At the dawn of the 20th century, the Universe was synonymous with the Milky Way Galaxy. The collection of billions upon billions of stars that make up our spiral galaxy was thought to be all there was in the Universe. Now we know that the Universe is much, much bigger than that, and contains billions upon billions of independent galaxies, each one containing as many or more stars as our own galaxy. The story of this realization involved work by many astronomers, one of which is Edwin Hubble.

After studying law and realizing his passions were in astronomy, Edwin Hubble completes his PhD thesis in 1917 on the classification of “spiral nebulae”. This was the term given to what we know of today as other galaxies, but at the time were considered curiously shaped clouds of gas in the Milky Way galaxy.

While early astronomers, such as Immanuel Kant in the 18th century, contemplated the idea of other galaxies, a crucial scientific observation would be the distance of these “spiral nebulae”. Distance is a notoriously difficult measurement in astronomy since the scale of our Universe is so large. Key to this puzzle is the use of stars that vary in brightness in a specific way, known as Cepheids, which can be used to calculate their distance. This relationship was identified by Henrietta Swan Leavitt in 1908.

Making distance measurements to these “spiral nebulae” became a scientific endeavor for many astronomers between 1910-1925. Many astronomers, including Heber Curtis, Harlow Shapley, Knut Lundmark, and Vesto Slipher were using different techniques to show that these nebulae were too far away to be within our own Milky Way galaxy. In 1925, Edwin Hubble published two papers that used Cepheids in the Andromeda galaxy to confirm it is extragalactic.

At the suggestion of George E. Hale, whose father had endowed a lecture series at the National Academy of Sciences, a lecture would be devoted to “The Distance Scale of the Universe”, with Heber Curtis (age 47) and Harlow Shapley (age 34) presenting opposing viewpoints. This event, which occurred on April 26, 1920 in Washington DC, would later be known as ”The Great Debate“. While resolution of the issue would occur after further observations, this event garnered much attention and clearly identified a leading astronomical question of the time.

The measurement of the motions of nebulae and galaxies revealed the extreme speeds by which they are moving away from us due to the expansion of the Universe. These measurements are made by noting the shift in wavelength of light due to the Doppler Effect, only observable via a spectrum, where the light of a distant galaxy is subdivided into the contributions of different wavelengths (like a prism). Spectroscopic observations by many astronomers, who were not aware or looking for evidence of an expanding Universe (including Edwin Hubble) ultimately resulted in the discovery of a linear relationship between distance and the speed of recession away from us, now known as Hubble’s Law. Several guestbook signatories published important new observations on this work, including George Paddock in 1916, Willem de Sitter in 1917 and 1930, Harlow Shapley in 1919, Knut Lundmark in 1920, 1924, 1925, Edwin Hubble in 1929, 1931, and Jan Oort in 1931.

Edwin Hubble was honored with many awards for his astronomical research during his life and the distinction of being the namesake of arguably NASA’s most famous telescopes, the Hubble Space Telescope. This large, versatile, and productive telescope remains in operation today, after more than 25 years of continuous use. Wesleyan astronomers are frequent users of this facility. Prof. Seth Redfield is using spectroscopic observations of nearby nebulae to study the properties and motions of these clouds of interstellar gas as they drift amidst the Sun and its nearest neighbors.