Amazing is an understatement when it comes to how much we've learned about our Universe in the last century. 90 years ago we confirmed general relativity, and got a whole new theory of gravity as a result. But, we still thought our Milky Way was the only galaxy in the Universe.

There were a few types of weird objects we knew of in great detail, such as the original 103 Messier objects. There were weird, spherical clusters of stars, like M2:

There were planetary nebulae, known to be the afterglows of dying or exploding stars, such as M27:

And then there were the much-debated "spiral nebulae," such as M31:

What were these Spiral Nebulae? Were they also dead stars, or clusters of some type, or (as was thought at the time) forming solar systems, or were they something different?

There was a Great Debate over the nature of these spiral nebulae, with no consensus reached. On one side, they were thought to be objects within our own galaxy, but the other side considered them island Universes, or objects outside of the Milky Way. In 1929, astronomer (and former boxing champion) Edwin Hubble set out to find the answer.

He watched this object for Novae, which had been seen in this object before. He figured that by keeping track of them, he could learn something about it. He saw one, and then two, and then three. And then, the unexpected happened. He saw a fourth one in the exact same location as the first! Why is this so unexpected? Because novae take thousands and thousands of years to recharge! That meant that this couldn't be a nova, it had to be a variable star! I've managed to track down a copy of Hubble's original data, where he originally marked it as a nova (N), and then crossed it out, with much excitement!

This was incredibly exciting, because this type of star (a Cepheid variable) was well understood, and allowed Hubble to determine that this object was well outside the Milky Way -- over 2 million (not billion, thanks DaleP) light-years away -- and was a galaxy all unto itself.

But Hubble didn't stop there. He went and surveyed a bunch of these spiral nebulae, this time looking for variable stars instead of novae. By observing the period and brightness of these variable stars, he could deduce their distance. He found that the galaxies that were farther away were also moving away from us at a faster rate! He graphed this, and reached an astonishing (and correct) conclusion:

The farther away something is, the faster it moves away from us. Or, in other words, the Universe is expanding! Those of you who are sticklers for detail will note that Hubble's data only goes out to a whopping 2 Megaparsecs, and that Hubble got his units wrong on his graph (velocity should be in km/s, not km). By comparison, here's what a modern graph of velocity vs. distance looks like, out to hundreds of Megaparsecs:

It's only been the last 80 years that we've known the Universe is expanding and that there are other galaxies than our own. Aren't you impressed at how far we've come?