On October 5, 1923, Edwin Hubble identified the first Cepheid variable in M31, the Andromeda Galaxy. This proved that the Andromeda nebula was not part of the Milky Way and paved the way for measuring the expansion of the Universe.
Cepheid variables are luminous yellow stars which pulsate in brightness with periods of several days. The first was discovered by Edward Pigott on September 10, 1784 and by John Goodricke a few months later. In 1908, Henrietta Swan Leavitt discovered a relationship between their periods and luminosity after observing thousands of variable stars in the Magellanic Clouds – which are nearby irregular galaxies. Knowing the luminosity from the period and comparing with the apparent brightness enables the distances of Cepheids to be measured at very great distances. For example, in 1918, Harlow Shapley, who later became acting director of the Armagh Observatory, used Cepheids to estimate the size and shape of the Milky Way.
In 1920, a great debate emerged concerning the nature of the Milky Way and the spiral nebulae. At that time, Shapley argued that the ‘spiral nebulae’, like M31 in Andromeda and the Whirpool Nebula, were clusters of stars in the Milky Way, whilst Heber Curtis argued they were distant ‘island universes’. Meanwhile, the Estonian astronomer Ernst Öpik, who would also later become an acitng director of the Armagh Observatory, found a method to estimate the distance to the Andromeda Nebula, placing it at 1,500,000 light years and well outside the Milky Way.
Edwin Hubble’s discovery and subsequent observations of Cepheid variables in M31 with the recently built 100-inch Hooker telescope on Mount Wilson, California, firmly established that the Andromeda Galaxy is a distant star system, similar to the Milky Way but separated from it by 2.5 million light years. This was the first step in building up a sequence of distances to other galaxies. Commencing measurements of the Doppler shift in the spectra of nebulae in 1912, Vesta Slipher had by 1917 established that the spiral nebulae were (mostly) moving away from us at great speeds. Combining Slipher’s measurements and additional ‘red-shift’ measurements with distance measurements from Cepheids, Hubble and Humason established a law of proportionality between galaxies’ distances and redshifts – now known as the Hubble-Lemaitre law – firmly demonstrating that we live in an expanding Universe.