Astronomers were befuddled at the beginning of the 20th century. The nighttime sky was littered with at least 110 nebulous objects first cataloged by French astronomer Charles Messier in the late 1700s. Most were identified as star clusters, nebulae, supernova remnants, and vast clumps of glowing gases.

But 40 of the objects were mysterious whirlpools on the sky collectively called the realm of the spiral nebulae. Unlike the other Messier objects, they were all scattered across the sky. To compound the confusion astronomer Vesto Slipher used spectroscopy to discover that the light from all of the spiral nebulae was redshifted, leading to the puzzling interpretation they all were moving away from us.

Edwin Hubble thought that the spiral nebulae were extragalactic because they were not constricted to the plane of our galaxy. But he needed observational evidence to nail down actual distances. He went hunting for stars embedded inside the spiral nebulae. In particular he searched for a unique class called Cepheid variables because they pulsate at a rhythm that correlates with their intrinsic brightness. And, that can be used to calculate astronomical distances. When Hubble measured the distance to a Cepheid in Andromeda, called V1, it settled debate over whether those fuzzy spirals were just cosmic clouds, or whole other galaxies.

He discovered that the universe was much larger than some imagined by determining Andromeda was over 2 million light-years away, or 20 times our Milky Way's diameter. This was accomplished 100 years ago. It marked an intellectual phase transition in human knowledge by unveiling a mind-numbing scale of a universe full of external galaxies. They were all apparently rushing away from us in all directions, implying the universe has a finite age and was not eternal.

This opened a window to beholding a vast frontier that would be pursued over the next 100 years with the most powerful scientific instruments ever conceived. The capstone of this has been the Hubble Space Telescope that precisely determined the age of the universe to 13.8 billion years —based in part on far-flung Cepheid star measurements.