(455) Evolution of stars [IB Physics SL/HL]

Main Sequence Stars

• Definition: Stars mainly convert hydrogen into helium.

• Hydrostatic Equilibrium: Outward radiation pressure balances inward gravitational force.

• Stable Stars: Main sequence stars maintain a constant size when in equilibrium.

After the Main Sequence

• Hydrogen Depletion: Core runs out of hydrogen, leading to collapse from gravitational force.

• Helium Flash: Core collapses to fuse helium into heavier elements (like carbon and oxygen).

• Red Giant Phase: Outer layers expand as core collapses and heats up.

• Successive Fusions: Stars fuse increasingly heavier elements until iron is formed.

Iron and Element Formation

• Iron Limit: Fusion stops at iron (Fe) due to unfavorable binding energy.

• Supernova: A star with an iron core undergoes a supernova explosion, scattering heavy elements into space.

Element Creation Beyond Iron

• Need for Supernova: Heavier elements are formed in intense environments, like supernova explosions.

Final Stages of Star Life

• RemnantMass Impact: The star's fate depends on its remnant mass:

  • White Dwarfs: Less than 1.4 solar masses (Chandrasekhar limit).

  • Neutron Stars: Remnants between 1.5 to 3 solar masses (Oppenheimer-Volkoff limit).

  • Black Holes: Greater than 3 solar masses, collapsing into a point of infinite density.

Neutron Stars and Pulsars

• Formation: Core collapses, creates a neutron core; mass bounces back causes supernova.

• Pulsars: Rapidly spinning neutron stars emitting beams of radiation.

Time Dilation Near Black Holes

• Event Horizon: Boundary beyond which nothing escapes, including light.

• Effect on Time: Time slows near black holes, leading to significant differences in time perception compared to distant observers.