We have seen with astonishment shapes in the heavens that are nothing other than systems of such fixed stars limited to a common plane, such milky ways, if I may express myself in this way, that exhibit elliptical shapes in different positions in relation to the eye with a weakened shimmering as is appropriate to their infinite distance; they are systems of, so to speak, infinity times infinity greater diameter than that of our solar system,h but that, without doubt, are generated in the same way, ordered and arranged by the same causes, and that maintain themselves by the same mechanismi as this one in its constitution.—Immanuel Kant
Immanuel Kant wasn’t the first to posit that the “spiral nebulae” observed by many astronomers, such as Charles Messier, were actually galaxies like to the Milky Way. Theologians, such as Emanuel Swedenborg, had proposed that a limitless God wouldn’t limit creation to a single galaxy. The British astronomers William and Caroline Herschel had discussed the possibility, and their hypothetical proposals influenced Kant. Kant was the most prestigious philosopher of his time, and his endorsement gave the idea heft.
Copernicus and Kepler had shifted the view of the universe from geocentrism to heliocentrism, but up until Kant’s argument it was accepted that the universe was little larger than the Milky Way. Kant created room for the possibility that nature contained many galaxies equivalent to the Milky Way.
165 years later, in the early 1920s, the question was still being debated. Those who believed the Milky Way way the only galaxy argued that spin had been observed in the spiral nebulae. That spin would be impossible to observe if they were as vast as the Milky Way since the outer stars would have to be moving faster than the speed of light. They also argued that novae that had been observed in them were so bright that they outshone the rest of the nebulae. That would put the energy output from the novae so high as to be incomprehensible. Another problem with the novae was that more had been observed in the spiral nebulae than in the Milky Way, implying that the Milky Way had a different composition than the distant nebulae.
On the other side were those who took Kant’s position. Their most powerful argument was that of homogeneity–that the Milky Way wasn’t special, and that the rest of the universe would be very much like the part of it we can observe. They also argued that dark regions in the spiral nebulae strongly resembled regions of darkness in the Milky Way caused by vast expanses of dust hundreds of light years across.
The Milky Way is immense, estimated at 100,000 light years across, but visually it doesn’t fill the night sky. In fact, the band of stars, clusters, and dust doesn’t even cross the sky completely. Despite it’s enormity, is visible. It’s imaginable. It can be held in the mind. If the entire universe consisted mainly of the Milky Way and a few thousand nebulae caught in it’s orbit, then it’s still small enough and simple enough to be grasped in the human mind. On the other hand, if there are thousands or millions of island universes like the Milky Way, then the universe is so large it’s beyond comprehension.
The question was answered in 1924 by Edwin Hubble, through the use of a giant telescope, and a spectrometer. But before discussing the answer to the question, we need to discuss how it was answered–with a little thing known as a universal candle.