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Discovering Cepheids: Luminous Pulsating Stars in Space

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  1. CEPHEIDS

  2. What are Cepheids? • Stars that “pulse” and change luminosity • Very bright (100,000x luminosity of Sun) • Used to measure extreme distances in space • Their brightness and pulsing allows for measurements to distant galaxies

  3. Cepheids- history • First discovered by John Goodricke in 1784 • Named after the first star (Delta Cephi) in the constellation Cepheus (the King) • Was not known at that time the important significance they would play • Henrietta Leavitt (1912) studied over 500 Cepheids in the Magellanic Cloud Henrietta Swan Leavitt

  4. History- continued • Harlow Shapely (1915) charted size of Milky Way using Cepheids • Leavitt and Shapely determined that the period of a Cepheid varied directly with its luminosity- (i.e., the brightest Cepheids have the longest periods) • Edwin Hubble’s work with Cepheids settled the debate of whether the Milky Way represented the entire universe, or was merely one galaxy of many

  5. Cepheid Characteristics • “Classical Cepheids” are Yellow Super Giants • Drifted off the main sequence into the “instability zone” • Pulsation is an actual change in temperature and diameter of the star • Typically ~25% change in size • Classical Cepheids are 4-20x more massive than the Sun, and 100,000x or more brighter pulsating stars

  6. HR diagram showing location of Cepheid variables compared to main sequence stars "HR-diag-instability-strip" by Rursus - Own work.

  7. Classical vs. Type II Cepheids • Type II Cepheids (AKA Population II Cepheids) are: • Metal-poor • Old stars • Smaller than the Sun (~1/2 solar mass) • Further subdivided by period length (1-4 days, 5-10 days, 10-20 days, 20+ days)

  8. Cepheids Graphs • Remember the direct relationship: the longer the period, the brighter the Cepheid

  9. Cepheids Graphs • What is the period of the Cepheid in this graph?

  10. Nearest Cepheid • Our nearest Cepheid is Polaris (See location on HR Diagram) • Classified as a classical Type I Cepheid • Recent evidence has shown Polaris is more than 100 light years closer than previously thought Polaris Closer Than Thought

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