On October 5, 1923, Edwin Hubble recognized the primary Cepheid variable in M31, the Andromeda Galaxy. This proved that the Andromeda nebula was not a part of the Milky Approach and paved the way in which for measuring the enlargement of the Universe.
Cepheid variables are luminous yellow stars which pulsate in brightness with intervals of a number of days. The primary was found by Edward Pigott on September 10, 1784 and by John Goodricke just a few months later. In 1908, Henrietta Swan Leavitt found a relationship between their intervals and luminosity after observing 1000’s of variable stars within the Magellanic Clouds – that are close by irregular galaxies. Understanding the luminosity from the interval and evaluating with the obvious brightness permits the distances of Cepheids to be measured at very nice distances. For instance, in 1918, Harlow Shapley, who later grew to become performing director of the Armagh Observatory, used Cepheids to estimate the dimensions and form of the Milky Approach.
In 1920, an incredible debate emerged regarding the nature of the Milky Approach and the spiral nebulae. At the moment, Shapley argued that the ‘spiral nebulae’, like M31 in Andromeda and the Whirpool Nebula, have been clusters of stars within the Milky Approach, while Heber Curtis argued they have been distant ‘island universes’. In the meantime, the Estonian astronomer Ernst Öpik, who would additionally later change into an acitng director of the Armagh Observatory, discovered a technique to estimate the space to the Andromeda Nebula, putting it at 1,500,000 mild years and properly exterior the Milky Approach.
Central area of M31, the Andromeda Galaxy, photographed with the Armagh Robotic Telescope, March 2012. (Credit score: Jack Wright)
Edwin Hubble’s discovery and subsequent observations of Cepheid variables in M31 with the lately constructed 100-inch Hooker telescope on Mount Wilson, California, firmly established that the Andromeda Galaxy is a distant star system, much like the Milky Approach however separated from it by 2.5 million mild years. This was step one in build up a sequence of distances to different galaxies. Commencing measurements of the Doppler shift within the spectra of nebulae in 1912, Vesta Slipher had by 1917 established that the spiral nebulae have been (principally) shifting away from us at nice speeds. Combining Slipher’s measurements and extra ‘red-shift’ measurements with distance measurements from Cepheids, Hubble and Humason established a legislation of proportionality between galaxies’ distances and redshifts – now generally known as the Hubble-Lemaitre legislation – firmly demonstrating that we dwell in an increasing Universe.
Simon Jeffery
