The Universe is old. 13.798 billion years old, to be precise (give or take about 37 million years or so). That’s so old, it’s genuinely a little tricky to wrap your head around – 200 million years ago, dinosaurs roamed planet Earth. 580 million years ago, complex life emerged on our world. The earliest known evidence for life on Earth dates back to 3.7 billion years ago, a few hundred million years after the planet formed around 4.54 billion years ago. This, in turn, was shortly after the Sun itself formed, 4.57 billion years ago. But compared to some stars out there, the Sun itself is still young…
The unassuming little point of light in the centre of this image is something really quite special. Just 6000 light years away, it’s the oldest star ever discovered. Going by the designation of SMSS J031300.36-670839.3, this star is older than the very galaxy which we live in, and formed when the Universe itself was still young.
I’m not going to give an actual age for the star because, the thing is, we simply don’t know. Quoting Anna Frebel, a Stellar Archaeologist (which has to be one of the coolest job titles ever) – we don’t actually know the star’s age… quoting any age is pretty much made up. So how do we know this star is so old? Well, the thing about astronomy is that all we ever really have to go on are photons, but photons can tell us an awful lot…
This is a spectrum of this star (corrected to give a flat baseline). Spectra like this can show you an awful lot about a star – and in this case, it tells me that this star contains a lot of hydrogen and not a lot else. Those lines are absorptions, coming from different elements which make up the star – the more elements, the more lines. Stars like the Sun are full of all sorts of interesting chemical elements. Astronomers call anything heavier than helium a “metal”, and a star like the Sun, with all of its metals is said to have a “high metallicity”. This spectrum, however, shows a very different creature. There’s barely anything there. Stars like these are termed “metal-poor”.
The thing is, in the billions of years the Universe has been around, stars have been industriously forming heavy elements from hydrogen. Inside the Sun right now, even as you’re reading this, 9 x 10³⁷ nuclear reactions are happening every single second. That’s ninety billion billion billion billion reactions. All of those nuclear reactions convert 620 million metric tons of hydrogen into heavier atomic nuclei. Every second! And right now, there are 300 billion stars in our galaxy doing exactly the same thing. When stars die, they cast all of those metals they’ve created out into interstellar space. Metals are everywhere, and any new star which forms takes in whatever elements are in the clouds it forms from. To find a star which is so devoid of metals, it would need to have formed from clouds which were similarly barren. In other words, it must be old. Very old.
In fact, the venerable SMSS J031300.36-670839.3 shows all the hallmarks of being one of the second generation of stars ever formed. The first ever stars formed quite soon after the Universe was born. They were composed of little more than hydrogen and helium. As a result, they were massive, fast burning, and rapidly died as supernovae. Known as “Population III” stars★, these mysterious primordial stellar beasts have never actually been observed. However, we know they must have existed. All of those metals must have come from somewhere.
Immediately after these first stars died and exploded, they seeded the surrounding primordial gas clouds with the very first metals. Those clouds then began to collapse into the second generation of stars. Careful examination of the star you see above shows that it is indeed one of this second stellar generation – incorporating material from the first stars the Universe ever saw. And it’s one of no more than a small handful of such stars which are still burning. One of the last survivors of a bygone age of the Universe.
★ As with so much in science, these were named in the wrong order and the names have stuck. Calling them population I stars would be less confusing, but trying to change established nomenclatures is a little bit like trying to stop a runaway freight train.