A relic from the primitive Universe… in our own backyard! [Starts With A Bang]
“We have been forced to admit for the first time in history not only the possibility of the fact of the growth and decay of the elements of matter. With radium and with uranium we do not see anything but the decay. And yet, somewhere, somehow, it is almost certain that these elements must be continuously forming. They are probably being put together now in the laboratory of the stars.” -Robert Millikan
The Universe has been around for a long time, but without the Big Bang, we’d never have any of the matter or starlight that gives rise to practically all of our experiences in the entire Universe.
13.7 billion years ago — minus one microsecond — the Universe was filled with protons, neutrons, neutrinos and photons, and electrons and positrons. The temperatures were high enough that heavier elements could be fused together in this hot, dense crucible, but there were too many high-energy particles for them to remain; they were blasted apart almost instantaneously. By time the Universe had cooled enough so that the first heavy combination of these particles — a deuteron — would be stable, the Universe was over three minutes old, and the density had dropped by a factor of around a billion.
Protons and neutrons then fuse together furiously, building up heavy elements as best as they can, but because the expanding Universe is now so much sparser and cooler, it can only form deuterium, helium, lithium and beryllium; all the heavier elements are out of reach.
This process — known as Big Bang Nucleosynthesis — accounts for the lightest elements in the Universe, and is where all the matter today eventually comes from.
Yes, that’s right: all of it. Including things like you and me, made primarily out of carbon, oxygen, hydrogen and nitrogen, where most of the elements in our body did not exist at the beginning of the Universe. In order to create them — in fact, in order to create nearly all of the elements on our planet — it took generations of stars burning through their nuclear fuel, fusing light elements into heavier ones, and recycling those elements back into the interstellar medium to form new and subsequent generations of stars, now enriched with these heavy elements.
But this means that one of the great predictions of the Big Bang is that — if we’re fortunate enough — we can find some of this pristine gas, direct from the primordial Universe. Atoms that have never been inside of a star before; that have never been burned inside of a nuclear furnace, that have never been spit back out into the cosmos to light the next generation.
This is very difficult, of course, because stars form all over the Universe, and finding “unpolluted” regions should be extremely rare, and should only become increasingly rare as we come forward in time.
Last year, we got incredibly lucky. There’s a technique we have, where we look very far back in the Universe, where we not only see the light from the extremely distant object we’re observing, but we also see the absorption lines from anything between that distant object and ourselves.
This makes it possible — in principle, at least — to find a cloud of gas that has not yet collapsed to form any stars, so long as there’s a region of space behind it that has.
In a stroke of serendipity, last year we discovered two such clouds that appear to have the exact abundances of Hydrogen and Helium predicted by Big Bang Nucleosynthesis, and none of the heavier elements that would exist in gas that was polluted by prior generations of stars. These two clouds, labelled with green arrows in the image below, are the most pristine clouds of gas — and they’re totally star-free — we’ve ever found.
That “Z” on the y-axis is astronomer shorthand for “elements heavier than hydrogen and helium,” more formally known as metallicity. (Why? Because “X” is Hydrogen, “Y” is Helium, and “Z” is everything else combined. I know, I know, carbon, nitrogen, oxygen, etc., are not metals. To you. But they are to an astronomer; just roll with it.) So these intergalactic clouds are the most metal-free clouds we’ve ever found.
But what about stars? Sure, due to the way light becomes harder and harder to observe the more distant objects become, we’re really restricted to our own galaxy and the very nearby vicinity (e.g., globular clusters) if we want to measure an individual star. But we’ve found some very, very metal-poor stars, and I want to share with you the current record-holder.
This completely non-descript star, SDSS J102915+172927 (or, in plain english, Caffau’s star), was found by the Sloan Digital Sky Survey last year. While pretty much every other star we’ve ever found — from the metal-rich to the metal-poor — has some amount of lithium and carbon in it, this star is pretty much all hydrogen and helium.
How much is “pretty much?”
It’s the most pristine star ever discovered, and it couldn’t have formed the way stars normally formed; in order to do so, stars need to cool, and it’s thought to take metals to do that conventionally. If you want to form stars the way the first stars in the Universe were formed, you need some type of special cooling, such as through dust.
But just because science doesn’t understand all the details of how a star like this got to be here doesn’t mean this isn’t an amazing story: some star had to be the first to form in the Universe, and our best estimates are that if the Universe is 13.73 billion years old now, the first stars formed when it was maybe 13.68 billion years old. And this star definitely exists with the properties just described; figuring out exactly how it formed could be a window into the earliest of times in the Universe!
The paper announcing the discovery of the most metal-free star ever discovered is here, and it’s amazing how untouched a star can be after over 13 billion years in our galaxy. The star is at least 13 billion years old, has the mass of about 80% of the Sun (making it a K-type star), and fortunately, it won’t be alone for long. The team that found it expects to find five-to-fifty more stars just like it over the next decade using the same technique!
So say hello to the primitive Universe; a piece of it is right in our own backyard!