You will use photometry data, which measure the apparent brightness of stars in units of magnitude, of stars in a far away galaxy in the constellation Virgo. M100, a beautiful front-on spiral similar to the Milky Way, is one of a number of galaxies in a group called the Virgo Cluster.
Here is a Hubble image of this striking galaxy.
NASA and the Space Telescope Science Institute
And here is a ground-based image of this beauty!
M100 copyright Anglo-Australian Observatory, Photo by David Malin