D-30: The Mirror

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Today we’re going to talk about a small galaxy called Messier 33, also known as M33, Triangulum or the Pinwheel (although this nickname is also used for M101.) Messier 33 is like our own Milky Way: a spiral galaxy full of young, hot stars. It’s smaller than us, but is fairly nearby and has been known for centuries.


(Image courtesy ESA.)

When Messier first observed it in 1764 he made a common mistake: he assumed that it was a lot smaller and a lot closer than it is. This is an easy mistake to make in astronomy, because there’s a lack of other things nearby to give perspective. At the time, people didn’t know the difference between our galaxy and our universe. They noticed that we were surrounded by stars, and thought that was all there was. With most optical telescopes, that may as well be true. Distant galaxies look like nearby nebulae, or even like pulsar stars. In this case, he catalogued Messier 33 as being a nebula: a cloud of gas and stars. In a sense he was right, since that’s all that galaxies are, but he thought it was small and inside our own galaxy. For hundreds of years nobody had the telescopes or the maths to think otherwise.

By the 1920s, we had much bigger telescopes. Edwin Hubble (not the telescope but the Englishman after whom the telescope was named) studied various objects through it, amongst them Messier 33. One of the things he noticed was that some of its stars are what we call Cepheid Variables. There’s a trick you can do with Cepheid Variables that tells you how far away they are.

“Cool”, said Hubble, “I can use this method to tell how far away Messier 33 is.” Then he redid his maths because he was sure he’d made a mistake, and after that he sent telegrams to every other physicist and astronomer he knew, because this was weird. Messier 33 was at least 700,000 parsecs away (over 2 million light years if you’re a light years person.) That meant that because it was far away, it had to be enormous: almost 20,000 parsecs across.

“This can’t be right”, said Hubble. “The universe is only 34,000 parsecs across. Everyone knows that.”

Everyone did indeed know that, but you can’t argue with Cepheid Variable stars, and other astronomers confirmed it: Messier 33 was further away than anything else yet seen, and was almost two-thirds as wide as the universe itself.

A few years later they had the answer: the universe is not full of stars. It’s mostly empty. The stars cluster together in galaxies which are separated by enormous stretches of nothingness, like candles in a dark hall. The phrase “galaxy” became used for them, but for  while a more evocative name was used: “island universe.”

In America, a young man called H P Lovecraft read about these discoveries. He said later that his sudden realisation of how pitifully small he and his world were, and how terrifying it was to have his comfortable ignorance stripped away, were something that inspired him to write cosmic horror.

Meanwhile back in Europe, Hubble was thinking hard. If Messier 33 was like that… were we like that too?

It’s really, really hard to study a galaxy from inside it, because the galaxy gets in its own way. It’s only when we can see it from outside that we have a good perspective. As a result, the study of Messier 33 (and other galaxies like its neighbour, the enormous Andromeda) taught us what to look for in our own Milky Way.

Once we knew that galaxies might have centres and spiral arms, for example, we could start to work out that our own galaxy had them too. We would never have known this if we didn’t have a mirror like Messier 33 there, showing us what we look like.

This has its pitfalls too. Messier 33 isn’t like us in some ways. It’s smaller, for one thing. As we learn about the universe we’ve learned just how enormous and unlikely our own Milky Way is.

For another thing, Messier 33 has a round centre. For a long time, we thought that the Milky Way had a round centre too. It’s only recently that we realised we might have a bar centre, more like NGC1300.


(Image courtesy NASA.)

It’s good to have a mirror: it lets you know what you look like. However, the most subtle trap is that a mirror encourages narcissism. This was intended to be an essay about the spiral galaxy Messier 33. Instead, it became about Charles Messier, Edwin Hubble, H P Lovecraft, and human history. We looked in the mirror and saw only ourselves.


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