Astrophysics - ignore Flashcards
Nebula (2)
- Giant cloud of H gas and interstellar dust
- Gravitational collapse (denser & denser clumps)
Protostar (5)
- Grav collapse –> work done –> Ek & temp increase
- Large core of material
- Grav field attracts matter until pressure & temp high enough for fusion
- 4 protons –> 1 helium-4 nuclei, 2 positrons, 2 neutrinos, 2 gamma photons
- momentum of photons –> radiation pressure
Main Sequence (2)
- stable: radiation + gas pressure = grav pressure
- H –> He fusion in core
Red Giant (< 10M) (4)
- fusion stops as H in core runs out
- radiation pressure decreases –> core collapse
- temp increases: shell H burning, core He burning
- outer layers expand and cool
Planetary Nebula (3)
- He in core runs out –> collapse –> He shell burning –> increase temp
- C-O core nor hot enough to fuse heavier elements –> unstable –> collapse
- outer layers ejected
White Dwarf (4)
- solid core left behind
- collapses until no further
- very hot, dense remnant of a low mass star
- grav force = Electron Degeneracy Pressure, so no further collapse
Black Dwarf (4)
- fusion stops, only heavy elements remain
- radiates energy as photons leak away
- cools down
- when surface temp = few kelvin
Red Super Giant (3)
- brighter than red giant
- same process
- burning forms heavier elements
Supernova (6)
- iron core forms –> fusion stops –> unstable –> collapse
- immense grav pot energy force Fe protons + electrons –> neutrons
- releases huge energy
- triggers explosive blowing out of outer shell
- outer layers rebound off core –> shockwaves –> supernova
- temp high enough to fuse elements beyond iron
Neutron Star (3)
- core mass > Chandrasekhar (1.4)
- core intact after explosion
- very small, extremely dense core
Black Hole (4)
- core mass > 3-4
- pressure so great
- core collapses into singularity
- escape velocity > speed of light
Singularity
Theoretical point at which matter is compressed to an infinitely small and dense point and the laws of physics as we understand them break down
Electron Degeneracy Pressure (3)
- the pressure that stops the grav collapse of a low mass star
- core contracts, compression forces electrons into higher energy levels
- rush of electrons creates EDP
The larger the energy level transition
the longer the wavelength of the photon emitted
Value of energy level =
energy required to remove atom from that level
Continuous Spectrum (3)
- contains all wavelengths
- hot dense sources
- fusion in stars produce this
Absorption Spectrum (3)
- dark lines on continuous spectrum
- caused by light passing through an absorbing medium
- Sun’s atmosphere
Emission Spectrum
- frequencies of em radiation emitted due to electron energy level transitions in that element
- unique for each element
- hot low pressure gases
As surface temp increases
Peak of intensity occurs at lower wavelength
Ideal Black Body
absorbs and emits all wavelengths
Hotter/Cooler objects colours
Hotter: white/blue
Cooler: red/yellow
Astronomical Unit
mean distance from centre of Earth to centre of Sun
1.5x10^11m
Stellar Parallax
The apparent shifting in position of a star viewed against a background of distant stars when viewed from different positions of the Earth
The Doppler Effect (2)
- the apparent shift in wavelength/frequency caused by the relative motion between a wave source and an observer
- moving towards observer = longer wavelength
Hubble’s Law (4)
- showed universe expanding by observing red shift of absorption line spectra from light of distant galaxies
- distant galaxies moving away from Earth
- red shift more pronounced for further galaxies
- recessional velocity proportional to distance from Earth
Ober’s Paradox (2)
- darkness of night sky conflicts with assumption of an infinite and eternal static universe
- if true every line of sight = star, night sky as bright as Sun
Cosmological Principle
On a large scale universe is uniform:
- isotropic (same in all directions)
- homogeneous (uniform density)
- subject everywhere to same physical laws and models
Cosmic Microwave Background Radiation (4)
- originate from early Universe
- microwave radiation received from all directions
- max wavelength corresponds to 2.7K
- Big Bang theory: hot and dense = thermal radiation
The Big Bang Theory
Universe created from an infinitely hot, small, dense singularity where all the universe’s current mass was situated
Evidence for Big Bang Theory
- Hubble’s Law: if expanding, point where started expanding
- No BB = No MBR
General Relativity
Space and time connected together as space-time
Property that connects 3 Ds of space and 4th of time
Evolution of The Universe
- 0s: singularity –> Big Bang
- <1s: 4 forces originated
- 100s: fundamental particles (quarks, leptons, photons + anti) formed
- matter & antimatter annihilate (gamma)
- protons cool –> fusion –> helium nuclei
- 300,000,000 years: atoms
The Future of the Universe: Density too High (4)
Closed
- grav forces prevent galaxies going out forever
- slow and stop
- collapse in –> Big Crunch
The Future of the Universe: Density too Low (4)
Open
- galaxies reach terminal velocity when far apart so negligible grav forces
- separate forever
- cool as they do
The Future of the Universe: Density = Critical
Flat
- universe decrease in velocity as it expands
- reached final finite size after infinite time
- galaxies slow but never stop
Dark Matter (5)
- matter which cannot be seen & doesn’t emit/absorb em radiation
- expect velocity to decrease as move away due to weak g
- not case for galaxies
- mass spread out & not concentrated
- mass is concentrated at centre –> another type of mass
Dark Energy
Type of energy that permeates the whole universe & opposes the attractive force of gravitation between galaxies via exertion of a negative pressure
