Stars Flashcards
Define Nebulae
Gigantic clouds of dust and hydrogen gas
Will eventually give rise to the formation of stars and planets
Define Planets
Spherical bodies that orbit a star
Define Dwarf Planets
A planet that has not cleared its orbit of other objects
Define Moons
Spherical bodies that orbit planets
Define Asteroids
Small, irregularly shaped bodies, composed of dust and metal. Usually in near circular orbits about stars. Remnants of planets and contain no ice.
Define Comets
Small, irregularly shaped bodies, composed of dust and ice. Usually in eccentric orbits about stars. Remnants from the formation of a solar system. Contain ice.
Define a Solar System
A system of planets orbiting a central star
Define Galaxy
A collection of stars and planets (collection of solar systems
Define Universe
All the galaxies and all their mass and energy
Steps of the Formation of a Star
Nebulae form over millions of years as tiny particles of dust and gas come together under the force of gravitational attraction
Denser regions of a nebula pull in more matter, coming hotter as GPE is transferred to heat energy
A protostar forms in the nebula - a very hot and very dense cloud of dust and gas
If the protostar becomes massive, dense and hot enough, the gravitational force of attraction between particle is able to overcome the electrostatic force of repulsion between hydrogen nuclei
Nuclear Fusion of hydrogen nuclei begins and the protostar becomes a main sequence star
Beginning for all stars
Denser regions of a nebula form a protostar
Protostar become hot and dense enough, Nuclear fusion begins and the star becomes a main sequence star
Life Cycle of a low mass star
Hydrogen nuclei in core runs out
Rate of fusion decreases - star cools -> radiation pressure drops
Gravitational force collapses star - GPE -> KE -> Causing it to heat up again
Fusion of hydrogen nuclei in shells of core begins -> Shells expand cool, emit lower red EM waves -> Red Giant
Hydrogen nuclei in shells run out
Rate of fusion decreases -> star cools -> mass of shells lost to space and radiation pressure drops
Gravitational force collapses star -> Hot core remains -> White dwarf
White dwarf cools -> black dwarf
Electron Degeneracy Pressure in White Dwarfs
When fusion slows down in shells of star, radiation pressure decreases, star core collapses under gravity
As the core collapses, matter/atoms are forced together into a smaller volume
However, electrons are not allowed to occupy the same energy levels within atoms
A pressure is exerted by the electrons as the star collapses - electron degeneracy pressure
This pressure outwards counters the gravitational attraction inwards, and no further collapse of the core is possible
What is the Chandrasekhar Limit
Limit to mass of core that will be prevented from total collapse of star due to electron degeneracy pressure
Core/White dwarf can only be as massive as 1.44 times mass of star
A mass of greater than this will mean gravitational collapse is not prevented by electron degeneracy pressure
The Life Cycle of High Mass Stars
More mass, more GPE -> KE and hotter core
Leads to faster rate of fusion and shorter life time
Hydrogen nuclei in core run our
Rate of fusion decreases -> star cools -> radiation pressure drops
Gravitational force collapses star - GPE ->KE - causes it to heat up again
Fusion of helium nuclei in the core begins - star heats up and expands - fusion of hydrogen in outer shell begins - outer shells further expand and cool - emit lower red EM waves - Red Super Giant
Star develops an iron core - Iron nuclei in core cannot be fused together - Star becomes unstable and implodes to become a supernova
Outer shells shed
Dense core remains - either a neutron star or black hole
White dwarfs and Neutron Star
If mass of remaining core is greater than Chandrasekhar limit - further gravitational collapse forms a neutron star
Composed of neutrons - small volume - very dense
White dwarfs and Black holes
If mass of the remaining core is greater than 3 solar masses - further gravitational collapse of core creates a very dense body
Gravitational pull is very strong - photons/light unable to escape the gravitational pull
Low Mass Star life cycle
Protostar -> Main sequence star -> Red giant -> White dwarf -> Black dwarf
High mass star life cycle
Protostar -> Main sequence star -> Red supergiant -> Supernova -> Neutron Star/Black hole
What does the Hertzsprung - Russell Diagram show
The position of stars at various stages or their life cycle as a plot of their luminosity against temperature
Both axis are log scales
Temperature axis runs in reverse
Luminosity is often plotted in units relative to the luminosity of the sun
For what is the Hertzsprung Russel diagram used for
To determine what stage of its life cycle a star is
What is Luminosity
An absolute measure of the radiated EM power (Watts)
How much EM radiation is emitted per second
What does the luminosity of a star depend in
Both temperature and mass
High temperature and low mass - low luminosity - low output of energy per unit time
Low temperature but large mass - high luminosity - output lots of energy per unit time
Where are the hottest most luminous stars located on the Hertzsprung Russel diagram
Top left
Where are the coolest least luminous stars located on the Hertzsprung Russell diagram
Bottom right
Where are electrons only allowed to exist
At discrete energy levels about a nucleus in an atom
What is the lowest energy level of an atom
Ground state
n=1
How much energy must be transferred to an electron to escape the atom
The energy at a given level to attain zero electrostatic potential energy
How can an electron excite to a higher level
By absorption of a photon E=hf only if the E matches is exactly the difference between levels
How can orbital electrons be excited to higher energy levels
Interaction with accelerated electrons
Accelerated electrons energy does not have to match the difference between energy levels to cause excitation - simply has to have at least energy matching the difference
What happens to any electrons that excite to a higher energy level
Must de-excite to the original level either directly or in steps
Every de excitation / step down between energy levels emits a photon of energy equivalent to the difference in energy between the two levels
What happens during every de-excitation between energy levels
A photon is emitted with the energy equivalent to the difference in energy between the two levels
What is a Black Body
A theoretical body which absorbs all EM radiation of all frequencies incident upon it
A perfect absorber of EM radiation and a perfect emitter of EM radiation
Describe Black Body Radiation Curves
Illustrate how the intensity of radiation emitted by a black body varies with its temperature
Every black body emits a continuous range of EM radiation
As temperature increases - the peak intensity of EM radiation shifts to lower wavelengths and increases in relative magnitude
What is Wien’s Displacement Law
The wavelength at peak intensity is inversely proportional to the absolute temperature of the body
What is Stefans Law
The total power radiated by a black body at absolute temperature is proportional to its surface area and to its temperature^4
Wiens Displacement Law Equation
λmax T = constant
Wien’s Constant = (0.0029)m K = (2.9x10-3) metre-Kelvin
Stefan’s Law Equation
P = σAT^4
σ = Stefan’s Constant
5.67 x 10^-8 Wm^-2K^-4
For a star of radius, r its total power radiated is a measure of its luminosity, L
L = 4πr^2σT^4
What can Wien’s Law and Stefan’s Law be usedfor
Wien’s Law & Stefan’s Law can be used together
to estimate the radius of a distant star
Once radius is known, mass and density of the star can be determined using Newton’s Law of Gravitation.
