Last week, we discussed how, in 1755, Emmanuel Kant predicted the existence of "island universes" — what we now call galaxies — located at vast distances from our home galaxy, the Milky Way. Continuing the story: The Third Earl of Rosse William Parsons combined his love of theory (he graduated from Oxford with first-class honors in math) with a thoroughly practical bent, designing what was in the 1840s the largest telescope in the world, the Birr Leviathan. It was 52-feet long, with a 4-ton, 6-foot diameter, polished mirror made of speculum, a copper-tin alloy. Until April 26, 1848, no one had seen details in any of the fuzzy "nebula" which share the night sky with stars and planets. That night, Parsons saw enough to sketch what we now call the Whirlpool Galaxy. Over the next 40 years, he and his son went on to locate nearly 50 spiral galaxies.

Unable to figure out how far away these fuzzy spirals were, astronomers of the time were divided on their nature. If our galaxy, the Milky Way, is the cosmos, then they were simply oddities within it. Or, if they could be shown to be much farther away than Milky Way stars, it would confirm Kant's guess about there being a multitude of "island universes," our Milky Way being but one of them. Enter Cepheid variables.

In 1784, English astronomer John Goodricke noticed that a star in the constellation Cepheus "pulsed" — it brightened and dimmed regularly over a five-day period. Many other "Cepheid variables" were subsequently found. In 1908, Harvard researcher Henrietta Swan Leavitt made a discovery that transformed astronomy. She found that the period of a Cepheid variable star is a function of its intrinsic brightness. This is like reading the wattage printed on a light bulb from afar.

Over the next decade, she refined her measurements of nearly 2,000 Cepheid variables in the Magellenic Clouds (two small companion galaxies to our own). Meanwhile, astronomers had figured out the distance to Cepheid variables within the Milky Way by their parallax as Earth orbits the sun. Now the distance to any Cepheid variable could be calculated from its observed period.

What was needed was a sufficiently large telescope to photographically capture Cepheids in what were hypothesized to be distant spiral nebulae, or galaxies. In 1917, astronomers began making use of a powerful new instrument, Mount Wilson's 100-inch Hooker telescope. By 1925, Edwin Hubble had made sufficient observations of Cepheid variables in the Andromeda galaxy to confirm — using the Leavitt's "law" — that it's at least 20 times farther away than any stars in our own Milky Way. Other galaxies soon followed. Thus, Hubble and others showed that most "nebulae" are actually independent galaxies located far beyond our own. He would later say that Leavitt, who died of cancer in 1921, deserved the Nobel Prize for discovering the Cepheid period–luminosity relationship. Her work helped to relegate our sun from the center of the Milky Way to a distant spiral "arm" and to downgrade the Milky Way itself from the center of the cosmos.

We now know we live in just one of Kant's "island universe" galaxies that number, oh, about 200 billion.

Barry Evans (he/him, barryevans9@yahoo.com) likes to think that Henrietta Leavitt would have received a Nobel Prize had she lived another five years.