A field of stars.

Running on empty: universe short on gas

The Universe forms fewer stars than it used to, and a CSIRO study has now shown why - the galaxies are running out of gas.

  • 16 September 2011

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Learn more about why the Galaxies are running out of gas.

Glen Paul: G'day, and welcome to CSIROpod. I’m Glen Paul. When you look out into the night sky the stars appear to go on everlastingly, but around 15 years ago Astronomers discovered the rate of star formation peaked when the Universe was only a few billion years old, and has been declining steeply ever since.

This has raised the question as to why? CSIRO Scientists now have the answer, and it would seem the Universe has literally run out of gas. Stars form from clouds of molecular hydrogen – the less molecular hydrogen there is the fewer stars will form.

Joining me to discuss it is Dr Robert Braun, from CSIRO Astronomy and Space Science. Firstly, Robert, how do you really know the lights are going out?

Dr Braun: Well, this has been measured by several independent means over the last years, and it started out being in fact quite a controversial claim. But as you mentioned, about 15 years ago that first assessment was made, and the evidence has only been accumulating since that time that, as you mentioned, the star formation rate first came to a peak when the Universe was only a few billion years old, and has dramatically diminished ever since then.

And indeed that’s what prompted us to go out and try to understand why that might be happening, and we used the Mopra telescope near Coonabarabran, that CSIRO operates, to look at galaxies that were about five billion years in the past. We used that simply from the light travel time – it takes five billion years for the light to reach us, and so we can look in the past by that means – and in looking at those galaxies we were in fact quite surprised to see just how dramatically different they were in the galaxies around us, our own part of the Universe, mainly that they had ten times as much gas as the same type of galaxy today.

And these are the most actively star forming gas rich objects that we know about, and they are so dramatically different five billion years ago. And so that takes us some of the way towards understanding what is going on – namely that if we don’t have the raw materials to form new stars, they don’t form. And so it’s a piece of the answer to this puzzle of what is going on and exactly why.

Now, another piece of that puzzle that we’re still trying to work to understand is, well what’s driving all of this? Why did the star formation increase and then decrease? And now, as I’ve just said, we understand that it is closely coupled to the availability of gas, but what I think people probably don’t appreciate is that galaxies around us are only able to form stars for a fairly limited length of time.

What's available today is only sufficient to allow them to form stars at their current rate for about one or two billion years, and unless they’re being continually supplied with new gas from somewhere else, they will just run out and stop forming stars all together.

So where is that gas coming from? Well, the Universe hasn’t run out of gas as a whole – we currently think that about one third of all the atoms in the Universe is now locked up in galaxies, almost completely in the form of stars; another third is distributed in the more extended environments of galaxies; and then the final third is out in the very tenuous portions of the Universe, so far away from galaxies that it will never be captured.

Now, what the Universe began by doing was being very much dominated by gravity – gravity ruled – and as such it would pull in gas from the environment, and seemingly cause very efficient formation of stars. And something that we’ve also learnt quite recently is that the Universe changed from this gravity dominated mode, to one that is dominated by something termed Dark Energy – that is a repulsive force it seems, and is leading to an accelerated rate of expansion of the Universe.

So while the universe began in a big bang and has been expanding, for some time gravity reigned and was able to pull in material from around the galaxy cores that began, but since about eight billion years ago the tables have turned, and now it’s Dark Energy that rules, and is leading to this ever increasing rate of expansion of the Universe. And what that seems to have meant is that gas in the environments of galaxies, that was once within reach of gravitational attraction and collection in the galaxies, is no longer in reach.

And I think this is the underlying reason for the phenomenon that we’ve been talking about, namely both star formation and the availability of gas to fuel it. As the Universe is expanding ever more quickly, gas in galaxy environments is just out of reach now, and if this trend continues – and that is probably the simplest theory to consider at the moment, that this trend of ever increasing expansion at a constant rate will just continue – it means that galaxies literally will run out of the raw material for forming new stars, and as such the lights will eventually go out.

Glen Paul: So does that mean if we were able to travel forward in time, à la Hitchhiker's Guide to the Galaxy, to the restaurant at the end of the Universe, there’s not going to be a spectacular cataclysmic ending – and just a long slow fade to black?

Dr Braun: Well, as I said, that’s the simplest assumption based on just a continuation of the period that we're now in, in terms of cosmic expansion. But it’s conceivable that new theories may emerge, new understanding of the Universe, that would result in a rather different outcome of that evolution. It’s something that we certainly can’t predict now, but unfortunately we don’t understand, what is Dark Energy really? And perhaps when we do get a better understanding of what that is it would actually permit us to make rather different predictions about what’s in store for the Universe.

Glen Paul: Hmm. And there isn't a reserve of gas out there floating around in space between galaxies that could effectively...

Dr Braun: Oh, absolutely. At the current time about one third of all the atoms are essentially sitting there waiting for us, but they are now no longer in gravitational reach of our own galaxy and other galaxies like ours, because as I said, the Universe is flying apart at this ever increasing rate, so what was in reach gravitationally is no longer in reach, and so the galaxies are just flying apart and the gas between them is no longer accessible.

Glen Paul: Right. So, give it to me straight then, Doctor, how long have we got?

Dr Braun: How long have we got? Well, most galaxies, if we look around us, have only one or two billion years worth of gas supply if they kept forming stars as quickly as they do today. In fact the lights wouldn’t go out with just like turning of a switch quite, because as you have less gas, less stars would form, so it would kind of peter out, and that process might take twice as long as what it would take with just a constant rate of gas consumption. So rather than being one or two billion years, it might be twice that – two, or four.

But around about two or four billion years from now no new stars would really have a chance to form, and at that point the Universe would just become dimmer and dimmer, as more and more of the existing stars would end their lives, and essentially no new ones were being formed to replace them. And so the galaxies like our own would become a redder colour, which is the colour of these older stars, that the blue young stars would have all ended their lives, and indeed each galaxy would just become dimmer, and darker, and quieter, and the lights would slowly fade away. So on the time scale of say ten billion years, there would be very little left to illuminate our night sky.

Glen Paul: So we will go quietly into the night. Fascinating and mind boggling stuff, Robert. Thanks for discussing it with me today.

Dr Braun: Oh, you’re very welcome. My pleasure.

Glen Paul: Dr Robert Braun. For more information find us online at csiro.au. You can like us on Facebook, or follow us on Twitter at CSIROnews.