Overview
This lecture covers the life cycle of high mass stars, focusing on their fusion processes, stages, explosive ends (supernova), and the formation of neutron stars or black holes.
Lifecycle Stages of High Mass Stars
- High mass stars begin as protostars and enter the main sequence, burning hydrogen in their cores.
- Due to more gravity, high mass stars complete their main sequence faster than low-mass stars.
- After exhausting hydrogen, fusion halts briefly; the core contracts and heats up to ignite helium fusion.
- Each subsequent fusion (helium, carbon, neon, oxygen, silicon) occurs in the core as the previous element is depleted.
- Each new fusion stage adds additional burning shells around the core for the previous elements.
- The stage after the main sequence with multiple fusion layers is called the supergiant stage.
Supergiant Stage and Supernova
- In the supergiant stage, the star fuses elements up to silicon in the core, surrounded by shells fusing lighter elements.
- Silicon fusion produces iron, which marks the end of fusion, as iron does not release energy when fused.
- Once an iron core forms, gravity causes the core to collapse, resulting in a rebound explosion called a supernova.
- The supernova expels the star's outer layers, creating a supernova remnant.
After the Supernova
- The star's leftover core becomes a neutron star if the remnant is less than 2-3 solar masses.
- If the leftover core is more massive (over 2-3 solar masses), it becomes a black hole.
- Supernova explosions occur in about 10 seconds, much faster than low mass star deaths, but the remnant can last thousands of years.
Element Formation
- Fusion in the supernova’s explosion (briefly) creates heavy elements like gold, silver, platinum, and uranium.
- These elements are rare and valuable because they are only formed during supernova explosions.
Key Terms & Definitions
- Protostar — a contracting mass of gas in the early stage of star formation.
- Main Sequence — phase where hydrogen fusion occurs in a star’s core.
- Supergiant — a massive, bright star with multiple fusion shells around the core.
- Supernova — a massive explosion marking the death of a high mass star.
- Supernova Remnant — the expanding cloud of gas left after a supernova.
- Neutron Star — dense core left after a supernova if the remnant is less than about 2-3 solar masses.
- Black Hole — extremely dense core left if the remnant exceeds about 2-3 solar masses.
- Fusion — process of combining lighter elements into heavier ones, releasing energy.
Action Items / Next Steps
- Be ready to discuss white dwarfs, neutron stars, and black holes in the next class.
- Review the order of fusion stages in high mass stars (hydrogen, helium, carbon, neon, oxygen, silicon, iron).