Astrophysics

• Stars are formed from dust and gas particles in clouds called nebulae being attracted to each other due to gravity
• The cloud becomes hotter and more dense until Fusion starts to occur
• A star remains stable when the outward pressure from Fusion and the force of gravity pulling inward are balanced, known as the main sequence stage of its life
• When a star dies, the outward pressure increases causing it to expand into a red giant if it's a star similar in size to our sun, or a super red giant for stars much bigger than our sun
• A red giant will collapse once all the fuel for Fusion has run out, leaving a white dwarf and then a black dwarf once it has cooled
• A super red giant explodes in a Supernova, leaving a very dense neutron star at the center or a black hole
• The outer layers of the Supernova move away forming new nebulae from which new stars could be made
• Nuclei fuse together to make heavier elements, some of which can only be made as a result of the huge amount of energy released from a supernova
• Luminosity of a star is the total power output in Watts, given by Stefan's law: L = Stefan constant * 4πr^2 * surface temperature in Kelvin^4
• Stars can be treated as black bodies, emitting and absorbing all wavelengths of light
• The energy emitted by a star increases with shorter wavelengths, reaching a peak and then decreasing
• The peak wavelength is given by Wien's law: wavelength * temperature in Kelvin = Wien's constant
• Luminosity against temperature graph shows main sequence stars following a specific line, with white dwarfs being small and hot, and red giants being huge and relatively cool
• Astronomical distances are measured in parsecs or light years
• When observing distant stars and galaxies, their wavelengths appear longer and shifted towards the red end of the spectrum, indicating redshift and that they are moving away from us
• The redshift of galaxies suggests they are receding faster, supporting the Big Bang Theory and the expanding universe
• Cosmic microwave background radiation is detected from all directions, suggesting we are looking at the edge of the Big Bang and the universe is still expanding
• The Hubble graph shows the recessional velocities of observable galaxies against their distance, supporting the Big Bang Theory
• The Hubble constant (h0) is the gradient of the Hubble graph, with the reciprocal giving the age of the universe in seconds
• The shift in wavelengths on the emission spectrum from a star can be due to redshift, and lines appearing split can be from a binary star system
• The Hubble constant may be given in different units such as kilometers per second and mega parsecs, which need to be converted to seconds to the minus one for calculations