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Goal: To understand special stars.

Goal: To understand special stars. Objectives: To learn about Black holes To learn about Neutron Stars To understand Stars that erupt. To understand Variable stars. Variable stars. Usually stars are held in equilibrium. If they expand then they cool and that ends the expansion.

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Goal: To understand special stars.

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  1. Goal: To understand special stars. Objectives: To learn about Black holes To learn about Neutron Stars To understand Stars that erupt. To understand Variable stars

  2. Variable stars • Usually stars are held in equilibrium. • If they expand then they cool and that ends the expansion. • Their cores are stable. • The star is stable. • However, there is a region on the HR diagram where this is not the case – the instability strip. • In this region stars will pulsate – that is they will expand and contract. • This causes the star to get brighter and dimer.

  3. Variable types • RR Lyrae – lower mass stars after they undergo their Helium flash (the sun will do this someday). • RR Lyrae are Horizontal Branch stars. • Metal rich and Metal poor Cepheid variables (Type I and II). • These are the higher mass stars which pass back and forth through the instability strip.

  4. Observing RR Lyrae • RR Lyrae have periods of about 0.3 to 0.5 days. • A) Why do you think those periods are so short? • B) Why is this length of period a really bad thing when it comes to observing the star (hint, when can a good telescope look at stars?)?

  5. Absolute Magnitude • RR Lyrae have an average absolute magnitude of 0.75. • Why is that an advantage? • What is the disadvantage if you are looking at other galaxies?

  6. How it works: • Eddington Valve • Star gets smaller and hotter. • Helium fully ionizes. • The Helium then absorbs most of the light. • So, energy out does not = energy produced

  7. Result • Outer part of star heats up • That makes it expand • That makes it cool • That makes it go back to being only singly ionized • It can now radiate the energy

  8. However • The star has now over expanded (overshot the runway) • It can no longer hold up the outer part of the sun • So it falls back, and will overshoot the other way going too far in • The process then continues

  9. What stars become RR Lyrae? • Stars that will not supernova at the ends of their life become RR Lyrae stars • This is stars like our sun and up to a mass of 4 times our sun. • However they have lost a good fraction of their masses by this point • RR Lyrae stars are the stars in the middle of the horizontal branch

  10. Why variable stars are important • Variable stars have a relationship between their period of pulsation and their absolute brightness. • The longer the period, the bigger the star is, and the brighter it is (sort of like a bigger bell has a larger period of vibration). • This allows us to measure distances (especially since these are very bright stars which can be seen a LONG distance away)! • In fact, the distance to Andromeda was first attempted to be estimated using Cepheid variables.

  11. Distance to Andromeda • Edwin Hubble tried to estimate the distance to Andromeda using Type II Cepheids (metal poor). • Type II Cepheids are in the globular clusters. • However, he made a slight mistake.

  12. Type I Cepheids (metal rich): Mv = -2.81 log(Period in days) -1.43 • Type I Cepheids (metal rich) • ones in the disk of our galaxy • have a pretty exact relationship between variability period and average absolute magnitude. • The brightness of Type Is is 4 times greater than Type IIs

  13. Distance misestimated • So, Hubble underestimated the distance to Andromeda by a factor of 50.

  14. Even today • We still know the distance to the Andromeda galaxy using the Type I (metal rich) Cepheids.

  15. Profile of a Cepheid Variable • Cepheids expand and contract. • As they do they change color (and temperature). • As they expand they cool and turn redder. • As they condense they get hotter and turn bluer. • When do you think they are brightest?

  16. Anatomy of a Cepheid pulsation • When smallest, hottest, and bluest, the star is DIMMEST • When largest, coolest, and reddest, the star is BRIGHTEST • This is because the change in size is much greater than the change in temp

  17. What will become a Cepheid • For metal rich stars it is massive stars (> 4 solar masses) • For metal poor they seem to become Type II Cepheids and not RR Lyrae.

  18. Unresolved Issue • Clearly composition of the star has an effect on the equation for period vs absolute brightness • However this total affect has not been completely narrowed down • This does lead to sizable errors in measuring distances (10%) • This error connects to the errors in measuring the age of the universe

  19. Conclusion • Variable stars are quite simple to explain in general and can be used for very important distance calculations. • All stars

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