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Astrophysics and Cosmology Overview

Apr 24, 2025

A-Level Physics: Astrophysics and Cosmology

Definitions

  • Planets: Spherical objects with sufficient mass, no nuclear fusion, cleared orbit.
  • Dwarf planets: Like planets but orbit is not cleared.
  • Planetary satellites: Bodies orbiting a planet.
  • Asteroids: Small, irregularly shaped, orbiting sun.
  • Comets: Balls of rock, dust, and ice with elliptical orbits around the sun.
  • Solar systems: Stars with orbiting objects like planets.
  • Galaxies: Collections of stars, dust, and gas; contain ~100 billion stars.

Formation and Evolution of Stars

  • Nebulae: Birthplace of stars, clouds of dust and gas.
  • Protostar: Forms as nebula collapses, high pressure and temperature lead to nuclear fusion.
  • Main phase: Stable equilibrium of gravitational and radiation pressure. Larger stars have shorter main phases.

Evolution of Low Mass Stars

  • Core mass between 0.5M and 10M.
  • Transition to red giant as hydrogen depletes; then to white dwarf with a planetary nebula.
  • Chandrasekhar limit: White dwarf stability if mass < 1.44M.

Evolution of Massive Stars

  • Core mass > 10M leads to red supergiant formation.
  • Iron core leads to type 2 supernova and potential neutron star or black hole formation.
  • Hertzsprung-Russell (HR) diagram: Plots stellar luminosity vs. temperature.

Electromagnetic Radiation from Stars

Energy Levels of Electrons

  • Electrons exist in discrete energy levels, unique per element.
  • Excitation: Electron moves to higher energy state with external energy.
  • De-excitation: Electron releases energy as photon.

Spectra Definitions

  • Emission line spectra: Unique for each element, colored lines on black.
  • Continuous line spectra: All visible light wavelengths.
  • Absorption line spectra: Dark lines on light spectrum.

Emission Spectral Lines and Diffraction

  • Formula: ( E = \frac{hc}{\lambda} )
  • Diffraction gratings: Diffract light at different angles by wavelength.

Weins Displacement and Stefan's Laws

  • Weins Law: Peak wavelength inversely proportional to temperature.
  • Stefan's Law: Luminosity proportional to fourth power of temperature.

Cosmological Distances and Measurements

  • Astronomical Unit (AU): Average Earth-Sun distance.
  • Light-year: Distance light travels in a year.
  • Parsec (pc): Distance with parallax angle of 1 arcsecond.

Stellar Parallax

  • Measures distance to nearby stars, accurate up to 100pc.

Cosmological Principle

  • The universe is isotropic and homogenous.

The Doppler Effect and Hubble's Law

Doppler Effect

  • Shift in wavelength due to motion of source.

Hubble's Law

  • Recessional velocity proportional to galaxy distance.
  • Evidence for expanding universe; estimates universe age ~14 billion years.

The Big Bang Theory

  • Origin of universe: rapid expansion from singularity.
  • Evidence: Red shift, cosmic microwave background radiation.

Timeline of the Universe

  • Big Bang: Universe begins from singularity.
  • 10^-35 s: Inflation.
  • 10^-6 s: Fundamental particles gain mass.
  • 100 s: Formation of light nuclei.
  • 380,000 years: First atoms form.
  • 9 billion years: Solar system forms.

Current Theories

  • Dark Energy: Hypothetical form accelerating universe expansion.
  • Dark Matter: Invisible matter affecting galaxy dynamics.
  • Potential universe futures: open, closed, or flat, depending on density.