Stellar Evolution Stages to Know for Astrophysics I

Stellar evolution describes how stars form, live, and die, shaping the universe around us. From protostars to black holes, each stage reveals the complex processes that govern stellar life cycles and their impact on galaxies and cosmic structures.

1. Protostar formation

   • Occurs when a dense region in a molecular cloud collapses under its own gravity.
   • The temperature and pressure increase as material falls inward, leading to the formation of a protostar.
   • Surrounding material forms an accretion disk, which can lead to the formation of planets.

2. T Tauri stage

   • Represents a young star that is still in the process of contracting and heating.
   • Characterized by strong stellar winds and variability in brightness.
   • Marks the transition from a protostar to a main sequence star, lasting a few million years.

3. Main sequence

   • The longest stage of stellar evolution, where stars spend about 90% of their lifetimes.
   • Stars fuse hydrogen into helium in their cores, producing energy that balances gravitational collapse.
   • The position on the Hertzsprung-Russell diagram indicates the star's mass, temperature, and luminosity.

4. Red giant phase

   • Occurs when hydrogen in the core is depleted, causing the core to contract and heat up.
   • The outer layers expand and cool, giving the star a reddish appearance.
   • Helium fusion begins in the core, leading to the production of heavier elements.

5. Horizontal branch

   • A stable phase where stars fuse helium into carbon and oxygen in their cores.
   • Stars are found on the horizontal branch of the Hertzsprung-Russell diagram, indicating a balance between gravity and thermal pressure.
   • This phase can last for several hundred million years.

6. Asymptotic giant branch

   • Characterized by the star expanding and becoming more luminous after helium burning ceases.
   • The star undergoes thermal pulses, leading to the creation of heavier elements through nucleosynthesis.
   • The outer layers become increasingly unstable and are eventually ejected.

7. Planetary nebula

   • Formed when the outer layers of a star are expelled, leaving behind a hot core.
   • The ejected material creates a glowing shell of gas and dust, illuminated by the remaining core.
   • Represents the transition from a red giant to a white dwarf.

8. White dwarf

   • The remnant core of a low to intermediate-mass star, no longer undergoing fusion.
   • Composed mostly of carbon and oxygen, it gradually cools and dims over time.
   • Supported against gravitational collapse by electron degeneracy pressure.

9. Supernova

   • A catastrophic explosion marking the death of a massive star, occurring after iron builds up in the core.
   • Releases an enormous amount of energy, outshining entire galaxies for a short period.
   • Can lead to the formation of neutron stars or black holes, depending on the mass of the original star.

10. Neutron star

    • The incredibly dense remnant of a supernova explosion, primarily composed of neutrons.
    • Has a strong magnetic field and can emit beams of radiation, observed as pulsars.
    • Represents one of the possible endpoints for stars with a mass between about 8 and 20 solar masses.

11. Black hole

    • Formed when a massive star collapses under its own gravity after a supernova.
    • The gravitational pull is so strong that not even light can escape, making it invisible.
    • Can be detected indirectly through its effects on nearby stars and gas, or through gravitational waves from merging black holes.