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Cepheid Variables

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

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