Overview
This lecture covers the life cycles of stars, focusing on how stars evolve, die, and create heavy elements, with specific attention to white dwarfs, supernovae, neutron stars, and binary systems.
Structure and Evolution of Stars
- Stars are powered by nuclear fusion, with gravity pulling inward and pressure from fusion pushing outward.
- Main sequence stars fuse hydrogen into helium in their cores; this phase lasts ~90% of a star's life.
- Once hydrogen in the core is depleted, stars evolve rapidly; cores contract and outer layers may expand into giants or supergiants.
Death of Low and Medium Mass Stars
- Stars like the Sun expand into red giants, expel outer layers, and leave behind white dwarfs.
- Helium fusion in the core (triple alpha process) creates carbon; cores become degenerate prior to fusion.
- The helium flash occurs when degenerate cores ignite helium, causing a rapid, intense burst of fusion.
- Expelled outer layers form planetary nebulae; white dwarfs are dense, Earth-sized, supported by electron degeneracy pressure.
Death of Massive Stars
- Stars >8 solar masses undergo successive fusion stages: hydrogen, helium, carbon, neon, oxygen, silicon, up to iron.
- Fusion past iron is endothermic and cannot support the core, leading to rapid collapse.
- The core collapse results in a supernova explosion, dispersing heavy elements into space.
- Remnants become either neutron stars (electron/proton fusion forms dense ball of neutrons) or black holes if mass is high enough.
Binary Systems and Mass Transfer
- In close binary systems, mass transfer can occur between stars through Lagrange points and accretion disks.
- Novae occur when material transferred onto a white dwarf ignites in a sudden nuclear explosion on its surface.
- The Chandrasekhar limit (~1.4 solar masses) is the maximum mass for a white dwarf; surpassing this may lead to a black hole or supernova.
Star Clusters as Evidence for Evolution
- Star clusters provide evidence for stellar evolution, as cluster stars are the same age but differ by mass.
- HR diagrams of clusters show stars leaving the main sequence at different points, indicating age and evolutionary path.
Neutron Stars and Pulsars
- Neutron stars are extremely dense and exhibit properties like intense gravity, rapid spin, and strong magnetic fields.
- Some neutron stars emit beams of radiation (pulsars); the fastest are millisecond pulsars.
- Magnetars are neutron stars with extremely strong magnetic fields, producing massive flares.
Key Terms & Definitions
- Main Sequence — Stage where stars fuse hydrogen into helium in their cores.
- Degenerate Matter — Extremely dense gas where quantum pressure supports the star.
- White Dwarf — Dense, Earth-sized stellar remnant supported by electron degeneracy.
- Supernova — Explosive death of a massive star, dispersing elements.
- Neutron Star — Dense core of neutrons left after a massive star’s supernova.
- Accretion Disk — Spinning disk of matter transferred between stars in a binary system.
- Chandrasekhar Limit — 1.4 solar masses; max mass a white dwarf can have before collapsing.
- Planetary Nebula — Expanding shell of gas ejected by a dying red giant.
- Pulsar — Rapidly spinning neutron star emitting beams of electromagnetic radiation.
- Magnetar — Neutron star with an extremely strong magnetic field.
- Lagrange Points — Positions in a system where gravitational forces and orbital motion create stable regions for matter.
Action Items / Next Steps
- Review the structure and evolution of HR diagrams for star clusters.
- Prepare questions on stellar nucleosynthesis and supernova classifications for the next class.
- Attend office consultations (Natural Science Building 1, 5th floor, Office 68, 10:00–15:00).
- Study mass transfer in binary systems and effects on stellar evolution.