The Island Universe Theory The Milky Way: Our Island Containing the Sun Fuzzy Patches: Nebula or Galaxy? The ShapleyCurtis Debate Shapley believed Galaxy is over 10 times larger than (was then) believed. Sun not at center of galaxy Spiral nebulae are located within our galaxy Curtis believed The (back then) accepted size of galaxy is correct. Sun is at center of our galaxy Spiral nebulae are each their own galaxy (island universes) And the winner is FACTS: Galaxy is about twice as large as astronomers than believed. Spiral nebulae are galaxies outside of our own Milky Way galaxy. The sun is about 2/3 out from the center of our galaxy. Shapley (sort of) Curtis Shapley Measuring the Distance to Spiral Nebulae Cepheid Variable Stars Stars that vary in brightness with a regular period of many days Period is directly related to absolute magnitude (brightness) Can be used to calculate distance to them M = log (P) Measuring Distance Tasks 1.Find classical Cepheid variable stars in the Great Andromeda Nebula. 2.Measure the period of each classical Cepheid. 3.Use the period-luminosity relationship to calculate the distance to each star. 4.Average the results to get a reasonable answer for the distance to the Great Andromeda Nebula. The Results Edwin Hubble performed each of those outlined tasks and found The Andromeda is very far away, so it must be outside of our galaxy. He then started to measure the distance (like he did for Andromeda) and velocity (using the Doppler effect) of other galaxies. Hubble found a remarkable relationship between distance and velocity of galaxies Hubbles Graph PNAS March 15, 1929 vol. 15 no Hubbles Law Hubble concluded that all galaxies beyond a certain distance are moving away from us. The speed at which each galaxy is moving away is linearly proportional to the distance away from us. The slope of the line (called the Hubble constant, H 0 ) can be used to predict the velocities of, or distances to, distant galaxies. v = H 0 d What does that all mean? All of the galaxies are flying away from us! The universe is expanding (faster and faster)! NASA No, we are not special. All galaxies are moving away from each other. An alien astronomer on another planet would observe Hubbles Law the same way we do. NASA Island Universe Theory Activity