Stellar Evolution Overview

Stellar Evolution: Overview

• Stars form from giant molecular clouds, collapsing under gravity.
• Stars spend most of their lives in a stable state called the main sequence, where nuclear fusion converts hydrogen to helium.
• The lifetime of a star depends on its mass: more massive stars live shorter lives.
• After a star exhausts its hydrogen fuel, its evolution depends significantly on its mass.

Main Sequence Stars (Nuclear Fusion)

• Hydrogen fusion occurs in the core, converting hydrogen to helium.
• This fusion releases enormous amounts of energy that resist the inward pull of gravity.
• The inward pressure and the outward pressure due to fusion maintain the star in balance.

Stellar Clusters and Star Formation

• Stars are often found in clusters that formed at roughly the same time.
• By studying the cluster's main sequence stars, astronomers can estimate the cluster's age.

Low-Mass Stars: Red Giants and Planetary Nebulae

• When hydrogen in the core is depleted, the star expands and becomes a red giant.
• Helium fusion then occurs in the core.
• Red giants eventually fuse helium to carbon.
• Low-mass stars expel their outer layers during the final stages which form a planetary nebula.
• The remaining core becomes a white dwarf, which is very dense and gradually cools.

High-Mass Stars: Supernovae and Neutron Stars/Black Holes

• High-mass stars fuse heavier elements like carbon, oxygen, silicon, up to iron in their cores.
• Fusion of elements heavier than iron is not exothermic; the star's core collapses under its own immense gravity.
• This rapid collapse causes a massive explosion known as a supernova.
• After the explosion, the core may become a neutron star, which is extremely dense.
• If the remaining core has a mass greater than a certain limit (about 3 times the mass of the Sun), it becomes a black hole.
• When material falls into a black hole, nothing can escape its powerful gravity, not even light.

Hertzsprung-Russell (HR) Diagram

• Plots the luminosity and temperature of stars.
• Different stages of stellar evolution appear as tracks on the HR diagram.
• The main sequence is a diagonal band on the HR diagram where most stars reside during their stable phase.
• Red giants appear at the upper right of the HR diagram.
• White dwarfs appear at the lower left.

Comparing Star Types by Mass

• Lower mass stars have longer lifetimes because they consume their fuel more slowly.
• Higher mass stars fuse material more rapidly and produce more intense forms of radiation.

Stellar Nucleosynthesis

• The process of creating new atomic nuclei by combining existing nuclei.
• This process is responsible for the formation of most elements heavier than hydrogen and helium.
• It occurs primarily in the cores of stars during different stages of their evolution.

Supernova Remnants

• These remnants are the leftover material from a supernova explosion.
• They contain heavier elements produced in the star and scattered into space.
• These remnants can provide crucial information about the explosion process and composition of the progenitor star.

The Evolution of Stars in Binary Systems

• Two stars orbiting each other can have a significant impact on their evolution.
• Mass transfer between the stars can alter their evolution.