stars and cosmology Flashcards
What are nebulae
Gigantic clouds of dust and gas
how does a protostar form from a nebulae
Gravitational attraction pulls the dust together
As they get closer the collapse accelerates which creates denser regions which are hotter
Protostar forms (hot dense sphere of gas(
How does a protostar become a star
Once the electrostatic repulsion between the hydrogen nuclei is overcome by extremely high temperatures and pressures (kinetic energy)
Hydrogen nuclei are fused to make helium nuclei, fusion begins
What prevents a star from being compressed
The radiation pressure from photos emitted during fusion
The gas pressure from the nuclei in the core
Low mass stars
Once run out of hydrogen
Red giants - core of star begins to collapse
- Pressure increases cause fusion into shell around core and layers of hydrogen forming other elements
Element layers float off into planetary nebula
Core remains as white dwarf
Electron degeneracy pressure
Pressure created by electrons in the core of a white dwarf as they are being collapsed as they cannot exist in the same energy state
What is the Chandrasekhar limit
1.44 solar masses
If the core of a star has a mass less than this then the electron degeneracy pressure is enough to prevent gravitational collapse
Why do high mass stars have a different lifecycle
Mass is much greater therefore core is hotter
Consume hydrogen in much less time
Helium nuclei fuse into heavier elements
Red supergiants
Protostar expands
Temp and pressure high enough to fuse even massive nuclei together —> forms a series of shells inside the star
UNTIL star has iron core, cannot fuse , makes it unstable
Supernova
Supernova
Shockwave from a red super giant which ejects all the material into space
Neutron star
If mass of star is greater than Chandrasekhars limit (1.44 solar masses) gravitational collapse continues into a neutron star
Black hole
After supernova
If core has mass higher than 3M gravitational collapse continues
Results in gravitational field so strong nothing even photons can escape
Hertz-sprung russel diagram
What are the axis
Y axis is luminosity
X axis is temperature but going right to left
The astronomical unit (AU)
Average distance from Earth to the sun
How many arc minutes in 1 degree
1 arc second is how many degrees?
60
(60 arc seconds in each minute)
1 arc second = 1/3600 degrees
Parsec definition
The distance at which a radius of 1 AU subtends an angle of 1 arc second
Stellar parallax
Technique used to determine the distance to stars which are relatively close to earth
Find the parallax angle as the earth orbits
Describe the differences between absorbtion, continuous and emission line spectra
Emission: coloured lines on a black background corresponding to the wavelengths
Continuous: Conatins the complete spectrum of of vivible wavelengths and frequencies (heated solid metal)
Absorbtion: Continuous line passes through a cool gas, some electrons are absorbed as the gas gets excited (black lines on a continous spectrum)
d = 1/p only if?
p is measured in arcseconds
How is an absorbtion line spectrum formed
When the light from a continuous spectrum passes through a cool gas absorbs the same wavelengths of light as it would emit when it is hot. The wavelengths it absorbs appear black on the continous spectrum background
A star is receeding when
Receeding
Redshift
Longer wavelength
the cosmological principle
the universe is
1. isotropic
- (eye) looks the same to any observer at any point
2. homogenous
- matter is distributed evenly
3. The laws of physics are universal
Hubbles Law observations
- The light from galaxies was red shifted
- Further away the galaxy is the faster it moves
Evidence for big bang
- Hubbles law
- Microwave background radiation
- universe first existed largely of high energy gamma photons, expansion of the universe stretched these high energy photons, now observe as microwaves
