Stellar Relic

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…

SMSS-J031300.36-670839.3

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…

ancient-spectrum

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.

About invaderxan

Molecular astrophysicist, usually found writing frenziedly, staring at the sky, or drinking mojitos.
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19 Responses to Stellar Relic

  1. just for fun, could you indicate the approx. location of SMSS J031300.36-670839.3 on your “you are here” map of the Milky Way? thanks!

  2. philip says:

     

    9000?

    Or am I missing something? …

  3. Pingback: [BLOG] Some Tuesday links | A Bit More Detail

  4. chris says:

    Can someone explain how a *lack* of metals implies old age?
    From the article, “… All of those nuclear reactions convert 620 million metric tons of hydrogen into heavier atomic nuclei. Every second! […] To find a star which is so devoid of metals, it must be old. Very old.”
    That sentence seems to imply the exact opposite of the conclusion. If a star has few or no metals it must be very young because it hasn’t had time to convert the hydrogen to heavier elements, right? What am I missing?

    • Stephen says:

      Hi Chris,

      Because this star is presumably very small, the rate of metal creation is very low. This is related to how this star can be so old and still shine. Stars convert H/He to heavier elements and die when that abundant energy source starts to fade away and it can no longer support its own weight with nuclear fusion. Larger stars consume H/He at a higher rate than smaller stars, meaning smaller stars can live much longer without running out of H/He.

      Additionally, most of the heavier elements, and in particular Iron and heavier, are created due to supernovae. This means that any later-generation stars that formed post-supernova will have more metals than older stars that formed out of pristine material.

      So, young stars in our galaxy will have metals because they are made up of dirtier recycled material, while old stars are metal-poor, relfecting their environment at birth.

    • invaderxan says:

      Oh sorry, I guess I was a little unclear on that.

      Basically, stars are doing this all the time, and then when they die all of those metals they’ve created are cast off into interstellar space. The interstellar clouds which form new stars already contain plentiful metals from every previous generation of stars which died and had its material recycled back into interstellar space. In this way, newly formed stars will already be enriched in metals from the cloud which formed them.

      So the only way for a star to be so sorely lacking in metals is if there weren’t any around when it formed – just the small handful of heavy elements which were formed by the first population of stars.

      I’ve edited that paragraph a little to be clearer. Does that all make sense? :)

      • cwarth says:

        yes, thanks to you both for clearing up my misunderstanding. Most of the heavier elements for a small star like this come from initial formation, not from fusion reactions during its lifetime.

        I’m guessing that if a star is fusing hydrogen to make helium, it won’t start making even heavier elements until it has used up the hydrogen, then starts fusing helium, and so on up the periodic table, with a substantial blowoff and collapse before each phase change. So if you see a star with a substantial amount of both hydrogen and heavier metals, you know those metals came from initial composition.

        • invaderxan says:

          Exactly. And you’re quite right – a star won’t start fusing helium until after it’s used up all of its hydrogen. That said, fusing heavier elements requires higher pressures and temperatures, which a smaller star may never be able to attain. I forget the exact stellar masses involved, but smaller stars like SMSS J031300.36-670839.3 will never be able to create heavier elements, simply because it isn’t massive enough.

          Additionally, stellar fusion happens inside a star’s core, and all of the metals it creates tend to stay there too. Until it begins to die and turns into a red giant, you don’t see much of those newly formed metals at a star’s surface…

  5. This begs the question: If it’s so very, very old, why isn’t it dead yet?

    • invaderxan says:

      Surprisingly enough, 13.8 billion years isn’t that long for a star to live. The Sun (a class G star) has a life expectancy of around 12 billion years. Cooler class K stars may get a few billion years more on top of that. Tiny class M red dwarfs may live for up to a trillion years – no red dwarf has ever died in the Universe, because the Universe itself isn’t old enough yet.

      There isn’t any information available on what class this star is (I suspect no one’s found out just yet), but I’m pretty sure it must be smaller and cooler than the Sun. When it formed, it would have likely been the runt of the litter, so to speak. It seems there’s a moral to that story hidden in there somewhere.

      (Though there are potentially more outlandish ideas which could also fit…)

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