D-19: Large, Part Two

The following was originally published on the Stellaris forums.

 

Hello once again everyone, and welcome to the second part of our series on large things. Last time we discussed things which are very large: galaxy clusters and dark matter filaments. Now let’s discuss something that’s even larger.

We live in the Laniakea supercluster. On every side this supercluster is surrounded by colossal voids, empty spaces as large as the supercluster itself in which there are almost no stars, let alone galaxies. Beyond one of them in the direction we arbitrarily call “north” is the Hercules supercluster, one of our neighbouring clusters. The Hercules supercluster sits close to several others in a web that’s referred to as the Great Wall. Beyond them…

…is nothing.

Welcome to the Boötes Void, also called simply the Great Void. This is a roughly spherical empty space which is so large as to defy description. There are no words for how large it is. There are numbers, but they are almost meaningless to humans. Its diameter is more than one-four-hundredth of the diameter of the universe. Is that meaningful to you? No. It’s not meaningful to me either, and I study this stuff. It’s colossal. The only suggestion of scale I can give is to say that entire web of superclusters called the Great Wall forms only part of one side of it, but even that doesn’t get it across.

Here’s a map. There are some nearby stars in the way but you can see the Great Void past them.

(Image courtesy Atlas of the Universe.)

The Boötes Void is not, intriguingly, entirely empty. In the 1980s we started to discover a few isolated galaxies in the middle of it. By the late 1990s we had concluded that there were 60 galaxies there, strung out in a rough line, possibly along a fragmentary dark matter filament. If someone lived on one of those galaxies, they would need mid 20th century astronomy tech to be able to tell that there was anything else in the universe except them.

In the 1920s, before the term “galaxy” became popularised, they were sometimes called “island universes.” That’s such a fantastically evocative name that it’s a shame we no longer use it (even if it’s a terrible name from an astronomical point of view) and it perfectly encapsulates what it must be like to live in a galaxy in the middle of the Boötes Void.

Even worse would be an isolated star. The sky around it would simply be black, even with an optical telescope. There’s a novel called Against A Dark Background which takes place on planets orbiting such a star. It could indeed happen, and here’s where it could happen.

Out beyond the Boötes Void, five times as far away as it is, there’s another big void. It’s even bigger in fact: it’s 3-4 times the diameter. We call this the Giant Void, because once you have a Great Void there’s really not many words you can use to one-up it. This is… look, it’s big, okay? Big and empty. There isn’t a great deal I can say about it. I study stars, and a void is a place where there are few if any stars. It’s not my field. It’s a Giant Void. It says so on its name plate. Done. Next.

We think that the structure that consists of voids, dark matter filaments, and galaxy clusters in places where the dark matter meets, is the basic structure of the universe. Picture it, if you will, as a sponge. In fact, you don’t have to picture it because here’s a picture I found on Wikipedia (of all places) of what it looks like.

(Image courtesy NASA.)

Here’s another one of how we think it formed.

(Image courtesy University of Chicago.)

See that? The mixture of energy in the early universe gathers around the strands and walls, abandoning the areas which become voids. The Boötes Void and the Laniakea Supercluster, and other places like them, were formed by the way energy blobbed around in the early universe.

Would you like to see a picture of the early universe? Good, because we can give you one.

Astronomers don’t just look out into the sky, they also look back into the past. When we see the Moon we are seeing something slightly more than one light second away; this means we’re seeing slightly more than one second into the past. The giant star WR104 which I covered before is 7500 light years away. The Boötes Void is hundreds of millions of light years across. When we look at them, we’re looking into the distant past.

If we look far enough, we can see far back enough to see this. You’ve seen it before. Now, hopefully, you might be able to make more sense of it.

(Image courtesy European Space Agency.)

This picture is too early for galaxies to have developed yet. What we’re seeing is patterns of energy, high and low, which will one day form the strands of galaxies and superclusters we live in. As the universe aged it shrank around its dark matter structure. Matter began to accumulate and would one day gather into stars and other things. The places between that matter became the space between them: the great emptiness between stars, galaxies, galactic clusters and galactic superclusters.

We will never travel across this great emptiness. Heartbreakingly, faster-than-light exploration is probably not possible, which means we can only gaze longingly at the void and know in our hearts that we are part of something vastly greater than ourselves. But we can dream, and we can learn, and we can teach, and we can tell stories.

A story told to another is a story that never dies, and that is truly the largest thing I know of.

D-20: Large, Part One

The following was originally published on the Stellaris forums.

 

Hello everyone and welcome once again to what has been dubbed the Astroknowledge series. I’m your host, EJ, and today we’ll be starting a two-part series to talk about very, very large things.

(Image courtesy photography-on-the-web.com)

Picture a spiderweb coated in dew. The web itself is invisible, but the strings of droplets along it show us that there’s something there. Where the strands meet, we see more and larger droplets. Spiderwebs are small, and so are dewdrops.

Galaxies are, by any reasonable definition of the word, large. Despite this, galaxies aren’t found alone: they exist in clusters. Until recently we thought that we lived in a cluster called the Virgo Supercluster, but we have discovered that Virgo is only part of a vastly larger cluster called the Laniakea Supercluster. (If you’re wondering why we only found this out recently, it’s because it’s surprisingly difficult to study something whilst being inside it.)

We’ve known for a while now that most galaxies are far heavier than the number of stars within them would suggest, and that this can’t be explained away by assuming large numbers of black holes. The normal theory is to suppose that galaxies have a lot of extra matter in them which we can’t see. Because we can’t see it, this is known as “dark” matter.

In 2012, Jörg Dietrich’s team studied two distant superclusters, Abell 222 and Abell 223, which are near one another. Using gravitational lensing and other techniques, they found that the two clusters of galaxies were connected by… something. Something very heavy, but something which light passed through as if it wasn’t there.

Picture that spiderweb again. We see two droplets. Between them there’s a strand. We can’t see the strand but we know from its effects that it’s there. Now picture a similar strand between two clusters of galaxies, invisible but holding them together. We think that throughout the universe there’s a network of such strands, called “dark matter filaments”, and that galaxies either form on them or are attracted to them. The Laniakea Supercluster itself is, we think, the intersection of several filaments, which is why so many galaxies have collected here.

In the middle of the Laniakea Supercluster is a point which was discovered before we mapped our supercluster, a place called the Great Attractor. We noticed a long time ago that lots of things are being pulled faintly but surely towards the Great Attractor, and theorised that it might be very heavy. A lot of hypotheses were formed about it. Nowadays we think that it’s the centre of gravity of the supercluster, hence the gravitational attraction. If this is the case then it’s great because it replaces a mystery (a strange point in deep space) with an explanation we understand very well (the centre of gravity of lots of heavy stuff.)

(Image courtesy Nature.)

Dark matter sounds very mysterious, and people are often more mystical about it than they need to be. It’s not actually that difficult as a concept, however, even if the maths is annoying. If you can imagine something which you can’t see or touch, so it could pass right through you and neither of you would ever notice, then you’ve basically got a pretty good understanding of how dark matter is thought to behave.

The Milky Way is thought to have more dark matter, proportionally, than many other galaxies our size. Over 90% of the Milky Way is thought to be dark matter, and it’s concentrated out in the disk rather than in the core. This may be why we’re such a big galaxy and have attracted other, smaller galaxies and star clusters to orbit us.

Those filaments of dark matter are thought to connect throughout the universe, joining all the visible matter together into strands and sheets of light which separate vast dark voids. In a sense, we have begun to discover the skeleton of the universe. And the universe is the biggest thing we have yet discovered.

D-30: The Mirror

The following is new content.

 

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.