Andromeda Nebula: Var!

Explanation: In the 1920s, using photographic plates made with the Mt. Wilson Observatory's 100 inch telescope, Edwin Hubble determined the distance to the Andromeda Nebula - decisively demonstrating the existence of other galaxies far beyond the Milky Way . His notations are evident on the plate shown above (the image is a negative with stars appearing as black dots against the white background of space). By intercomparing plates, Hubble searched for “novae”, stars which underwent a sudden increase in brightness. He found several on this plate and marked them with an “N”. Later he discovered that one was actually a type of variable star known as a cepheid - crossing out the “N” he wrote “Var!” (upper right). Thanks to the work of Harvard astronomer Henrietta Leavitt , cepheids, regularly varying, pulsating stars , could be used as “standard candle” distance indicators . Identifying such a star allowed Hubble to show that Andromeda was not a small cluster of stars and gas within our own galaxy, but a large galaxy in its own right at a substantial distance from the Milky Way. Hubble's discovery is responsible for our modern concept of a Universe filled with galaxies .

Information: The Scale of the Universe Debate in April 1996