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Thread: RR Lyrae stars

  1. #1 RR Lyrae stars 
    Forum Masters Degree thyristor's Avatar
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    I jst read the book "Big Bang" by Simon Sing. In the fifth chapter he writes about an astronom called Walter Baade who was looking for some cepheid look-a-like stars called RR Lyrae stars in the Andromeda Galaxy.
    Sing said that according to Baades calculations he should be able to see the stars but he didn't why he made the conclusion that the known distance to the galaxy must be wrong. But, if I couldn't see the stars how the heck could he make such statements?


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  3. #2  
    Administrator KALSTER's Avatar
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    RR Lyrae stars can be used as standard candles to measure distances out to about 2.5 million light years. Since he could not see the stars it meant that Andromeda had to be more than 2.5 million ly distant. I am assuming that he calculated a shorter distance, while recent results from the Hipparcos satellite have given a larger distance of between 2.5 to 3 million light years to the Andromeda Galaxy.


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  4. #3  
    Forum Masters Degree thyristor's Avatar
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    Why can't you see them at a farer distance than 2.5 million light years?
    Have they got too low intensity?
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  5. #4  
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    Yes. RR Lyrae variables are relatively dim to begin with. Normally Type Ia supernovae are used to determine galaxy distances. Determining distance from a known light source follows an inverse square rule, so a 100 watt light is only one fourth as bright at twice the distance, etc.
    (You can actually see the Andromeda Galaxy with the naked eye on a clear night.)
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  6. #5  
    Forum Masters Degree thyristor's Avatar
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    Ok, thanks for the help :-D
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