Stars and Their Evolution
Stars follow distinct evolutionary paths determined by their mass. These paths are understood through models of stellar evolution, incorporating nuclear physics, thermodynamics, and gravitational dynamics.
Formation
Stars begin their life in nebulae, clouds of gas and dust. When gravitational forces overcome thermal pressure, these clouds collapse, forming protostars. Once the core temperature reaches around 10 million Kelvin, nuclear fusion ignites, marking the star’s entry into the main sequence.
Main Sequence and Beyond
During the main sequence, stars convert hydrogen into helium in their cores. Massive stars (above 8 solar masses) burn their fuel quickly and expand into supergiants. Lighter stars, like the Sun, evolve into red giants after their hydrogen supply is exhausted.
For massive stars, fusion continues with heavier elements until the core collapses into a supernova—a violent explosion dispersing elements into space. These explosions are crucial for the synthesis of elements heavier than iron, like gold and uranium, through processes like the r-process.
End Stages
Low-mass stars shed their outer layers, forming planetary nebulae, while the remaining core becomes a white dwarf. This remnant cools and fades over billions of years.
Massive stars may leave behind neutron stars or, if the core is above a critical mass, collapse into black holes.
The study of stellar remnants like white dwarfs, neutron stars, and black holes reveals critical information about the death throes of stars and the conditions in extreme environments.