Stellar Evolution

STELLAR EVOLUTION

Formation of Stars

  • Stars begin their life in nebulae, large clouds of dust and gas in space.
  • Dense regions within the nebulae collapse under their own gravity, leading to the formation of a protostar.
  • As the matter in the protostar continues to condense, temperature and pressure rise, leading to nuclear reactions. This starts the main sequence stage of the star. Stellar wind blows away the surrounding nebula.

Main Sequence Stars

  • Main sequence stars are mature stars, like our Sun, which are in a state of equilibrium. Gravity pulling inwards is balanced by the pressure from nuclear fusion in the core pushing outwards.
  • In the core of the star, Hydrogen is fused to form Helium by a process known as nuclear fusion, producing lots of energy in the form of light and heat.
  • The amount of time a star spends in the main sequence depends on its mass. Heavier stars burn through their Hydrogen fuel supply quicker, so their main sequence stage is shorter.

Red Giants and Red Supergiants

  • Once a star exhausts its Hydrogen fuel, it begins to expand and cool, becoming a red giant or, if it is very massive, a red supergiant.
  • The core contracts while the outer layers expand and cool. Fusion of Helium and heavier elements occurs, but eventually, the core’s fuel supply runs out.

White Dwarfs and Supernovae

  • For smaller stars, the outer layers are eventually ejected, leaving behind a hot, dense core known as a white dwarf, which cools and fades over billions of years.
  • For more massive stars, when fusion ceases, the core implodes, causing the outer layers to explode in a supernova. This releases huge amounts of energy and spreads elements throughout space.

Neutron Stars and Black Holes

  • The remnant of the supernova can become a neutron star, an incredibly dense object primarily composed of neutrons.
  • If the star’s core was greater than about three solar masses, the remnant core implodes completely to form a black hole, an entity of such immense gravity that nothing can escape, not even light.

Remember to revise the stages of stellar evolution in order, from nebulae to the final state of the star. Understanding this sequence will allow you to answer questions about the lifecycle of different types of stars.