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Space (Fate of the Universe (If the universe has been expanding at the…
Space
Fate of the Universe
Observed that spectra from galaxies alsmost all show red-shift - theyre all moving away from us (redshift compared to lab spectra)
Amount of galactic redshift gives recessional velocity - proportional to distance v=H(o)d which suggests that the universe is expanding
H(o) has a large amount of uncertainty due to difficulty in measuring astronomical distances (approx. 65-80)
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Universe is expanding and cooling meaning if you trace back far enough there will be Hot Big Bang
- Hot Big Bang Theory: the universe started very hot and very dense (perhaps infinitely) and has been expanding ever since
According to the HBBT, before the HBB there was no space or time, space-time began with the HBB when time = 0 and radius of universe = 0
Cosmological Red Shift - Glaxies are not moving through space away from us, instead space itself is expanding and light waves are stretched along with it
If the universe has been expanding at the same rate for its whole life, age = t = 1/H(o)
Absolute size of the universe is unknown but there is a limit to the size of the observable universe - sphere with Earth at its centre with radius equal to maximum distance that light can travel during its age
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Critical Density = density of mass in the universe that will mean gravity is just strong enough to stop expansion when t=inf. = 3(H(o))^2 / 8piG
If Density of the universe is less than critical density, gravity is too weak to stop the expansion and the universe will expand forever
If density of the universe is greater than the critical density, gravity is strong enough to stop the expansion and start the universe contracting again
In all three cases the rate of expansion slows meaning the rate of expansion in the ast must have been faster than currently
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Astronomical Distances
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Distances to Galaxies
Standard Candles
- Stellar objects with known luminosity
- Measure intensity on Earth and use inverse square law to calculate distance
Life Cycle of a Star
Clumps of dust and gas contract under force of gravity
- When clumps become dense enough, cloud splits into protostars that continue to contract and heat up
- Eventually temperature reaches few million degrees and hydrogen nuclei begin to fuse into helium which releases a large amount of energy and creates enough pressure to stop gravitational collapse
Main Sequence
- Pressure from hydrogen fusion in stars' core balances gravitational force trying to compress it
- When all hydrogen in the core is used up, nuclear fusion stops and outward pressure stops
- Helium core contracts and heats up under the weight of the star and the outer layers cool and expand
Low Mass Stars become Red Giants
- High luminosity and relatively low surface temperature ∴ huge surface area
- Helium core continues to contract until eventually hot and dense enough for helium fusion - releases huge amounts of energy which pushes outer layers further outwards
White Dwarf
- Once all helium in the core is fused, star begins to collapse under its own weight
- Core of low-mass star will not get hot enough for further fusion so continues to contract
- Once core shrinks to ≃ Earth sized,
High Mass Stars become Red Supergiants
- Have lots of fuel in main sequence that is used up faster
- Can keep fusing elements up to iron
Supernova
- Star explodes into supernova and leaves behind a Neutron Star, or if star massive enough, a Black Hole
- When core of star runs out of fuel, contracts to form a white dwarf core
Neutron Star - (incredible dense, very small, rotate very fast)
- If star massive enough, it keeps contracting (>1.4x mass of sun)
- Electrons squashed into atomic nuclei and combine with protons to form neutrons and neutrinos
- Core suddenly collapses to become neturon star which outer layers fall onton
- When outer llayershit surface, they rebound setting up shockwaves that rip star apart causing supernova
Black Hole - (infinitly dense point in space) - so dense that normal laws of physics break dow and gravitational strength so strong that even light is pulled in
- If core of star is >3x suns mass, neutrons cant withstand the gravitational forces and the star collapses to form a black hole
Neutron Stars and Black Holes do not show up on HR diagrams as Neutron Stars have low luminosity, and black holes no luminosity
HR Diagrams 
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Doppler Effect
Doppler Effect - Change in frequency and wavelength due to the velocity of the source relative to the observer
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