Late in the evening of October 5, 1923, Edwin Hubble sat at the eyepiece of the Hooker telescope at the Mount Wilson Observatory, atop the mountains overlooking the Los Angeles basin. He was observing an object in the northern sky. To the unaided eye, it was visible as a faint smudge. But through a telescope it sharpened into a brilliant ellipse called the Andromeda Nebula. To settle a debate about the size of the Milky Way — which was then thought to be the entire universe — Hubble needed to determine Andromeda’s distance from us.
In the telescope’s field of view, Andromeda was a giant. Hubble patiently captured several exposures covering many glass photographic plates, and in the early hours of October 6, he made a 45-minute exposure on a small glass plate and scrawled “N” where he saw three new stars, or novas. But when he compared his image with photographs captured by other astronomers, he realized that one of his new novas was actually a Cepheid variable star — a type of star that can be used to measure astronomical distances.
He scratched out one “N” and wrote “VAR!”
Hubble used this pulsating star to calculate that Andromeda was 1 million light-years from Earth, a distance much greater than the diameter of the Milky Way (he was slightly off; Andromeda is about 2.5 million light-years away). And he realized that Andromeda was no mere nebula but an entire “island universe” — a galaxy distinct from our own.
With the cleaving of the cosmos into a home galaxy and a larger universe, the study of our finite home — and how it exists within that universe — could begin in earnest. Now, a century later, astronomers are still making unexpected discoveries about the only cosmic island we’ll ever inhabit. They may be able to explain some of the Milky Way’s characteristics by reimagining how it formed and grew in the early universe, by scrutinizing its uneven shape, and by studying its ability to form planets. The latest results, amassed over the past four years, are now painting a picture of our home as a unique place, at a unique time. [Continue reading…]