🪐Intro to Astronomy Unit 22 – Stars from Adolescence to Old Age

Key Concepts and Definitions

• Stellar evolution traces the life cycle of stars from birth to death
• Main sequence stars fuse hydrogen into helium in their cores (Sun)
• Stellar mass determines a star's evolutionary path and ultimate fate
• Hertzsprung-Russell (H-R) diagram plots stellar luminosity against temperature
• Stellar remnants include white dwarfs, neutron stars, and black holes
• Nucleosynthesis produces heavier elements through nuclear fusion in stars
• Stellar populations categorize stars based on age and composition (Population I, II, III)

Star Formation and Early Stages

• Stars form from gravitational collapse of molecular clouds composed of hydrogen and helium
• Protostellar phase begins when a collapsing core becomes opaque and pressure builds up
• Accretion disks surround protostars and can give rise to planetary systems
• T Tauri stars are pre-main sequence stars with strong magnetic activity and stellar winds
• Hayashi track represents the early evolutionary path of low-mass stars in the H-R diagram
• Henyey track describes the evolution of intermediate-mass stars before reaching the main sequence
• Herbig Ae/Be stars are massive pre-main sequence stars with circumstellar disks

Main Sequence Stars

• Main sequence stars are in hydrostatic equilibrium, balancing gravity and internal pressure
• Stellar mass determines a star's position on the main sequence (O, B, A, F, G, K, M)
• Main sequence lifetime depends on mass, with more massive stars having shorter lifetimes
• Stellar magnetic activity, including starspots and flares, is common in low-mass stars
• Stellar winds are outflows of charged particles that can affect a star's evolution
  • Solar wind is an example of a stellar wind emanating from the Sun
• Convective and radiative zones transport energy within stars
• Stellar rotation rates decrease over time due to magnetic braking

Stellar Evolution and Energy Production

• Nuclear fusion powers stars, converting lighter elements into heavier ones
• Proton-proton chain dominates energy production in low-mass stars like the Sun
• CNO cycle is the primary fusion process in massive stars
• Stellar evolution is driven by changes in the star's internal structure and composition
• Main sequence stars evolve into red giants when hydrogen fusion in the core ceases
• Helium flash occurs in low-mass stars when helium fusion ignites in the core
• Asymptotic giant branch (AGB) stars are cool, luminous stars in late stages of evolution
  • AGB stars undergo mass loss through stellar winds and pulsations

Giant and Supergiant Phases

• Red giants are cool, inflated stars that have exhausted hydrogen in their cores
• Horizontal branch stars are low-mass stars fusing helium in their cores (RR Lyrae variables)
• Asymptotic giant branch (AGB) stars are highly luminous and undergo mass loss
• Dredge-up events mix nuclear fusion products to the surface of AGB stars
• Planetary nebulae form from ejected material during the AGB phase
• Supergiants are massive, luminous stars that have evolved off the main sequence
  • Examples include Betelgeuse (red supergiant) and Rigel (blue supergiant)
• Supernova explosions mark the end of life for massive stars

Stellar End States

• White dwarfs are the remnants of low- and intermediate-mass stars
  • Electron degeneracy pressure supports white dwarfs against gravitational collapse
• Chandrasekhar limit ($\sim$1.4 solar masses) is the maximum mass of a white dwarf
• Neutron stars form from the collapse of massive stars during supernova explosions
  • Neutron degeneracy pressure supports neutron stars against further collapse
• Pulsars are rapidly rotating neutron stars with strong magnetic fields
• Black holes are the end state of the most massive stars
  • Event horizon is the boundary beyond which nothing, including light, can escape
• Stellar mass black holes form from the collapse of massive stars
• Supermassive black holes reside at the centers of galaxies and can power quasars

Observational Techniques and Tools

• Telescopes collect and focus light from distant stars and galaxies
  • Reflecting telescopes use mirrors (Hubble Space Telescope)
  • Refracting telescopes use lenses (Yerkes Observatory)
• Spectroscopy analyzes the wavelengths of light emitted or absorbed by stars
  • Doppler shift of spectral lines reveals stellar motion and composition
• Photometry measures the brightness and colors of stars
• Astrometry precisely measures the positions and motions of stars
• Interferometry combines light from multiple telescopes to achieve higher resolution
• Space-based observatories avoid atmospheric distortion and absorption (Chandra X-ray Observatory)
• Adaptive optics corrects for atmospheric distortion in ground-based telescopes

Real-World Applications and Current Research

• Stellar evolution models inform our understanding of the universe's history and future
• Nucleosynthesis in stars explains the origin of elements heavier than hydrogen and helium
• Exoplanet research seeks to find and characterize planets around other stars
  • Transit method detects exoplanets by measuring dips in stellar brightness
  • Radial velocity method detects exoplanets through stellar wobble
• Gravitational wave astronomy detects mergers of compact objects like neutron stars and black holes
• Stellar forensics uses stellar composition to trace the history of galaxies
• Astrobiology explores the potential for life in the universe, including around other stars
• Stellar archaeology studies ancient stars to understand the early universe
• Solar astronomy investigates the Sun's structure, dynamics, and effects on Earth