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.