Habitable Galaxies - Part 1: Galaxy types and origins

Hello blog readers, I'm back :-)

I've talked about habitable planets and habitable parts of our galaxy before. Now I am going to talk about the possible habitability of other galaxies. It's a somewhat lengthy topic so it'll be spread out over a few weeks. Today, some background.

Miscellaneous types

There are two main classes of galaxies: spiral and elliptical. (Also known as late type and early type, but those names are silly as will soon become apparent.) These are called morphological types because they relate to the physical shape of the galaxy. You can also get dwarf galaxies, which are very small—on the scale of galaxies, although they're still billions of times the mass of our sun compares with tens of trillions of suns for galaxy like ours.

Once we get past galaxy shapes there are all sorts of interesting things galaxies can be doing. For example, you can have active (or not active) galaxies, galaxies which are merging, galaxies where stars are actively being born, galaxies filled with old, dying stars, galaxies with high or low metallicities, isolated galaxies and galaxies part of giant groups or clusters. There's a lot of variety, even within each of these (not always mutually exclusive) classifications. You think the two hundred billion* or so stars in our galaxy are a lot? There are eighty billion galaxies just in the observable universe. Which ones, other than carbon copies of our own, can we live in? Let's first have a look at where galaxies come from.

* I use billion here in the American sense to denote 1 000 000 000 = 10⁹ for three reasons: 1. it's most commonly used this way in the scientific community, 2. the word "milliard" (as in the British counting convention) has fallen into disuse and 3. "thousand million" (the replacement for milliard) sounds clunky.

Galactogenesis

As Dougalas Adams said, in the beginning there was nothing, and then it exploded. After that, if we fast forward a bit (which, current theories suggest, the universe did), matter which at this stage consisting mostly of hydrogen with a little bit of helium that was fused in the primordial fires of the big bang, starts to clump together.

(At this point I feel the need to mention that I'm more or less ignoring dark matter because in this context it needlessly complicates things. But it's there too. Just so you know.)

These clouds then collapsed into discs and clumpiness within the discs led to star formation (that is, clumps of hydrogen collapsed in on themselves to make the first stars). So the first galaxies were all disc galaxies, another name (more or less) for spiral galaxies. However, not all clumpy areas are the same size, so you might get a clump of galaxy-sized clumps and then get a bunch of galaxies forming close together. Over time, gravity will pull these galaxies closer together and they will eventually merger (I'll get to mergers shortly). On the other hand, you could have an area of the universe where not much matter ended up and there was only enough stuff to make a few small galaxies.

If you look at our Local Group, there are two big spiral galaxies—ours and Andromeda—a medium-sized galaxy—the Large Magellanic Cloud—and a slew of tiny dwarf galaxies. Some of these dwarf galaxies are in the process of raining down onto the Milky Way or Andromeda. Others are further away and are falling into one of the big galaxies much more slowly. Andromeda and the Milky Way will merge on a time-scale of five or so billion years. Eventually, all the galaxies in the Local Group will merge into one giant Local Structure.

But I'm getting ahead of myself.

So current theory has galaxies being born as discs. Where do the elliptical galaxies, the round or oval balls of stars, come from? The currently popular answer is through mergers.

Mergers

On a cosmic time-scale, small galaxies are constantly falling into (accreting onto) larger galaxies. Every so often two medium-sized or larger galaxies might merge. These events are called major mergers. There are two possible outcomes of major mergers between spiral galaxies:

1. A new, larger, spiral galaxy. There will often also be a period of more vigorous star formation after a major merger like this thanks to the gas and dust in the galaxies being shaken up and combined with more gas from the other galaxy. It is thought this outcome happens when there is enough spare gas in the two progenitor galaxies.
2. An elliptical galaxy. When there isn't enough spare gas in the two merging galaxies, the stars don't rearrange themselves in a nice flat pattern (exactly why is unclear, I think) and we're left with a roundish blob of stars.

There is no question that minor mergers—small galaxies accreting onto larger ones—don't pose much of a threat for life. After all, small galaxies have been accreting onto the Milky Way far longer than we've been around. Whether planets will survive major mergers is another question. In principle, most planets should be more or less OK. Despite the billions or trillions of stars involved, they are actually quite spaced out within galaxies (for example, the nearest stars to our sun are more than four light years away). Therefore, the chances of stars colliding are very small, at least at the sort of distances from the centre of the galaxy where we find ourselves. If the stars don't pass too closely together, their planets will also remain unharmed.

Of course, it's possible that a particular planetary system could be unlucky, but then it could also be unlucky enough to have a supernova explode nearby. In fact, if enough gas nearby is stimulated into forming stars during and after the merger, it could be that a future supernova (from a newly formed massive star) is more of a threat than the actual merger itself.

Finally, will the Earth survive Andromeda and the Milky Way merging? Not exactly, but not because of the actual merger. You see, the sun is scheduled to turn into a red giant before we get to the merger stage, so Earth as we know it will be long gone already.

But wait, there's more

This isn't the full story, of course. Mergers also lead to increased activity in galactic nuclei, but that's a topic I'll be covering next week. Watch this space!