On New Year’s Day, Thursday, January 1, 1925, at a scientific conference in Washington D.C., a paper written by Edwin Hubble, a 35-year-old staff astronomer at Mount Wilson Observatory in Los Angeles County, was read to conference attendees. It was titled, “Cepheids in Spiral Nebulae.” The paper was read by noted Princeton astronomer Henry Norris Russell, because Hubble himself was apparently not confident enough to appear in person for the conference. To anyone not familiar with astronomy, it probably sounded like a lot of boring scientific jargon. To astronomers, however, the paper was earth-shattering. Hubble’s findings at Mount Wilson had settled a centuries-old debate about the form and size of our universe. How big was our universe? Was the Milky Way Galaxy all there was?
Until that time, astronomers could see many spiral “clouds” among the stars of our galaxy. Some of these, such as the Andromeda spiral, were even visible without a telescope. Some speculated that these were cosmic gas clouds in our galaxy or, perhaps, even adrift star groups. Many found it difficult to believe that anything actually lay outside the Milky Way. Hubble, however, subscribed to the theory that other galaxies existed beyond our own. Unlike other prominent astronomers promoting this theory (such as Vesto M. Slipher of Lowell Observatory), Hubble had, at his disposal, the largest telescope in the world at the time, the 100-inch Hooker telescope at Mount Wilson Observatory. With this tool, he was able to precisely view and measure details of Andromeda - the seemingly closest of the mysterious spirals. Among the details he discovered was a variable star known as a Cepheid, a type of star that consistently brightens and dims. The light from this star allowed Hubble to measure, over time, its distance from earth. His estimate came to 930,000 light years, a mind-blowing distance prior to that time (and revised considerably upwards since). As Hubble presented it, the Andromeda system (and billions of other spiral star systems) was far beyond the bounds of our own Milky Way and itself was a distant galaxy.
To say that Hubble’s findings dramatically expanded astronomy’s view of our universe could be said to an understatement. As Marcia Bartusiak illustrated the significance of this discovery in The Day We Found the Universe, “In more familiar terms, it's as if we had been confined to one square yard of Earth's surface, only to suddenly realize that there were now vast oceans and continents, cities and villages, mountains and deserts, previously unexplored and unanticipated beyond that single plug of sod.”
Astronomy came to accept the Milky Way as just one of an innumerable number of galaxies. Hubble’s work further led to the discovery of the expanding universe.
Also see: January 1, 1925: The Day We Discovered the Universe by Corey S. Powell
The Day We Found the Universe by Marcia Bartusiak.