Chapter 19 - Stars Flashcards
Nebulae
Clouds of gas and dust, often hundreds of times larger than our solar system. They compress due to the gravitational attraction and become hotter
Planets
An object in orbit around a star that:
i) Has a mass large enough for its own gravity to give it a round shape
ii) Has no fusion reactions
iii) Has cleared its orbit of most other objects
Planetary Satellites
A body in orbit around a planet
Comets
Small, irregular bodies made up of ice, dust and small pieces of rock. Orbits a star in strongly elliptical orbits.
Solar Systems
Contain stars and all of the objects that orbit them
Galaxies
A collection of stars and interstellar gas and dust
Lower mass stellar evolution
Nebula -> Protostar -> Main sequence -> Red giant -> Planetary nebula -> White dwarf
(0.5-10)M☉
Higher mass stellar evolution
Nebula -> Protostar -> Main sequence -> Red supergiant -> Supernova -> Neutron star or Black hole
M > 10M☉
M☉
1 solar mass
Star birth
It starts with a cloud of gas and dust (nebula) where the particles are pulled together by gravitational attraction
Denser regions form where more gas and dust are pulled in and gravitational energy is transferred to thermal energy, increasing the temperature
A hot and dense protostar will form in one part of the cloud
Main sequence
The main phase of a star’s life, outward gas pressure and radiation from the fusion is balanced by the inwards gravitational force
The duration is dependent on the mass of the star, a larger star has a hotter car that releases more energy but has a shorter life
Red Giant
As the hydrogen runs low, the gravitational forces overcome the gas pressure and it begins to collapse
It increases the pressure so fusion happens in shells around the core but not in the core as the temperature is too low to overcome the electrostatic repulsion between helium atoms
The star expands and cools, causing the red colour
White Dwarf
As a red giant gets larger, the outer layers drift off leaving a planetary nebula
Leaves an extremely hot and dense core made of helium
No fusion takes place and the only energy emitted comes from leaking photons produced by earlier reactions
Electron Degeneracy Pressure
When a star is compressed, it creates a pressure that prevents any further collapse. Wolfgang Pauli
Chandrasekhar Limit
The maximum mass of a white dwarf, 1.44M☉. The maximum mass such that the electron degeneracy pressure prevents any further collapse
Red Supergiant
Have enough heat to overcome the electrostatic repulsion between helium so it can fuse into heavier elements
They fuse in shells around the core as enough energy is released to fuse further elements
Continues until there is an inert iron core and non-fusing hydrogen on the surface
Supernova
The inactive iron core makes the star incredibly unstable and it violently implodes
Creates a shockwave that ejects all of the core material into space, ‘explosion’ is called a type II supernova
Elements above iron are formed and scattered through the universe
Neutron Star
M > 1.44M☉
The collapse continues and a neutron star is formed almost exclusively made of neutrons
Diameter 10km and mass 2M☉
Black Hole
M > 3M☉
Gravitational collapse continues past the point of a neutron star
The result is a gravitational field such that an object would need an escape velocity greater than the speed of light
Vary in mass up to several million M☉
Schwartzchild Radius
The radius of an imaginary sphere sized so that if all the mass of an object were compressed into it the escape velocity would be greater than the speed of light
Schwartzchild Radius Formula
r = 2GM/c^2
s
Hertzsprung-Russell diagram
A classification system for stars, graphs the luminosity against surface temperature for all stars in our galaxy
The temperature decreases
from left to right
Luminosity
The total radiant power output
1 Ls
The luminosity of the sun (3.85 x 10^26 W)
Appearance of a HR diagram
A tan shaped curve from the top-left to bottom right, fairly narrow - main sequence stars
At a low surface temperature and average luminosity, a circle branches containing giants
An oval all the way along the top of supergiants
A cluster in the bottom left for white dwarfs
HR axes
y: Luminosity 0.001 to 100000 Ls
x: Temperature: 40000 to 2500 K
Logarithmic scale
Gas Atoms
Electrons bound to an atom can only exist in discrete energy levels
The energy levels are negative as external energy is required to remove an electron from an atom
An electron with 0 energy is free from the atom
Ground State
The energy level with the most negative energy
Moving between energy levels
When an electron moves to a higher energy level it is excited
When an electron moves from a higher energy to a lower one it loses energy
The electron moves down from its excited state, releasing the energy gained as a photon with an energy equal to the difference in energy between the two energy levels
Each atom has its own energy levels
Moving up multiple levels
It could settle at any or none of the remaining energy levels until the ground state, releasing a photon for each it stops at
Emission line spectra
Show a series of lines at different wavelengths to show the wavelengths that each element emits (the difference between energies of energy levels an electron moves between)
Continuous spectra
A spectrum showing all the wavelengths of visible light, produced by atoms of a heated solid metal
Absorption line spectra
A continuous spectrum with a series of dark spectral lines at specific wavelengths overlayed, the wavelengths of light absorbed by the star (the same as an element’s absorption lines)
Making absorption line spectra
Light from a source that produces a continuous spectrum passes through a cooler gas
Photons with energies equal to the difference between energy levels are absorbed
When they are re-emitted, it is in all directions with a lower intensity
These can be compared to an elements emission spectra to determine the composition of gases around a star
Diffraction grating vs double slit
A diffraction grating has sharper fringes and a clearer and brighter diffraction pattern
Brighter as more light passes through
Grating equation
sin(θ) = nλ/d
where d is the distance between adjacent bright fringes and n is the order of the maxima
Calculating the highest order maxima
d/λ
Black Body
An idealised object that absorbs all EM radiation that hits it and then re-emits as a distribution of wavelength dependent on their temperature, almost any object can be modelled as a black body
Wien’s Displacement Law
λmax T = constant
The peak wavelength of light emitted from a black body
Higher temperature link to peak wavelength
Lower peak wavelength, sharper peak on intensity-wavelength graph
Stefan Boltzmann Law
L = 4ℼr^2σT^4
Stefan constant (σ)
5.67 X 10^-8 W m^-2 K^-4
