Image credit: www.hubblesite.org

My eye was caught by this article on the BBC earlier. That said, I couldn’t help but notice that they immediately tried to sensationalise the subject-matter:

    Our Sun’s position will be disturbed but the star and its planets are in little danger of being destroyed.

Note the ominous subsection there: ‘…little danger of…’, that is, implying that there is indeed some ‘danger of’, not ‘no’ danger of.

Actually, the planet’s not in any danger at all. And we human beings are in none whatsoever – by the time this happens, something else will certainly have finished us off long beforehand. The Milky Way/Andromeda collision is interesting, but it’s not really our problem in any direct sense.

That said, what is happening? And why?

Stars are small compared to the spaces between them; the Sun is a bit under a million miles wide, but it’s nearest stellar neighbour is about 25 trillion miles away. Or to put it another way, the Sun is about 1 part in 30 million as wide as the gap between it and its nearest neighbour. Out here in the galactic disk, the stars are well-spaced. (Note that this isn’t true inside dense environments such as globular clusters or the galactic bulge itself – however, we don’t live in the bulge or a cluster!)

Well, strictly, it’s stellar *systems* that are well-separated; you do get binary and multiple systems where two or more stars are in close orbit around each other, and some of them can be very tight indeed. (There are such things as contact binaries, which are exactly what they sound like.) However the separation between double and trinary and x-nary systems and their neighbours are (usually) large compared to the system itself. (There are some exceptions; because these ‘wide binaries’ are such rare and weird freaks, we’re quite interested in just how exactly they develop in the first place.)

Anyway, consequently, because stars are so well-spaced, collisions between stars are pretty rare. And when they do happen, it’s usually stars that are already in binary and multiple systems, so that they’re in very close proximity to each other. The vast majority of galactic-disk stellar systems will probably never come within even as much as half a parsec of their neighbours.

Consequently, stellar collisions are rare enough that we can entirely disregard them when considering the life-cycles of stars. Almost all stars will die of something else before a collision would ever occur.

Galaxies, however, are a different matter.

The Milky Way – ours! – is currently reckoned to be on the order of about ~100,000 light-years wide. Now, there are big uncertainties on what we know of the structure of this galaxy, mainly because we’re in it and thus literally can’t see the wood for the trees. (There are stars and gas and dust-clouds blocking many of our sight-lines.) And there are certainly uncertainties on how big it is – but, they’re not factor-of-10 uncertainties; the current ‘best guess’ range is about 100-120,000 LY.

Now, the Andromeda Galaxy is another spiral, like ours. Only bigger. It has about five times as many stars as ours does, and it’s about 220,000 LY from one side to the other. (Weirdly enough, we arguably have its overall structure better-mapped than we do for ours – wood and trees, remember?) Saying where a galaxy stops is a bit difficult – there isn’t exactly a binary cut-off, rather a sort of fuzzy fade-out. (The stuff I work on lives in that vague fade-out, also known as the stellar halo.) However, it’s pretty clear that Andromeda is about 2.2 million LY away from us.

Or, to put it another way, it’s about one tenth as wide as the distance between here and it. One tenth, not one thirty-millionth as for the Sun and Proxima. Galaxies aren’t ‘small’ compared to the distances between them.

Consequently, while stars hardly ever collide, galaxies basically do it all the time. In fact collisions and mergers are a key part of their life-cycle.

When two spiral galaxies collide, the stars in each aren’t destroyed. The gas and dust within each, however, are hugely-disrupted. Gas-clouds get ‘shocked’ by the collisions, and effectively splash out of the galaxy. This in turn disrupts spiral structure, and also puts a nice spanner into star-formation. A few cycles of this the post-merger galaxy doesn’t look anything like a spiral anymore. In fact you end up with an elliptical. Basically elliptical galaxies are the puddles of stars left behind after several spiral galaxies have merged and scrambled themselves into complete randomness as they do it.

Incidentally, I’ve always found something a bit creepy about elliptical galaxies; they’re old, populated mainly with old and dying stars and they’re not making any new ones anymore. (The gas is all gone, knocked out by various mergers.) They also have that bland, smooth sort of shape, unlike the much more active and structured patterns you find in spirals. Oh, and they’ve cannibalised themselves into existence out of the corpses of younger, fresher galaxies. Creepy or what?

And guess what? Right now, the Milky Way and Andromeda are falling toward each other. And they’re doing it at several hundred kilometres per second. But it’s a process that’s both fast and slow; although the absolute numbers on the speeds are huge, there’s also a lot of space to cover.

This is where the new research comes in. (In fact, you can find the three papers in question here, here and lastly, here.)

Basically, it’s been known for a long time that Andromeda’s light is blue-shifted, which means it’s moving toward us. However, what hasn’t been known has been what the lateral component of the motion is – for lots of reasons, that’s been difficult to measure. (First off, seen from ~2million LY, any lateral component will be pretty small, and then there’s the problem I mentioned before, of deciding where the galaxy in question stops – get that wrong and it’ll *really* stuff up your measurements! And I’ve done proper motion work, so I’m kind of the voice of grim experience here…) Depending on its value, the lateral motion could have been enough to prevent a collision – in that case, Milky Way and Andromeda sort of siddle past each other, if you will.

What the researchers here have done, basically, is that they’ve finally managed to measure that lateral motion. And it turns out that it’s not enough to prevent a collision. It’s official; the Milky Way and Andromeda are going to smack into each other. Every time we look at the night sky, we’re watching a trainwreck in progress!

However, the key words are ‘in progress’.

The researchers have used the new measurements to estimate when the collision is due. It turns out to be due in 5.86 billion years (plus or minus a billion or so either way). So set your alarms clocks now!

This, incidentally, is why it’s not going to be *our* problem; the Sun is due to expand off the main sequence in about 5 billion years. Depending on precisely how that plays out – and there are a lot of uncertainties in red-giant expansion – the Earth itself may possibly still exist, but if it does it’ll be a scorched and lifeless husk. At the peak of the red giant phase, the Sun will be several hundred times brighter than it is now; it’s pretty unlikely that anything living here now will be able to survive a noon-time temperature of ~2,000 Celsius!

However, the Sun itself will still be there, in some form at least. It’s strange to think of it as a fading white dwarf, floating through the vast elliptical ghost-galaxy left behind by the Milky Way/Andromeda merger.


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