Star Formation Basics

Stars originate from a nebula, a vast interstellar cloud composed of hydrogen gas and dust.
Gravity causes particles in the nebula to aggregate, forming a protostar, characterized by increased density and temperature due to frequent particle collisions.

A protostar evolves into a main-sequence star when nuclear fusion begins, fusing hydrogen nuclei into helium and releasing heat and light.
At this stage, the star achieves hydrostatic equilibrium where gravitational forces are balanced by the outward pressure from fusion reactions.
The star's future trajectory depends on its mass, leading to different life cycles.

A low-mass star (up to 8 solar masses) transitions into a red giant as hydrogen fusion decreases after billions of years.
The core shrinks and heats up, initiating helium fusion, leading to an expansion of the outer layers resulting in a red giant appearance.
Once helium fusion ends, the star ejects its outer layers, forming a planetary nebula.
The core remnant becomes a white dwarf, gradually cooling and diminishing in energy output until it turns into a black dwarf, eventually fading from visibility.

High-mass stars (greater than 8 solar masses) evolve into red supergiants after a few million years as hydrogen fusion wanes.
Similar to low-mass stars, the core undergoes periods of contraction and expansion, forming heavier elements up to iron.
The inevitable end of fusion in a red supergiant leads to a supernova, a colossal explosion that ejects outer layers and creates a dense neutron star at the core.
The neutron star may further collapse into a black hole, an extremely dense region from which not even light can escape.

The nebula formed from supernova remnants can lead to the birth of new stars and planetary systems.
The process of fusion reactions is crucial for a star's stability, influencing its structure and eventual fate.