Principal Stages and Timescales of Stellar Evolution

Stellar Birth and Protostars

A star’s life begins in dense, cold molecular clouds composed of gas and dust in the interstellar medium.
These clouds collapse under gravitation, forming a protostar, the earliest stage of stellar evolution.
Protostars are hidden in their birth clouds, detectable mainly through infrared radiation.

Main Sequence Stars

As pressure and temperature rise in a protostar’s core, nuclear fusion of hydrogen commences, marking the birth of a main sequence star.
This stage is the longest in a star’s lifetime, with time scales ranging from a few million years for the most massive stars, to tens of billions of years for the least massive.

Red Giants and Red Supergiants

When a star exhausts its core hydrogen, it starts to collapse under gravity, increasing core temperature and causing the outer layers to expand.
This results in a red giant or a red supergiant, depending on the original mass of the star.
Red giants burn for around a billion years, while red supergiants exist for a shorter timescale of a few million years.

Advanced Stages: White Dwarfs, Neutron Stars, and Black Holes

A low-mass, main sequence star sheds its outer layers to become a white dwarf after being a red giant.
White dwarfs shine for billions of years before eventually cooling and darkening into black dwarfs.
A high-mass star undergoes core collapse and explodes as a supernova, leaving behind a dense neutron star, or for the most massive stars, a black hole.
The supernova phase is short but explosive, lasting only a few months, while the end products, neutron stars or black holes, can exist indefinitely.

Summary of Timescales in Stellar Evolution

Stellar evolution timescales depend on a star’s mass: high-mass stars evolve more quickly than their low-mass counterparts.
For low-mass stars like our Sun, the main sequence stage lasts approximately 10 billion years, while for high-mass stars it may be a few million years.
The post-main sequence stages are shorter, on the order of millions to billions of years, depending on the stage and the star’s mass.