🚀Astrophysics II Unit 3 – Late Stages of Stellar Evolution

Key Concepts

• Stellar evolution driven by nuclear fusion in the core converting lighter elements into heavier ones
• Main sequence stars fuse hydrogen into helium in their cores (Sun)
• Post-main sequence evolution depends on initial stellar mass leading to different end states
• Massive stars undergo core collapse and explode as supernovae leaving behind neutron stars or black holes
• Lower mass stars expel their outer layers forming planetary nebulae with white dwarf remnants
• Stellar remnants provide insights into extreme physics (high density, strong gravity, intense magnetic fields)
• Supernovae enrich the interstellar medium with heavy elements enabling formation of planets and life
• Observational techniques (spectroscopy, photometry, neutrino detection) probe stellar interiors and evolution

Stellar Lifecycle Recap

• Stars form from gravitational collapse of molecular clouds consisting primarily of hydrogen and helium
• Protostars contract until core temperatures reach ~10 million K initiating hydrogen fusion
• Main sequence phase lasts until hydrogen exhausted in the core (timescale depends on stellar mass)
• Post-main sequence evolution:
  • Lower mass stars (<8 solar masses) become red giants with inert helium cores and hydrogen burning shells
  • Massive stars (>8 solar masses) become red supergiants with onion-like structure of fusion shells
• Red giant phase ends with helium flash in low mass stars as core reaches 100 million K and helium fusion begins
• Asymptotic Giant Branch (AGB) phase follows with alternating hydrogen and helium shell burning
• Thermal pulsations in AGB stars drive strong stellar winds and mass loss

Late-Stage Stellar Evolution Processes

• Helium fusion in red giant cores produces carbon and oxygen (triple-alpha process)
• AGB stars undergo thermal pulses and dredge-up events enriching surface with fusion products
• Mass loss via strong stellar winds in AGB stars (up to $10^{-4}$ solar masses per year) expels outer layers
• Carbon-oxygen core left behind in AGB stars becomes a white dwarf
• Massive stars develop iron cores after successive stages of fusion (carbon, neon, oxygen, silicon burning)
• Iron core collapse triggered when core exceeds Chandrasekhar mass limit ($\sim$1.4 solar masses)
• Core collapse supernova explosion occurs as infalling matter rebounds off ultra-dense neutron core
• Supernova shock wave and neutrino heating expel outer layers of the star

Types of Stellar Remnants

• White dwarfs: remnants of low to intermediate mass stars (<8-10 solar masses)
  • Supported by electron degeneracy pressure with no fusion reactions
  • Typical mass ~0.6 solar masses and radius ~Earth-size resulting in high density ($10^6$ g/cm$^3$)
• Neutron stars: remnants of massive stars (>8-10 solar masses) after core collapse supernovae
  • Supported by neutron degeneracy pressure with densities comparable to atomic nuclei ($10^{14}$ g/cm$^3$)
  • Rapid rotation (millisecond periods) and strong magnetic fields ($10^{12}$ Gauss) in some neutron stars (pulsars)
• Black holes: remnants of the most massive stars (>20-25 solar masses) or from mergers
  • Formed when stellar remnant exceeds maximum neutron star mass (~3 solar masses)
  • Characterized by event horizon boundary where escape velocity equals speed of light
  • Supermassive black holes (millions to billions of solar masses) exist at the centers of most galaxies

Supernovae and Their Aftermath

• Core collapse supernovae mark the explosive death of massive stars
• Neutrino emission carries away ~99% of the gravitational binding energy released in the collapse
• Heavy elements synthesized in the stellar interior (silicon through iron) are ejected into space
• Supernova remnants (Crab Nebula) expand into the interstellar medium shocking and enriching the gas
• Supernova shock waves can trigger new star formation in nearby molecular clouds
• Type Ia supernovae result from thermonuclear explosions of white dwarfs in binary systems
  • Occurs when white dwarf accretes matter from companion star and exceeds Chandrasekhar mass limit
  • Used as "standard candles" to measure cosmic distances due to their consistent peak luminosity

Observational Evidence and Techniques

• Stellar spectra provide information on surface composition, temperature, and wind outflows
  • Emission lines from circumstellar material indicate mass loss (P Cygni profiles)
  • Absorption lines reveal surface abundances of fusion products (technetium in AGB stars)
• Light curves (brightness vs. time) distinguish different types of supernovae
  • Type II supernovae show hydrogen in their spectra and have extended plateaus in light curves
  • Type Ia supernovae lack hydrogen and have consistent peak brightness allowing distance measurements
• Neutrino detections from SN 1987A in Large Magellanic Cloud confirmed core collapse models
• Supernova remnants studied across electromagnetic spectrum (radio, infrared, optical, X-ray)
• Gravitational wave observations (LIGO/Virgo) probe mergers of stellar remnants (neutron stars, black holes)

Astrophysical Implications

• Chemical enrichment of the Universe driven by stellar evolution and supernovae
  • Elements heavier than iron formed through neutron capture in AGB stars and supernovae
  • Planetary systems and life based on elements produced in stellar interiors
• White dwarfs constrain age of the Universe and serve as probes of dark matter (microlensing)
• Neutron stars test physics under extreme conditions (strong gravity, high density, intense magnetic fields)
• Supernova feedback regulates star formation and shapes evolution of galaxies
• Compact object mergers (neutron stars, black holes) are primary sources of gravitational waves
• Stellar remnants in binary systems (X-ray binaries, cataclysmic variables) exhibit rich phenomenology

Cutting-Edge Research and Open Questions

• Supernova explosion mechanisms and role of neutrinos, rotation, and magnetic fields
• Formation and evolution of the most massive stars (>100 solar masses)
• Equation of state of ultra-dense matter in neutron star interiors
• Origin of heavy elements beyond iron (r-process nucleosynthesis) in neutron star mergers and supernovae
• Nature of supermassive black holes and their role in galaxy evolution
• Progenitor systems and explosion mechanisms of Type Ia supernovae
• Fate of the most massive stellar remnants (intermediate-mass black holes?)
• Surveys to discover and characterize new stellar transients (LSST, ZTF)