Cepheid Variables


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Cepheid Variables

  1. 1. Cepheid Variables
  2. 2. How does a star work ? <ul><li>Gravity tries to compress the star. </li></ul>
  3. 3. <ul><li>Heat from nuclear fusion pushes out </li></ul>
  4. 4. <ul><li>Star in a position of equilibrium ( balanced forces. </li></ul>
  5. 5. <ul><li>Many stars pass through a period of instability during their lives. </li></ul>
  6. 6. <ul><li>Cepheid Variables are yellow supergiants with a mass over 3x the mass of the sun </li></ul><ul><li>They pulse in periods of a few days. </li></ul>
  7. 7. <ul><li>Their brightness can be measured </li></ul>
  8. 8. Why does it pulse ?
  9. 9. <ul><li>So how can we measure the distance of stars? </li></ul>
  10. 10. What do we know ? <ul><li>1 . The further a star is away, the dimmer it will seem. </li></ul><ul><li>2 . The size of the star is important in deciding how bright it is. </li></ul><ul><li>3 . We need to know the type of star ( e.g. White dwarf, red giant) </li></ul>
  11. 11. <ul><li>So its temperature is important too </li></ul>
  12. 13. <ul><li>Red Giants are large but cool </li></ul><ul><li>White dwarfs are small but hot. </li></ul>
  13. 14. <ul><li>So The distance and the temperature are key factors in deciding the brightness of stars. </li></ul>
  14. 15. <ul><li>So why are Cepheid variables important? </li></ul><ul><li>We can use them to measure their distances from Earth. </li></ul><ul><li>Gives us a capability of measuring more distant objects . </li></ul><ul><li>Better than parallax. </li></ul>
  15. 16. <ul><li>From their period, we can calculate their Absolute magnitude ( apparent magnitude it would have if it is 10 parsecs from Earth) </li></ul><ul><li>We can measure its apparent magnitude. </li></ul><ul><li>The distance can be calculated from the Absolute and apparent magnitude using a simple formula. </li></ul>
  16. 18. Remember from P1 <ul><li>The Great Debate of 1920 </li></ul><ul><li>Harlow Shapley said nebula were part of the Milky Way. </li></ul><ul><li>Heber Curtis claimed that spiral nebulae were star systems outside the Milky Way. </li></ul>
  17. 19. <ul><li>Shapley won the debate but Curtis was later proved to be right. </li></ul><ul><li>Edwin Hubble used a 100 inch telescope to discover Cepheid variables in the Andromeda nebula and in other spiral clusters of stars. </li></ul><ul><li>These turned out to be much further away than stars in our own galaxy. </li></ul>
  18. 20. <ul><li>Hubble used Cepheid variables to measure the distances of many local galaxies e.g. 2.5 million light years away. </li></ul><ul><li>Most galaxies were too far away to pick out Cepheid variables </li></ul><ul><li>The Milky Way is 100,000 light years in diameter. </li></ul>
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