P7 2

P7 2

1. Parralax

   • Things appear to move when you are moving
   • We can work out the distance to stars that are close with parallax using trigonometry
   • We can do this as we know the distance from the earth to the sun
   • The bigger the parallax angle the closer the star
   • 1 degree = 60 minutes
   • 1 minute = 60 seconds
   • Closest star is 0.76 seconds parallax angle
   • Seconds written as 6"
   • Minutes written as 6'
   • Parallax doesnt work for stars really far away
   • 1 Parsec = distance of object with parallax angle of 1 second
   • Distance in parsecs = 1/parallax angle seconds

2. Hudgens method

   • The further away a star the fainter the light
   • Light gets weaker with distance as the photons spread out
   • Problems
     • 1. Some stars naturally brighter and bigger
     • 2. He wasnt taking measurements
     • 3. He was assuming all stars are the same
   • Luminosity = how bright a star actually is
   • Luminosity = energy given out per second as light radiation
   • Luminosity depends on how hot and big a star is
   • Observed brightness is how bright it appears to be from earth
     • Depends on - Stars luminosity - Distance from earth - Dust or gas blocking it
   • A cepheid is a star that varies in its brightness
   • By measuring a cepheids period you can work out the luminosity and the distances very quickly
   • Correlation between luminosity and period length in a cepheid
   • So people can find out the distances and luminosity of stars with cepheids

3. Hubble

   • Shapleys argument
     • 1. Universe is a gigantic galaxy
     • 2. Sun and solar system are far off the center of the universe
     • 3. Nebulae are huge clouds of dust and are part of the milky way
   • Curtis's argument
     • 1. Universe is made up of many galaxies
     • 2. Our galaxy is smaller than shapley suggests
     • 3. Spiral nebulae are the other distant galaxies completely separate form the milky way
   • Hubbles conclusion
     • 1. Shapley was right that the solar system is far from the center of the universe
     • 2. Curtis was right about the nebulae, they are far away in separate galaxies
   • He did this using cepheids to see how far away they are
   • 1 mega parsec = 1 million parsecs
   • Hubble used the cepheid technique to determine the distances of other galaxies
   • By red shift he also found that the galaxies are moving away from us
   • A spectrometer is a device that splits light into all the colours produced by the star
   • All stars contain hydrogen
   • A spectra from stars has black lines because different elements absorb particular wavelengths of light
   • The black lines for hydrogen are in every star
   • If the black lines are shifted slightly to t he left towards red) then they are moving away
   • The further away the galaxies the faster they are moving away from us
   • Hubble compared red shift of galaxies and how far away they are and it was basically directly proportional
   • Speed of recession = Hubble constant x distance
   • km/s = km/s per Mpc x Mpc
   • Hubble constant = 70.6 km/s per Mpc

4. Understanding stars

   • Stars are not all the same colour
   • The colour depends on how hot it - the hotter the more white-blue - the colder the more red
   • The peak of the graph shows the actual colour and heat of the star
   • The more to the left the peak is the hotter the star as it releases radiation at a higher frequency
   • The higher the peak the more luminous it is as it releases more radiation
   • The frequency is increasing as it gets more to the left (opposite x axis)

5. Emitting and absorbing photons

   • When you put the light of the sun through a spectrometer there are lots of black Fraunhofer lines
   • Some wavelengths are missing
   • Burning different elements give different colour flames
   • Different elements give out different wavelengths of light when heated - eg sodium gives out two of yellow
   • The wavelengths that is emits is called an elements emission spectra
   • No two emission spectra are the same
   • The emission spectra is the same as the elements absorption spectra
   • You can look at the spectra from a star and see which elements are present
   • Helium was discovered using this
   • Basically when an atom gives out or absorbs light the electrons are moving up or down energy levels
   • When an electron absorbs light it goes up an energy level
   • When it emits light it goes down an energy level
   • The amount of energy a photon has depends on the difference between energy levels occupied by the electron
   • The energy levels are arbituary mathematical values
   • Different elements have energy levels in different places
   • Different jumps of energy levels mean a different wavelength of light it emitted
   • Electrons will only absorb photons with the exact energy to get it up an energy level
   • There are as many Fraunhofer lines (black lines) as there are combinations of different energy levels jumps

6. How stars and solar systems form

   • Scientist believe all stars were formed by clouds of gas slowly being brought together by gravity
   • After a long time when the gas starts to get close together it starts spinning
   • As it spins around the temperature increases
   • As everything collapses the particles spin around faster and it gets hotter
   • Eventually, it will get hot enough to fuse hydrogens and then it will be classified as a star
   • Over time some of the mass/gas doesn't make it to the star - they are left outside and form planets and mooons
   • A protostar is a star that isn't hot enough for fusion
   • When it starts fusing hydrogen it is a main sequence star
   • Differences between stars is due to their masses

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