Variable stars

About half the stars we can detect in the sky are binaries (Cornell astronomy) you wouldn"t know that from looking up at night but there we are, This makes them pretty important.A lot of early work on Cepheids was done by Henrietta Leavitt. She took two pictures of the stars at different dates, one was a positive (white stars black sky and one a negative (black stars white sky). When these were put on top of each other...

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