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Goal: To understand special stars. Objectives: 1) To learn about Black holes 2) To learn about Neutron Stars 3) To understand Stars that erupt. 4) To understand Variable stars 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. 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. 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?)? 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? 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 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 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 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 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. 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. 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 Distance misestimated • So, Hubble underestimated the distance to Andromeda by a factor of 50. Even today • We still know the distance to the Andromeda galaxy using the Type I (metal rich) Cepheids. 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? 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 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. 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 Conclusion • Variable stars are quite simple to explain in general and can be used for very important distance calculations. • All stars