Spiral galaxy NGC 3370, HST image. This galaxy is around 98 million light years from Earth. It has a nucleus of mainly older yellower stars, surrounded by spiral arms containing younger, hotter, bluer stars. This galaxy was home to a type 1a supernova in 1994. Such supernovae have a known brightness, which allows their distance to be calculated. Studying type 1a events in galaxies provides information about the expansion of the universe. This image was taken in 2003 during a survey of Cepheid variable stars in the galaxy, the most distant such stars ever measured at the time. Cepheids provide very accurate distance data.