Stars Flashcards
Planet
Object in orbit around a star
It must have a mass large enough for its own gravity to give it a round shape, have no fusion reactions and have cleared its orbit of most other objects
Planetary satellites
A body in orbit around a planet. This includes moons and man made satellites
Comets
Small irregular bodies made up of ice, dust and small pieces of rock. They orbit around the sun in highly eccentric elliptical orbits. They range from a few hundred meters to tens of kilometers
Solar system
A star and all the objects that orbit it
Galaxy
A collection of stars and interstellar dust and gas
How is a protostar formed
Nebulae are formed over millions of years as gravitational attraction pulls particles together.
As dust/gas gets closer together, gravitational collapse accelerates and denser regions begin to form
These regions get hotter and denser until a protostar forms
Nebulae
Clouds of dust and gas
How does a protostar become a main sequence star
Nuclear fusion needs to start, requiring extremely high pressures and temperatures to overcome electrostatic repulsion.
Once hydrogen to helium fusion starts, radiation pressure from emitted photons and gas pressure from nuclei in the core push outwards. Gravitational forces inwards are balanced
Life cycle of stars with low mass
They remain in the main sequence for longer because temperature is lower so fusion happens slower
Stars with lower mass (between 0.5 and 10 solar masses) form red giants then white dwarfs
Red giants
When fusion decreases in a low mass main sequence star, the star collapses inward.
As it shrinks, pressure increases enough to start fusion in a shell around the core. The core is inert as there is very little hydrogen.
Outer layers slowly move away from the star, cooling as they expand.
White dwarfs
Outer layers of red giant drift off leaving the hot core. This is very dense but no fusion takes place
The core is prevented from collapsing by the electron degeneracy pressure due to the Pauli exclusion principle
Pauli exclusion principle
Two electrons cannot exist in the same energy state. This is only true up to the Chandrasekhar limit(1.44 solar masses - this is the maximum mass of a stable white dwarf)
Life cycle of more massive stars
More massive stars become red supergiants. Then, a supernova occurs. Stars above the Chandrasekhar limit become neutron stars. Stars above 3 solar masses become black holes
Red supergiants
Star collapses, but as it is larger it becomes much hotter so fusion of helium into heavier elements occurs.
A series of shells form inside the star, with lighter elements fusing in the outer shells and heavier elements in the inner shells
This continues until the star develops an inert iron core
Neutron star
Gravitational collapse continues, forming a star made almost entirely out of neutrons
Black hole
Gravitational collapse continues until gravitational field is so strong that light cannot escape
The Hertzsprung-Russel diagram
Graph of luminosity against surface temperature(increasing right to left)
Where do different stars fall on HR diagram
Main sequence stars form a curved line from top left to bottom right -hotter stars are more luminous
White dwarfs are very hot and very dim so they are bottom left
Red supergiants are very luminous because of their size but have relatively low temperature. They are found around the top right
Smaller red giants split off from the main sequence in the middle
Energy levels in atoms
When electrons are bound to their atoms in a gas, they can only exist in discrete energy levels
Energy levels are negative because external energy is required to remove an electron from the atom
An electron with zero energy is free and maximum negative energy is known as the ground state
Exciting an electron
When an electron moves from a lower to a higher energy level it is excited. This happens when it absorbed a photon of a specific frequency
E=hf
Emission line spectra
Each element produces a unique emission line spectra because of its unique set of energy levels. Atoms in a gas are are excited, then when electrons drop back into lower energy levels they emit photons
Absorption line spectra
Dark spectral line against the background of a continuous spectrum
Formed when light from a source that produces a continuous spectrum passes through a cooler gas
Some photons are absorbed by the gas, creating a pattern where specific wavelengths are missing from a continuous spectra
Photon will be reemitted but in a random direction so intensity is greatly reduced
Diffraction grating
When light passes through a diffraction grating it is split into a series of narrow beams depending on wavelength. White light is therefore split up
Diffraction grating equation
Nλ = d sin(θ)
Where N is the order of maxima, d is the grating spacing and θ is the angle between the zeroth order maxima and the Nth order maxima
Wien’s displacement law
Peak wavelength is proportional to absolute temperature
For all black body emitters, peak wavelength x temperature = constant
Black body radiation
Idealised object that absorbs all the EM radiation that hits it and emits distribution of wavelengths
Stefan’s law
L = 4πr ^2σT^ 4
How does intensity against wavelength graph change with temperature
As temperature increases, peak wavelength decreases and peak becomes sharper
