In the early 1900s, astronomer Henrietta Leavitt discovered an odd but powerful fact about one class of variable stars, which are known as Cepheid variables after their prototype star, Delta Cephei.
She was looking at the “pulses” of several Cepheids in the Large Magellanic Cloud, a small companion galaxy to the Milky Way that is about 160,000 light-years away. Because the stars were all in another galaxy, they were roughly the same distance from Earth, which allowed Leavitt to discover an important fact about the stars: There is a relationship between a Cepheid’s brightness and the length of its pulse. Stars with longer pulses are brighter than those with shorter pulses.
Leavitt’s discovery, known as the period-luminosity relationship, had a profound implication: If astronomers could accurately measure the distances to a few of these stars, which would provide their true brightness, then they could measure the distances to all Cepheids just by measuring the length of their pulses.
In the 1920s, Edwin Hubble used this relationship to settle a major astronomical disagreement. Some astronomers thought that the bright motes of matter that were shaped like spirals or fuzzy footballs were inside the Milky Way galaxy. Others, including Hubble, thought they were separate galaxies of stars far beyond the Milky Way.
Using the 100-inch telescope at Mount Wilson, California, Hubble studied several Cepheids in M31, which was then known as the Andromeda Nebula. His measurements showed that the Cepheids, and therefore the surrounding “nebula,” was about 850,000 light-years away from Earth, placing it far outside the confines of the Milky Way. This single discovery confirmed that the universe contains many individual galaxies, each of which contains millions or billions of stars.
Additional measurements have since pushed M31 even farther away, to about 2.5 million light-years. And instead of the Andromeda Nebula, it is known today as the Andromeda Galaxy.