Billions and billions of galaxies?
Our picture of the universe has changed dramatically during human history. 500 years ago we knew that the Earth was at the center of the universe, the sun went around the earth, and the stars were points of light set in a crystalline sphere. 100 years ago we knew all the stars in the universe were in a disk we call the Milky Way galaxy.
Today we know that our galaxy is just one of many billions. To help understand this latest change in our world picture we can look at the museum’s Photographic History Collection. This collection includes many astronomy related photographs, most of them taken more than 100 years ago. Because our Milky Way galaxy was thought to be unique at that time, all the photographs in the collection of faint deep sky objects are identified as star clusters or nebulae (faint gas clouds lit by embedded stars).
Around 1880, the arrival of photographic dry plates made a medium available to astronomers that could be exposed for several hours. The result was that faint objects, including stars in the Milky Way, star clusters, and nebula could be photographed for the first time. Our collections include many photographs from this early period of deep sky photography.
Left: M81 nebula. Right: M101 nebula.
The numbers ‘M81’ and ‘M101’ come from a catalogue of faint deep sky objects made by the astronomer, Charles Messier, in the late 18th century. When these two images were taken (before 1910) most astronomers thought they were patches of gas illuminated by stars in our galaxy.
However, in April 1920, a debate took place at the Smithsonian’s National Museum of Natural History between the astronomers Heber Curtiss and Harlow Shapley that challenged accepted views. Shapley argued for the accepted view that the spiral nebulae shown in the photographs above were gas clouds inside the Milky Way; Curtiss claimed they were distant island universes resembling the Milky Way.
The issue was settled in the 1925 by Edwin Hubble discoveries with the 100 inch Mount Wilson and 200 inch Mount Palomar telescopes. Hubble showed that many nebulae, especially those with the distinctive spiral structure shown in the above images, were indeed distant objects with a similar structure to our own Milky Way galaxy. In making these discoveries he probably changed our understanding of the Universe more than any other person in history.
In the 1980s the astronomer and writer Carl Sagan became closely associated with the phrase “billions and billions.” He often used the term to refer to the number of galaxies in the observable universe. Thirty years later we have become familiar with space photographs like the one below that show multitudes of galaxies everywhere in the sky.
Abell 1689 galaxy cluster. Credit: X-ray: NASA/CXC/MIT/E.-H Peng et al; Optical: NASA/STScI.
Today we live in a universe of “billions and billions of galaxies” instead of just one, and when the the two images (above) in the Photographic History Collection are viewed, they are now recognized as spiral galaxies. In just one century our understanding of the universe has expanded more than any previous time in human history.
Anthony Brooks is a volunteer in the Photographic History Collection at the National Museum of American History.