Stellar evolution

Representative lifetimes of stars as a function of their masses

The life cycle of a Sun-like star (annotated)

The life of Sun-like stars

Star Life Cycle Chart

Starlifesimple

Stellar evolution L vs T

Stellar evolution is the process by which a star undergoes a sequence of radical changes during its lifetime. Stellar evolution is driven by the never-ending battle between the force of gravity, which seeks to pull the star's material inward, and the pressure of nuclear fusion reactions in the star's core, which push outward. The stages of stellar evolution vary depending on the mass of the star.

Formation

Stellar evolution begins with the collapse of a molecular cloud, typically within a molecular cloud complex. These regions are dense enough to collapse under their own gravity, forming protostars. This initial stage is known as the formation phase. As the protostar accumulates mass from its surrounding cloud, it heats up until the core temperature is sufficient to start nuclear fusion, at which point it becomes a main-sequence star.

Main Sequence

The main sequence phase is the longest period in the life of a star. During this phase, the star fuses hydrogen into helium in its core. The amount of time a star spends on the main sequence depends on its mass. High-mass stars burn through their hydrogen fuel quickly and have shorter lifespans, while low-mass stars, like the Sun, consume their fuel more slowly and can last billions of years.

Post-Main Sequence

After a star has exhausted the hydrogen in its core, it leaves the main sequence. What happens next depends on the star's mass:

Low to Intermediate-Mass Stars

Stars with up to about eight times the mass of the Sun become red giants. In this phase, the core contracts and heats up, causing the outer layers to expand and cool. The star then starts to fuse helium into carbon. After exhausting the helium, the core contracts again, and the outer layers are ejected to form a planetary nebula, leaving behind a white dwarf.

High-Mass Stars

Stars with more than eight times the mass of the Sun become supergiants. They undergo a series of nuclear fusion processes, producing elements up to iron. Since iron fusion consumes energy rather than producing it, the core cannot support itself against gravity once it is mostly iron. This leads to a supernova explosion, which can leave behind a neutron star or, if the star was massive enough, a black hole.

End Stages

The final stages of stellar evolution depend on the remnant left behind:

• White dwarfs slowly cool and fade over time.
• Neutron stars may be detected as pulsars due to their rapid rotation and strong magnetic fields.
• Black holes represent the end point of stellar evolution for the most massive stars, where gravity is so strong that not even light can escape.

Importance of Stellar Evolution

Understanding stellar evolution is crucial for various fields of astronomy and astrophysics. It helps scientists determine the ages of stars, the chemical composition of galaxies, and the distribution of heavy elements in the universe. Stellar evolution also plays a key role in the life cycle of galaxies and the possibility of life on planets orbiting other stars.