Physics
Define the doppler effect
The change in wavelength caused by the relative motion between a wave source & an observer
Define 'Parallax'
A measure of the apparent displacement of the star against a fixed background
What is the 'Parallax angle'
The angle between the earth & sun from a near by star (mreasured in arc seconds)
What is 1 arc second in degrees
1/3600 degrees
Define a 'Parsec'
The distance which 1 AU will subtend an angle of 1 arcsecond
Give the formula for distance in parsecs
distance (parsecs) = 1 / parallax(arcseconds)
What is Hubble's Law
The redshift of a distant object is proportional to the recessional velocity of that object
Give the formula for speed of recession
Speed of recession(kms^-1) = Hubble's constant(kms^-1)(Mpc^-1) * Distance(Mpc^-1)
Give the value of hubble's constant
70(kms^-1)(Mpc^-1)
Define 'Homogenous'
Where matter is distributed uniformly across the universe & the has a uniform density
Define 'Isotropic'
Where the universe looks the same in all directions to every observer
Define 'Stellar Parallax'
The apparent shift in position of a nearby star relative to more distant stars viewed from earth during it's orbit around the sun
What happens when a wave source moves towards an observer
The wavelength of the waves will appear to decrease
What happens when a wave source moves away from an observer
The wavelength of the wave will appear to decrease
When comparing the light spectrum produced by the sun and a distant galaxy, what evidence is there that the universe is expanding?
There are more lines on the red end of the spectrum (Of the distant galaxy compared to the sun)
Give the Doppler equation
Δλ/λ ≈ Δf/f ≈ Δv/c
What does Hubble's Law show to us?
The further away a star is from earth, the faster its moving away from us
The closer a star is to earth, the slower it moving away from us
Explain the evolution of the universe
- The universe started off as a hot, dense and small saturated with high energy gamma photons
- Once the big bang occurred & as the universe was expanding, the gamma photons wavelength also increased (redshifted)
- The wavelength has increased up to the microwave region of the spectrum
- As the CMB of the universe decreased, the temperature also decreases.
- The intensity of the CMB is uniform and has a value of 2.7K
Explain the formation of a star
Nebula - Clouds of dust & Gas are pulled together via gravitational attraction & form denser clumps
Protostar - Gravitational collapse causes the gas to heat up & glow, forming a protostar (WD on particles by collisions causes an increase in their KE causing an increase it temp)
Nuclear fusion - Temperature will become very high & then hydrogen fuses to helium. 4 hydrogen are fused into 1 helium nucleus, producing gamma ray photons, two neutrinos & positrons.
Main Sequence Star - The star reaches a stable state where the inwards(gravitational collapse& outwards(gas pressure) forces are in equilibrium
Explain how a low-mass star forms
Masses between 0.5-10 times the suns mass
Red Giant - The hydrogen fuelling the star runs out, fusion stops & the star shrinks due to the unbalanced forces (gravitational pull > gas pressure + radiation pressure)
Planetary Nebula - The outer layers of the star are released. The carbon-oxygen core isn't able to fuse heavy elements resulting in the star being unstable and collapsing. Electron degeneracy prevents further collapse (ON STARS SMALLER THAN THE CHANDRASEKHAR LIMIT)
White Dwarf - core remains(3000K) photons leak and dissipate heat
** White dwarfs continue to radiate photons & after billions of years it cools down until it doesn't emit heat/light(Black dwarf)
Explain how a massive star forms
Masses greater than 10 times the mass of the sun
Red SuperGiant - Temperature is high enough for helium fusion, so heavier elements are made (layers are increasingly heavy)
Supernova - Iron core becomes unstable & a shockwave ejects materials from the outer shells and the core collapses. Elements heavier than iron are formed
If the star is greater than 3 times the suns mass, a black hole is formed. Gravity is so strong that the escape velocity is bigger than the speed of light. Photons can no longer escape
If the star is between 1.44 - 3 times the suns mass, a neutron star is formed p+s and e-s combine to form neutrons forming a small dense neutron star
Define the 'Chandrasekhar Limit'
The max mass a white dwarf can be whilst stable (1.4 times the suns mass)
Why can e-'s not have an an energy value between 2 levels?
e-'s only exist as discrete values
Explain how photons are emitted
e-'s move from a lower energy level to a higher energy level (they get excited), using external input energy, e.g. heat. The e-'s get de-excited and in the process releases energy in the form of a photon
When is an emission line spectrum produced & what produces it?
When an e-'s moves from a higher energy level to a lower energy level & emits a photon with an energy equal to the difference of the energy levels. Only certain energy levels are allowed so only certain frequencies are seen. Hot and low pressure gases
What must happen when during the de-excitation process?
Energy must be conserved so that photons can be emitted with a discrete frequency/wavelength
Give the formula to calculate the energy level of a hydrogen atom
E = -13.6/n²
What is the continuous spectra & how are they produced?
Where photons emitted from the core of a star have all wavelengths & frequencies of the EM spectrum. Produced from hot dense sources
What is the absorption spectra
Where white light passes thorugh a cool, low pressure gas & certain wavelengths are missing.
How an light from a star be analysed
Using a Spectroscopy
State Wein's law
The black body radiation curve for different temperatures peaks at a wavelength inversely proportional to the tempreature of the object
State Stefan's law
The total heat energy emitted from a surface is proportional to the 4th power of its absolute temperature & th surface area of the star
Give the formula for Luminosity
4πr²T^4,
When calculating Luminosity, what must you ensure
Temperatures are their absolute values (Kelvin)
Define 'Luminosity'
The radiant output power of a star
What does the HR diagram show to us?
Shows the luminosity of stars against their temperature
Explain what electron degeneracy pressure is?
Where the compression of a collapsing star forces electrons to fill any available space it can find in a higher energy level, resulting in an outwards pressure
What events happened after the big bang?
10⁻³⁵s - Universe has a phase of incredible acceleration (inflation)
10⁻⁶s - 1st fundamental particlesgain mass through higgs boson
10⁻³ - Quarks combine to form hadrons (protons and neutrons). Most mass is formed by pair production (High energy photons transform into particle & antiparticle pairs)
1s - Creation of matter stops & temperature drops to 10⁹K
100s - p+ & e- fuse to form deuterium, helium and a small quantity of lithium and beryllium
3.8×10⁵y - Universe is cool enough for first atoms to form. nuclei capture e-s & release EM radiation thats now CMBR
30×10⁶y - 1st star appears, elements heavier than lithium form via nuclear fusion
200×10⁶y - Milky way forms
9×10⁹y - solar system forms from nebula left by supernova
11×10⁹y - Primitive life is thought to begin
13.7×10⁹y - Temperature is 12.7K (NOW)
Define 'Dark Energy' and how much of the universes energy does it make up?
Energy that covers the entire universe & opposes the gravitational force between galaxies.
- Makes up 68% of the total universes energy
Define 'Dark Matter' and how much of the universes mass does it make up?
Matter that cannot be seen & doesn't absorb or emit EM radiation
- Estimated makes up 27% of the universes mass