When the earliest astronomers and philosophers looked up at the night sky, they never could have imagined a sight like this.

What if I told you there are only four single stars in this image?

That’s right. Four.

The rest are whole galaxies, full of billions of stars.

You can tell the foreground stars from the galaxies by the diffraction spikes–astronomer speak for those four bright spikes of light. Can you find them?

Here they are.

This image is known as the Hubble Deep Field: the first image taken of the vast universe of galaxies. It was taken when astronomers aimed a telescope at what appeared to be an empty spot of the night sky, only one-thirtieth the diameter of the full moon.

Not so empty after all, huh?

We spent the last couple of months exploring our own home galaxy in depth–and honestly, it’s hard to deny the Milky Way alone is a spectacular object! But there’s so much more…and now we’re ready to understand it.

Those four foreground stars are the only objects in this image that are part of our own galaxy. The galaxies themselves are incredibly far away–the nearest one, the Andromeda Galaxy, is 765 kiloparsecs (2.5 million light-years) from Earth.

You guys know what that means?

When you peer out at the Andromeda Galaxy (Messier 31) through a telescope…

…the light you see left its source 2.5 million years ago. You’re looking a couple million years back in time.

It’s extraordinary to think that light doesn’t travel instantaneously. It sure seems to, when you aim a flashlight at the ground! But over insanely long distances, the set velocity it travels at becomes obvious. That’s how far away other galaxies are.

In fact, they’re so far away that only the Andromeda Galaxy is visible to the unaided eye. And that made it pretty difficult for astronomers working as recently as the early 1900s to discover them, let alone figure out what they were.

Our story begins 200 years after the “discovery” of the Milky Way–in 1845, with William Parsons, third Earl of Rosse in Ireland.

Honestly, it’s no surprise to me that an Earl is credited as the first to observe galaxies. Plenty of less powerful people, like Hypatia, Cecilia Payne-Gaposchkin, and Clyde Tombaugh, have been credited (often posthumously) with important discoveries. But to build the world’s largest telescope, you need money…and space to put it.

Well…anyway. Back to my point.

By 1845, the Milky Way had been determined to be a disk-shaped star system, and sibling pair William and Caroline Herschel (what’s with all the Williams?) had begun to determine its size. It would be more than 50 years before Harlow Shapley would realize it was considerably bigger than previously thought.

Also, no one had begun to call it a galaxy. And as far as anyone at the time knew, the “star system”–as they called the Milky Way–was alone in the universe.

But Parsons’ 72-inch telescope was the first to be big enough to challenge that.

Here is one of Parsons’ drawings of his observations–of faint, blurry objects that he called spiral nebulae.

Today’s astronomers can recognize the obvious shape of a spiral galaxy in this early drawing. But in Parsons’ day, no one was sure what the heck this object could possibly be.

In the 1920s, Shapley’s work with globular clusters revealed that the Milky Way was quite a bit bigger than previously thought. And around the same time, German philosopher Immanuel Kant suggested something even more incredible: that perhaps, the Milky Way was not alone. Perhaps it was just one of a great many wheels of stars, called island universes.

Parsons welcomed the idea–perhaps these “island universes” could explain his “spiral nebulae.”

The stage was set for the greatest expansion yet of humanity’s view of the universe.

Enter Heber Curtis, astronomer of Lick Observatory, pictured on the left. On the right is Harlow Shapley himself, who did not accept the idea of “island universes.”

Shapley’s view was based on his own measurements of the Milky Way’s size. Based on inaccurate observations of variable stars in globular clusters, he argued that the Milky Way was much larger than previously thought, and didn’t it make more sense that the “spiral nebulae” were simply swirls of gas and faint stars within our own star system?

Heber Curtis took Kant and Parsons’ position: the spiral nebulae were island universes, wheels of stars separate from our own.

Curtis and Shapley presented their arguments at the National Academy of Sciences on April 26, 1920, in what astronomers now refer to as the Great Debate.

Problem was, at the time, there just wasn’t enough evidence to rule anything out.

That is…until Edwin Hubble came along.

You might recognize him as the guy the Hubble Space Telescope was named for. He’s also the guy who came up with the Hubble Law and figured out the universe is expanding…something we’ll get into very soon.

In 1924, however, Hubble was able to settle the Great Debate–with observations of Cepheid variable stars in the Andromeda Galaxy.

It was during Hubble’s time that the term galaxy was used to describe the “spiral nebulae.”

I spent entirely too much time trying to figure out who the heck first started calling the spiral nebulae “galaxies.” Turns out, “galaxy” isn’t a new word at all: its roots are in the Middle English galaxias or galaxie, which were ultimately borrowed from the Greek gála, meaning “milk.”

Makes sense to me–when we started using “galaxy” to describe vast star systems, we had long since been calling our own star system the “Milky Way.”

Anyway…on to my favorite part of this story.

Meet Palomar Observatory’s Hale Telescope, one of the most prominent fixtures of my childhood.

I can’t tell you how many hours I’ve spent wandering the observatory grounds and touring the areas where visitors are allowed. It’s filled with plaques detailing its history as the largest telescope of its time. But I never truly appreciated the novelty of its discoveries…until now.

In 1845, Parsons’ 72-inch telescope had been the largest telescope in the world. But the Hale Telescope–with a whopping 100-inch mirror–was able to resolve Cepheid variables 2.5 million light-years away. No one had ever peered so far into space before, at least not with such a clear view.

Edwin Hubble observed Cepheids within the Andromeda Galaxy with month-long pulsation periods, indicating that they were supergiant stars.

What’s a supergiant again?

Suffice to say…they’re absolutely enormous.

Supergiants are stars that have lost stability and expanded to around 100 to 1000 times the radius of our own sun–though some have been discovered that challenge our understanding of just how big these evolving stars can get.

As I’ve described before, a star’s total luminosity–its output of all wavelengths of electromagnetic light–depends on its size. The bigger the star, the brighter it appears.

But there’s something else that has an effect on the observed luminosity: how far the star is from Earth.

Think about it–if you see two cars with headlights that are exactly the same brightness, but one car is some distance down the street from you and the other is right next to you in your driveway, which one’s going to look brighter?

If the “Andromeda Nebula”–as it was called at the time–was actually a “spiral nebula” within the Milky Way, then Cepheids with month-long pulsation periods should have appeared very bright. After all, Cepheids’ pulsations work like bells. It takes a very large, very bright star to produce such a long-period, low-frequency gong.

But the Cepheids had apparent magnitudes of about 18 (on a scale where 0 is bright). That’s ridiculously faint for a supergiant star.

Clearly, these supergiants are very far away–meaning that “spiral nebulae” are not nebulae at all. They are separate galaxies, equaling and even rivaling the Milky Way in size. The “Andromeda Nebula” was redefined as the Andromeda Galaxy.

The discovery of galaxies was a critical expansion of humanity’s understanding of the universe. Identifying what galaxies were was the first step to exploring their histories–and what they can tell us about the evolution of the universe itself.

Soon, astronomers would begin to categorize galaxies’ shapes and realize that they were far more varied than just spirals. But we’ll explore that next up!