Physics Paper 1- Astrophysics

• Star formation

  An interstellar dust cloud collapses under the force of gravity, the gravitational potential energy decreases, kinetic energy and temperature increase, until hydrogen nuclei fuse into helium at 10 million Kelvin, and radiation/gas pressure balances gravity
• Star life cycle

  • Sun-like star: Main sequence -> Red giant -> White dwarf

  Massive star: Main sequence -> Supergiant -> Supernova -> Neutron star or Black hole
• Hertzsprung-Russell diagram

  Plots luminosity vs temperature, with main sequence, red/blue supergiants, and white dwarfs
• Chandrasekhar limit

  Maximum mass of a white dwarf star, 1.4 solar masses. Above this, a neutron star or black hole forms.
• Electron energy levels

  Electrons can only occupy certain discrete energy levels

  Ground state is the most negative energy level

  Electrons can absorb energy to jump up levels, or emit energy to drop down levels
• Electron energy level transition

  Absorption of a photon causes electron to jump up an energy level

  Emission of a photon causes electron to drop down an energy level
• Calculating photon energy/frequency/wavelength

  Use ΔE = hf or ΔE = hc/λ to relate energy level changes to photon properties
• Emission spectrum

  Hot gas emits photons at only certain discrete wavelengths, producing a spectrum of coloured lines
• Absorption spectrum

  Hot gas absorbs photons at only certain discrete wavelengths, producing a spectrum of dark lines on a continuous background
• Diffraction grating

  Device that spreads light into a diffraction pattern, allowing measurement of wavelength via d sin(θ) = nλ
• Analyzing light using a diffraction grating

  1. Light passes through the diffraction grating

  2. Light spreads resulting in a diffraction pattern

  3. Equation: d sin(theta) = n lambda
• d

  1 over the number of lines per meter in the diffraction grating
• Theta

  The angle that corresponds to the order of the diffraction
• n

  The order of diffraction
• Lambda

  The wavelength of the light
• Wien's displacement law

  The predominant wavelength emitted (lambda_max) is inversely proportional to the temperature of the star
• Stefan's law

  Luminosity (L) is proportional to the surface area (4*pi*r^2) and the temperature (T) raised to the power of 4
• The sun has a peak wavelength of 550 nm and surface temperature of 6000 K