Island Universes, Part 1 

"We thus observe, that with each successive increase of optical power, the structure has become more complicated and more unlike anything which we could picture to ourselves as the result of any form of dynamical law, of which we find a counterpart in our [Milky Way] system."

— William Parsons, Third Earl of Rosse, on the Whirlpool Galaxy

If historians ever assign a date to our species coming of age, cosmologically speaking, my vote would be April 26, 1848. On that night, Parsons wrote in his notebook: "6-feet instrument. Saw the spirality of the principal nucleus very plainly; saw also spiral arrangement in the smaller nucleus." He was referring to M51, the Whirlpool Galaxy, a favorite target of amateur astronomers. It lies just below the end of the Big Dipper's handle.

This was the first time that a so-called nebula — one of hundreds of fuzzy "clouds" that share the night sky with pinpoints of stars — was resolved into a spiral structure. As he later wrote, per the lead quote, this was "unlike anything ... in our system." Our system being the Milky Way, with the implication that what he saw (and drew) must be outside and separate from our home galaxy.

Astronomers of his age had barely come to terms with the fact that our sun was one of uncountable other stars that together make up the Milky Way, so the idea that our galaxy was but one of many was almost impossible to conceive. Almost. A couple of far-sighted thinkers had speculated on this idea nearly a century before Parsons' breakthrough observation of another "island universe."

That catchy phrase originated with German philosopher Immanuel Kant nearly 100 years earlier, long before he turned his attention to the weighty matters of rationalism and empiricism. The youthful Kant read, in a Hamburg journal, a review of the 1750 book An Original Theory, or New Hypothesis of the Universe by Thomas Wright, an English sailor and surveyor who'd turned his attention to the heavens. Wright was actually more interested in the religious overtones of his various models of the universe — where God's throne was located and where Hell is — than in serious cosmology. Serendipitously, the review distorted his ideas, so what Kant saw wasn't quite what Wright proposed, but was closer to what we now know to be true: The reason we see the Milky Way as a ghostly band across the night sky is that our galaxy is a flattened disc of stars, of which our sun is one of many. When we look into the plane of the disc, we see many stars, i.e. the Milky Way. When we look perpendicularly to the disc, we see comparatively few.

Kant reasoned that if such a body were seen from afar, it would appear either "as a patch of space whose figure will be circular if its plane is presented directly to the eye, and elliptical if it is seen from the side or obliquely." The latter, he theorized, was the case with the Andromeda nebula (our closest large neighboring galaxy, just visible with the naked eye), which was "nothing else than a mass of many fixed stars." It was faint because of its distance, he correctly surmised. That is, Andromeda is a distant replica of our own Milky Way, an island universe in its own right and perhaps one of many. The quotes are taken from Kant's book Universal Natural History and Theory of the Heavens, published in 1755.

Nearly 100 years later, William Parsons' sketch of the Whirlpool Galaxy would be the first direct confirmation of Kant's speculations, as we'll see next week.

Barry Evans (he/him, [email protected]) would have enjoyed the young (astronomical) Kant more than the old (philosophical) Kant.