Story So Far

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Story So Far

  1. 1. Story so far
  2. 2. Ancient Greeks <ul><li>Had the Earth at the centre of the Universe. </li></ul><ul><li>Evidence– Planets, stars ,sun, moon all appeared to rise in the East and set in the West. </li></ul><ul><li>Conclusion– They must be going around us. </li></ul>
  3. 3. <ul><li>Ptolemy as the ancient Greeks was aware that the Sun, moon and 5 planets moved against the star background. </li></ul><ul><li>This led to the two sphere model with the Earth at the centre. </li></ul><ul><li>The variable movement of the planets and retrograde motion caused a problem with this model. Epicycles were used to explain retrograde motion.(p5) </li></ul>
  4. 4. <ul><li>Copernicus changed the model ,having the Sun at the centre. The planets moved in a circular path around it. </li></ul><ul><li>This didn’t quite agree with observations of the planets motion. </li></ul><ul><li>Kepler proposed a modification. Planets move in ellipses. </li></ul><ul><li>He also proposed his laws, using Newton’s law of gravitation to find the relationship between T and r. </li></ul>
  5. 5. <ul><li>Galileo used a telescope to support Copernicus. </li></ul><ul><li>He observed mountains on the moon– the heavens weren’t perfect. </li></ul><ul><li>Observations of the Milky Way showing millions of stars supported the Copernicus idea that stars were at a great distance from the Earth. </li></ul>
  6. 6. <ul><li>Galileo discovered that Venus goes through a series of phases. </li></ul><ul><li>Jupiter has 4 orbiting moons. </li></ul><ul><li>These strongly supported a Copernican model. </li></ul>
  7. 7. <ul><li>Perturbations in Uranus’s orbit helped to find Neptune. </li></ul><ul><li>Newton’s laws helped to find Neptune. </li></ul><ul><li>There was a problem with the stars which were believed to be static. </li></ul><ul><li>They should cause the universe to collapse. </li></ul><ul><li>It hasn’t so matter must be uniformly </li></ul><ul><li>spread through an infinitely large space . </li></ul>
  8. 8. How do stars die? <ul><li>Sun type </li></ul><ul><li>E= mc ² so any small change in the mass of a star ,produces an enormous amount of energy. </li></ul><ul><li>In the sun at 15 million ºK , hydrogen nuclei fuse to produce helium nuclei. </li></ul><ul><li>4H = He + 2 neutrinos + 2 positrons </li></ul><ul><li>0.7% of the initial mass is converted to energy </li></ul>
  9. 9. <ul><li>With a star </li></ul><ul><li>1. The energy produced by thermonuclear fusion exactly matches the energy radiated by the star. Temperature therefore is constant. </li></ul><ul><li>2.The thermal pressure outwards balances the gravitational forces inward keeping the size constant. </li></ul>
  10. 10. Time on main sequence <ul><li>0.5 x Sun’s mass 200 billion years </li></ul><ul><li>Sun 10 billion years </li></ul><ul><li>3 x Sun’s mass 15 million years </li></ul><ul><li>25 x Sun’s mass 3 million years </li></ul>
  11. 11. Death of a sun type <ul><li>Hydrogen burning ceases in the core. </li></ul><ul><li>It contracts giving a loss in PE and a gain in KE so it gets hot. Expansion follows. </li></ul><ul><li>Outer layers will cool as a result . </li></ul><ul><li>A red giant is formed. </li></ul><ul><li>As the core continues to contract, Helium burning occurs.(p38) </li></ul>
  12. 12. <ul><li>When this stops ,the core will collapse again . </li></ul><ul><li>What happens next depends on the star’s mass. </li></ul><ul><li>If it is below a critical mass like the sun, it will be peaceful. </li></ul><ul><li>If more it will be spectacular and violent. </li></ul>
  13. 13. <ul><li>If less than 3 x the mass of the sun , there will be no more thermonuclear reactions. </li></ul><ul><li>The star will be unstable and shed the outer layers ( about half its mass) </li></ul><ul><li>The glow of this layer due to radiation from the core is a planetary nebula. </li></ul><ul><li>The core will collapse until the electrons are closely packed enough to generate Fermi pressure. </li></ul>
  14. 14. <ul><li>This prevents any further collapse. </li></ul><ul><li>We now have a star 1% the diameter of the sun called a white dwarf. Not very bright but very dense. </li></ul><ul><li>There is an upper limit to the size of a white dwarf. </li></ul>
  15. 15. <ul><li>If a white dwarf has a mass 1.4 x the mass of the sun ( called the Chandrasekhar limit) then even the Fermi pressure will not stop it collapsing. </li></ul><ul><li>Neutrons will be formed and this final stage takes only a few seconds. Giving a rapid rise in temperature. </li></ul>
  16. 16. <ul><li>A red giant of this mass doesn’t collapse immediately after it stops helium burning. </li></ul><ul><li>Further thermonuclear reactions occur. </li></ul><ul><li>Each one will produce a period of equilibrium. This will happen until all the fuel is exhausted. </li></ul><ul><li>Then the neutrons will be compacted as far as they will go. </li></ul>
  17. 17. <ul><li>The intense radiation pressure from a very hot core causes the star to explode. </li></ul><ul><li>A supernova occurs. </li></ul><ul><li>It may emit as much radiation as a whole galaxy for a few days. </li></ul><ul><li>It will then become a nebula. </li></ul><ul><li>In this explosion, extreme pressures and temperatures can cause further fusion . </li></ul>
  18. 18. <ul><li>This can lead to elements more massive than iron being formed. </li></ul><ul><li>The remnant can become a pulsar. </li></ul><ul><li>A rapidly spinning neutron star with a strong magnetic field. </li></ul><ul><li>This would accelerate charged particles leading to a beam of radio waves being emitted. </li></ul>
  19. 19. <ul><li>Hence as it spins, it produces regular pulses which can be detected. </li></ul><ul><li>Some pulsars emit X rays. These can switch off for a few hours indicating a companion is blocking the radiation. It must be part of a binary system. </li></ul>
  20. 20. What are Quasars? <ul><li>Radio brightness 10 million times the brightness of a whole galaxy. </li></ul><ul><li>Massive red shift indicating a distance of 18 billion light years away. </li></ul><ul><li>Optical brightness 100 x that of a galaxy. </li></ul><ul><li>They often fluctuate in brightness . </li></ul><ul><li>They must be a few light days in diameter. </li></ul>
  21. 21. <ul><li>Their huge power can only be explained by matter orbiting or going into a black hole. </li></ul>

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