As big as our Sun is — its radius is about 100 times that of Earth, meaning that around one million earths can fit inside it — there are many even larger ones. And more exotic: for instance, the Cepheid variable stars expand when their internal pressures and temperatures rise, and then contract from that expanded state when the pressures and temperatures subside. One of these stars — found on a routine search by astronomer Edwin Hubble — became critical evidence in the debate over the size and movement of our universe.

On this day, October 5, in 1923, Hubble began his studies of the distant Andromeda galaxy through the 100-inch telescope at Mount Wilson Observatory in California. What he found was at first thought to be an exploding star, but after Hubble observed regular flare-ups, he concluded it was variable. Hubble designated it “VAR!” in his star notes.

Hubble used the star’s brightness to calculate its path and distance from earth. The calculations yielded 100 million light-years — more than three times the sizes of our Milky Way galaxy. The findings were the best evidence against the “Island Universe” theory that held our Milky Way galaxy was the entire universe, and Andromeda and other nebulae were located within it.