Galaxies, part 1: Crash Course Astronomy #38

We live in a pretty cool neighborhood: the Milky Way galaxy. We’re out in the suburbs,

sure, but it’s still an interesting place, buzzing with activity. Stars, nebulae, stellar

clusters of various sorts, the occasional supernova. It’s a happening place.

In the earliest part of the 20th century, astronomers were just starting to figure this

all out. But there were a handful of objects that were puzzling: Dotting the sky here and

there were faint fuzzies displaying a variety of shapes. Some were round, some elongated,

and some even seemed to have spiral arms. Even with big telescopes they looked smoky,

so they were simply called “nebulae.”

Their existence was puzzling, though. What were they? How did they form? Were they big,

small, near, far? Eventually, astronomers had uncovered the key to these objects, and

in one fell swoop our Universe got a lot bigger.

A LOT.

In 1920, there were two competing ideas about the Universe. One was that our Milky Way was

IT, and that everything we saw was in it. The other was that the “spiral nebulae”

seen in the sky were also like our Milky Way; “island Universes” in their own right.

Two astronomers debated this controversy in that year. Harlow Shapley argued that the

Milky Way was all there is, while Heber Curtis was of the opinion that we were one of many

galaxies. It wasn’t a debate as such; more of a presentation of ideas. And there was

no clear winner; both sides had fragmentary data and – we now know – some shaky observational

evidence that turned out not to be correct.

For example, Shapley noted that one of the spiral nebulae had been seen to rotate, so

it must be small. It turns out that was just wrong, dead wrong. On the other hand, Curtis

noted that if galaxies were as big as Shapley claimed – hundreds of thousands of light

years across – then other galaxies must be impossibly far away. But, HELLO, galaxies

really are that big, and they truly are mind-numbingly distant.

The observation that finally unlocked this mystery was made just a few years later when

Edwin Hubble and Milton Humason observed the great spiral nebula M31 in Andromeda, using

what was at the time the largest telescope in the world. They found dozens of pulsating

stars in it, literally stars that changed their brightness in a regular, periodic fashion.

These are called Cepheid variables, and they were critically important, because it was

known that the time it took them to pulse was directly related to their luminosity,