“It’s a brilliant surface in that sunlight.” – Neil Armstrong
Indeed, all that glitters so brilliantly in the cosmos does so because of the stars that have formed throughout it.
Over the 14 billion-or-so years that our Universe has been around, we’ve formed hundreds of billions of stars in our galaxy alone.
Given that our galaxy is just one of at least hundreds of billions in the observable Universe, the number of stars that have formed over our Universe’s history is a tremendous number, when you add them all up.
But one of the fun things we discover is — by looking back at the younger galaxies in the Universe — the star formation rate back then was much higher than it is now! A typical galaxy from long ago is forming more stars on average than a galaxy now.
This galaxy — the Sunflower Galaxy — is typical of galaxies today. You can identify star-forming regions in galaxies from the characteristic pink glow that star-forming regions give off, thanks to their ionized hydrogen.
Do you see how, above, there are only a few, small pink regions in that galaxy? This is a classic example of a mature spiral galaxy, where it’s full of gas, dust, and stars, all clearly visible in the snapshot here, but only a few sparse regions are currently forming stars.
This was not always the case for this galaxy, and it won’t always be the case for this galaxy going forward into the future, either. Why not?
Because events are going to happen that cause this gas and dust to contract and form stars. It can happen in large bursts, like due to a gravitational interaction (above), it can happen gradually over time, triggered by something like a nearby star’s explosion, or it can happen in the most spectacular way imaginable: in a huge rush caused by a major merger with a comparably-sized galaxy.
In this last case, the entire galaxy will become a star-forming region, and this is known as a starburst galaxy.
Incidentally, this will be us in about 4 billion years, when the Milky Way and Andromeda undergo a major merger. Our night sky will look something akin to this, as our entire system of merging galaxies will be forming new stars.
But — you may be curious — how long can this process go on for? Sure, we’ll form stars in great, periodic bursts when rare, catastrophic events occur, and very slowly and intermittently otherwise. But at some point, we’re going to run out of the hydrogen gas that — at one point — comprised 92% of the atoms in the Universe. Because stars work by fusing light elements into heavier ones, at some point in the future, we’ll have fused all the elements we’re going to form.
Well, here’s what you definitely shouldn’t do.
You might say that the universe has been suffering from a long, serious “crisis”: cosmic GDP output is now only 3% of what it used to be at the peak in star production! If the measured decline continues, then no more than 5% more stars will form over the remaining history of the cosmos, even if we wait forever.
Let me tell you something about our galaxy. Our unexceptional, unremarkable, uninteresting-save-that-it-contains-us galaxy. This guy, shown vertically so you can get a good look at it.
Sure, with your eyes, you’re going to notice mostly the stars, and — in pink — the small and sparse star-forming regions. But you may also notice the dust lanes!
Here’s the thing: if you add up all the normal matter in our galaxy — all the protons, neutrons, and electrons — most of it is still neutral hydrogen gas! We’re in no danger of running out anytime soon.
However, we went through intense periods of star formation in the distant past. We observe these — in and around our galaxy — all across the Universe as still being an ongoing thing.
When this happens, only about 10% of the gas that made up these star-forming regions actually gets locked up in stars, with the remaining 90% evaporating and getting blown back into the interstellar medium, where it will someday form stars again in the future.
Furthermore, most of the stars (in terms of mass) that form will eventually die in either a supernova or a planetary nebula, returning a huge fraction (perhaps half of a star’s worth) of unburned fuel back to the interstellar medium on top of the large gas fraction that never formed stars during the initial starburst!
Since it’s in a gravitationally bound system — a galaxy — it’s only a matter of time and gravity, neither of which are going anywhere, before all of the gas eventually forms stars.
The thing is, it’s going to take a looooong time — many trillions of years, by my estimates — until we’re out of fuel. Why’s that? Because, when Sobral says “If the measured decline continues,” that’s his big flaw. Yes, there’s an initial burst of star formation that’s huge, and occasional bursts like that will punctuate the timeline of the Universe and dominate the measured star formation rate. But there’s a slow, steady component on top of that, and as long as gas is abundantly present within our galaxy, that measured decline will not continue arbitrarily far into the future.
And the thing is, Sobral’s a good enough astronomer that he knows it.
Yes, it’s interesting that the star formation rate has declined, and it’s interesting that it’s declined at the rate we’ve observed. But it’s not going to drop to zero any time soon, and if you sum up the total number of stars in our Universe’s future, it’s actually far greater than the number of stars that have already existed up until this point in time, a far cry from the “only 5% more than we have now” figure you may have read.
Although we might be approaching the peak of star density within our galaxy, we can very strongly say that the vast majority of stars that will ever call our galaxy home haven’t been born yet.
We won’t live long enough to see them, either, as many trillions of years into the future is far too ambitious to count on, even for those of you counting on the singularity. But based on the physics and astronomy we know, there will be new stars for ages and ages to come, outnumbering even the full complement of stars that have ever existed up until today.