What type of variable star did Hubble use to measure distances to galaxies?

The type of variable star Hubble used to measure distances to galaxies was the Cepheid variable. Cepheids brighten and dim in a regular rhythm, and their pulsation period is linked to their true luminosity.

That period–luminosity relationship makes a Cepheid a standard candle. By comparing how bright a Cepheid truly is with how bright it appears from Earth, astronomers can calculate its distance. Henrietta Swan Leavitt established the crucial relationship while studying Cepheids in the Small Magellanic Cloud, giving later astronomers a powerful distance tool.

In 1923, Hubble identified classical Cepheid variables in what was then called the Andromeda Nebula. Their calculated distance showed that Andromeda lay far beyond the Milky Way, helping settle the Great Debate over whether the Milky Way was the entire universe. Cepheids are not the same as RR Lyrae stars: both are variable stars used in distance measurements, but they have different luminosities, ages, and period–luminosity calibrations. Hubble's work used classical, or Population I, Cepheids.