Stages of Stellar Evolution

Stars go through fascinating stages from birth to death, shaping the universe around us. Understanding these stages, from molecular clouds to supernovae, helps us grasp the life cycles of stars and their impact on the cosmos.

1. Molecular cloud and protostar formation

   • Molecular clouds are dense regions of gas and dust where star formation begins.
   • Gravitational forces cause regions within the cloud to collapse, forming protostars.
   • Protostars are characterized by their increasing temperature and pressure as they accumulate mass.

2. Main sequence

   • The main sequence is the longest phase in a star's life, where it fuses hydrogen into helium in its core.
   • Stars spend about 90% of their lifetime in this stable phase, maintaining a balance between gravitational collapse and nuclear fusion.
   • The position of a star on the Hertzsprung-Russell diagram indicates its temperature, luminosity, and size during this phase.

3. Red giant phase

   • After exhausting hydrogen in the core, stars expand and cool, becoming red giants.
   • Helium fusion begins in the core, while hydrogen fusion occurs in a surrounding shell.
   • This phase leads to significant changes in a star's structure and luminosity.

4. Planetary nebula (for low-mass stars)

   • Low-mass stars shed their outer layers, creating a colorful shell of gas known as a planetary nebula.
   • The core left behind becomes increasingly hot and luminous, ionizing the ejected material.
   • This process enriches the interstellar medium with heavier elements.

5. White dwarf (for low-mass stars)

   • The remnant core of a low-mass star becomes a white dwarf, a dense and hot stellar remnant.
   • White dwarfs no longer undergo fusion and gradually cool over billions of years.
   • They are supported against gravitational collapse by electron degeneracy pressure.

6. Supergiant phase (for high-mass stars)

   • High-mass stars evolve into supergiants after exhausting hydrogen in their cores.
   • They undergo multiple stages of fusion, creating heavier elements up to iron.
   • Supergiants are characterized by their large size and high luminosity.

7. Supernova explosion (for high-mass stars)

   • When a supergiant's core collapses, it triggers a catastrophic explosion known as a supernova.
   • This explosion disperses the outer layers into space, enriching the surrounding medium with heavy elements.
   • Supernovae are among the most energetic events in the universe and can briefly outshine entire galaxies.

8. Neutron star or black hole (for high-mass stars)

   • The remnants of a supernova can become either a neutron star or a black hole, depending on the mass of the core.
   • Neutron stars are incredibly dense, composed mostly of neutrons, and can exhibit strong magnetic fields and rapid rotation.
   • Black holes form when the core's mass exceeds a certain limit, creating a region of space with gravitational pull so strong that not even light can escape.