Chapter 19,20 - Astro and Cosmology Flashcards
Define planets
Objects with mass sufficient enough for their own gravity to force them to take a spherical shape, the object exhibits no nuclear fusion and has cleared its orbit of other objects
Define dwarf planets
planets where the orbit has not been cleared of other objects
Define planetary satellites
bodies that orbit a planet
Define asteroids
objects which are too small and uneven to be planets, with a near circular orbit around the sun
Define comets
small, irregular shaped balls of rock, dust and ice. They orbit the sun in eccentric elliptical orbits
Define solar systems
system containing stars and orbiting objects like planets
Define galaxies
a collection of stars, dust and gas. Containing around 100 billion stars
Describe the formation of a star
- Nebulae are gigantic clouds of dust and gas.
- Over millions of years the gravitational attraction between dust and gas particles pull them together
- gravitational collapse becomes greater the denser parts get
- The gravitational energy is converted to thermal energy heating up these clouds
- as heating continues the dense cloud of gas starts to glow, forming a protostar
- As gravity continues to pull together particles, the temperature inncreases until 1 million kelvin where hydrogen gas nuclei overcome electrostatic forces of repulsion and create nuclear fusion.
- hydrogen fuses into helium producing a star
Describe the stars life after formation but before evolution
- initially the star remains in stable equilibrium where gravitational forces producing fusion equal the repulsive forces of radiation from the fusion
- This is known as the main phase
- smaller stars stay in the main phase for a while
- however larger mass stars use up fusion alot faster due to their heat, and so run out of available hydrogen nuclei quickly.
factors affecting life cycle of a star
mass of the stars core
Evolution of a low mass star like our sun
- low mass star is about from 0.5 solar masses to 10 solar masses
- this star has a smaller, cooler core and remains in the main sequence for longer
- Once hydrogen supplies are low, the gravitational forces inwards overcome forces due to fusion outwards
- so the star collapses inwards.
- It evolves into a red giant
- the core is too cool to fuse helium but the pressure in the outer core is great enough for fusion to occur there.
- as helium runs low the red giant evolves into a white dwarf and the outer layers drift off into space to form a planetary nebulae
- core remains a very dense white dwarf of around 3000K
- no fusion occurs in a white dwarf
- the white dwarf will be stable if below 1.44 solar masses, the Chandrasekhar limit
Evolution of a massive star to a supernova
- a star with a mass of larger than 10 solar masses
- hydrogen supplies deplete
- as mass is larger, temperature is high enough in the core for helium fusion into heavier elements
- forming a red supergiant
- The red supergiant has layers of increasingly heavy elements produced by fusion, with an inert iron core
- As the core cant fuse anymore the star becomes unstable and collapses rapidly inwards
- this collapse bounces off the dense core in an explosion out to space forming a supernova
possible evolution beyond the supernova
if the remaining core has a mass of 1.44 solar masses, ie the Chandrasekhar limit, then the protons and electrons combine to form neutrons
- this produces a neutron star
- very small but high density
if the core has a mass of over 3 solar masses, a black hole is forms
- this is where the gravitational pull causes escape velocity to be greater than the speed of light
- huge mass
- negligible volume
- infinite density
- not even photons can escape
why do electrons absent from atoms have zero energy
energy is said to be the work done required to remove an electron from its atom
describe an emission line spectra
each element produces a unique emission line spectra due to their unique set of energy levels associated with its electrons. It appears as a series of colourful lines on a black background
describe a continuous line spectra
all visible wavelengths are present. They are produced by atoms of solid heated metals e.g. lamp filament
describe an absorption line spectra
a series of dark spectral lines against the background of the continuous spectrum. The dark lines have exactly the same wavelength as the bright emission line spectrum lines of the same gas atoms
describe what energy levels are
- an electron cannot have energy of between two levels
- energy levels are negative because external energy is required
- an electron with 0 energy is free from the atom
- the energy level with the most energy is the ground level or state
What does an electron being excited mean
it moves up energy levels
what happens when an electron is de excited
- energy is conserved so the electron emits a photon of specific wavelength. The energy of the emitted photon is given by E=hc/λ. E is the difference in energy of the two levels.
What are diffraction gratings
components with regularly spaced slits that can diffract light
equation for determining wavelength of light by diffraction
dsinθ = nλ where d is the slit separation and n is the order maxima
what affects the stars colour
surface temperature
what can stars be modelled as
idealised black bodies that emit radiation across a range of wavelengths. There is a peak wavelength when graphed which shows the colour of the star
Weins law
peak wavelength is inversely proportional to the temperature of the object.
weins formula
λmax α 1/Temp so λmax x Temp = Weins constant
what is weins constant
2.9x10^-3 (metresKelvin)
where λmax is the peak wavelength used to determine the colour
Stefans Law
radiant power output per unit area is proportional to Temp^4
- power output is also proportional to surface area
stefans equation
L = SAT^4σ L = 4πr^2T^4σ
what is stefans constant, σ
5.67x10^-8
What does absolute temp mean and why is it important
- When using temperatures in stefans and weins equation we must use absolute values
- this means the temperature must be in Kelvin
Define an astronomical unit
average distance from the earth to the sun
Define the light year
the distance light travels in one year
convert arc seconds to degrees
one arc second is 1/3600th of a degree
Define a parsec
distance at which a radius of one AU subtends an angle of of one arc second
how many metres is a parsec
3.1x10^16
outline stellar parallax
parallax is the apparent shift in position of an object against a backdrop that doesn’t appear to move. and so is used for nearby stars
equation to find distance using parallax angle
d = 1/p
where d is the distance away in parsecs and p is the angle in arcseconds
What is the cosmological principle?
the universe is isotropic and homogenous, and the laws of physics are universal
what does isotropic mean?
The Universe looks the same in all directions to every observer. There is centre or edge.
what does homogenous mean?
Matter is uniformly distributed - for a large volume of the universe, the density is the same
outline Olber’s paradox and what it means
the night sky should be bright due to the sky being of infinite stars, however it is dark therefore the universe is expanding and finite
Describe the doppler effect
the apparent shift in wavelength occurring when the source of the wavelengths is moving.
Describe what happens to wavelengths when the source of wavelength moves towards and away from the detector
When the source moves towards the detector, the wavelength appears to decrease. When moving away the wavelength is stretched and so increases.
Does the emitted wavelength change or is it just the wavelength at the detector
No, the source emits the same wavelength, the wavelength received differs
Equation for doppler effect
𝜟λ/λ = 𝜟f/f = v/c
where v is the velocity of the star relative to earth
𝜟λ is the change in hydrogen spectral line λ emitted by the star
How do we use the doppler effect and hubbles law together
v=HoD so we find v then plug into doppler equation
What is Hubble’s law
the recessional velocity (v) of a galaxy is proportional to the distance from earth
what did Hubble find
- Light from the vast majority of galaxies was red-shifted, so moving away
- Further the distance, the more the red-shift, so the faster the recession
How to estimate the age of the universe
1/Ho in seconds
conversion factor for converting Ho to seconds?
multiply by 3.23x10-20
Describe the conditions at the start of the big bang
temperature and density infinite volume 0 (singularity)
Evidence for the Big bang?
- Hubble’s shows the universe is expanding, through the red shift of light from distant galaxies. If time were to run backwards, the universe would eventually shrink to a singularity
- cosmic microwave background radiation at 2.7K, this was created when high energy gamma photons saturating the early universe were stretched due to the expansion
- gas clouds of primordial hydrogen and helium
Evolution of the Universe
- initially at 0s the universe is a singularity and is infinitely hot and dense
- at 10^-35 seconds, exponential expansion called inflation occurs, no mass only gamma photons with temperature 10^28
- 10^-6 we see the first fundamental particles producing a quark and letpon soup
- 10^-3 as mass exists gravity exists and now forms the first hadrons, such as protons and neutrons through pair production.
- at 1s the creation of mass stops, temperature at 10^9
- 100 seconds protons and neutrons fuse to form primordial helium and hydrogen
- 380 000 years the universe has cooled enough to allow atoms to form, gamma photons have now stretched to microwaves
- 30 million years, first stars appear as gravity hauls in mass and the solar systems and galaxies
- now, temperature 2.7K
Explain how the theory of dark matter came about
During observations of galaxies it was seen that the velocity didn’t always follow predictions
- it was seen that the further away from the centre of the galaxy, it’s velocity would decrease
- this is due to most of the “physical”mass is collated in the centre of the galaxy
- however this new observation suggest that mass is spread out over the galaxy and therefore there is some sort of matter that we can’t see
- this is dark matter and is said to make up 27% of the universe
outline the possible ends of the universe
hot death, collapse if actual density is greater than threshold density
cool death - increased expansion if actual density is less
- or constant if densities equal
