• Antonis

VAR! – Or how Edwin Hubble opened our eyes to the universe

Many times in the history of science, the most covert secrets of the universe seem to have revealed themselves in seemingly random situations. According to the famous American philosopher Thomas Kuhn, the development of scientific discoveries is not linear. Instead, science is punctuated by revolutions that signal major shifts in our understanding of our cosmos.

So when the famous astronomer Edwin P. Hubble was developing a photographic plate after his observation using the Hooker 100-inch telescope in Mt. Wilson back in 1923, little was he aware of what the cosmos had in store for him.

Hubble in ca. 1922 operating the 100-inch Hooker telescope at Mt. Wilson. [Edwin Hubble Papers/Courtesy of Huntington Library, San Marino, Calif.]

Hubble was studying our nearby Andromeda galaxy and had identified many novae, events that cause the sudden magnification of the brightness of a star, which then goes on to slowly fade back again after a few weeks or months. By photographing the same patch of sky during different nights and comparing the plates, Hubble was able to identify these apparently “new” stars.

However, it turned out that one of these Novae was actually a Cepheid variable, a type of star whose luminosity changes periodically with time. What is even more important is that the luminosity of the Cepheid star and its pulsating period are directly linked. So, knowing the luminosity and measuring the apparent brightness of the star, Hubble was able to calculate the distance to the Cepheid and thus to the Andromeda galaxy to 900,000 light-years. Although this calculation is significantly different from the currently established value of 2.54 million years, it nevertheless proved that the Andromeda galaxy lies way outside our Milky Way.

The famous Andromeda Galaxy (M31) VAR! plate. Hubble had first identified the star on the top right as a Nova (N), but he later realised that it is actually a variable Cepheid star and corrected it without hiding his excitement (VAR!) [Courtesy of the Carnegie Observatories]

Before this discovery, Hubble was convinced that these “island universes” resided way farther than our own Milky Way galaxy, having received heavy criticism for his ideas, mainly by the popular Harvard-based astronomer Harlow Shapley. Hubble was sure that by measuring the distance to any of these galaxies he could prove his theory. However, conventional methods such as parallax measurements were inefficient due to the tremendous distances that made the stars in those other galaxies appear static.

By proving that our Milky Way is just one of the many galaxies in the universe, Hubble moved the centre of our universe once again, almost 450 years after Copernicus had moved our Earth away from the center of our solar system.