud. Its core temperature and pressure rise dramatically.
The Main Sequence: A Star is Born
When the core of the protostar becomes hot and dense enough (around 15 million K), nuclear fusion begins. This marks the birth of a star and its entry into the main sequence phase.
During this phase, the star is in a state of hydrostatic equilibrium: the outward pressure from nuclear fusion in the core perfectly balances the inward pull of gravity.
The primary fusion process is the conversion of hydrogen into helium.
Stars spend about 90% of their lives on the main sequence. Our Sun is currently in this phase.
The Diverging Paths: Low-Mass vs. High-Mass Stars
What happens next depends on the star's mass.
1. The Fate of Low-Mass Stars (like the Sun):
When the hydrogen in the core is exhausted, fusion stops, and the core contracts under gravity.
The outer layers of the star expand, cool, and glow red, turning the star into a red giant.
The core becomes hot enough to fuse helium into carbon.
Eventually, the outer layers drift away into space, creating a beautiful planetary nebula.
The remaining core, a hot, dense ball of carbon and oxygen, is left behind. This is a white dwarf, which will slowly cool and fade over billions of years.
2. The Fate of High-Mass Stars (8+ times the Sun's mass):
These stars live fast and die young. They are much hotter and brighter than low-mass stars.
When they run out of hydrogen, they become red supergiants.
Their cores are massive enough to fuse heavier and heavier elements, up to iron.
Iron fusion does not release energy; it consumes it. This causes the core to collapse catastrophically in a fraction of a second.
The outer layers rebound off the collapsed core in a massive explosion called a Type II supernova. This explosion is so powerful it creates elements heavier than iron and scatters them throughout the galaxy.
The Remnants: Stellar End States
The supernova leaves behind an ultra-dense remnant.
If the original star was moderately massive, the remnant will be a neutron star—a city-sized object so dense that a teaspoon of it would weigh billions of tons.
If the original star was extremely massive, gravity will overwhelm all other forces, and the core will collapse indefinitely to form a black hole, a region of spacetime from which nothing, not even light, can escape.