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The oldest star discovery tells much about the early universe

The discovery of an ancient star formed approximately 13.6-billion years ago just after the Big Bang is telling us much about the early universe. The star – designated SMSS J031300.36-670839.3 – lies within…

An explosion in the universe (artist’s impression). www.shutterstock.com

The discovery of an ancient star formed approximately 13.6-billion years ago just after the Big Bang is telling us much about the early universe.

The star – designated SMSS J031300.36-670839.3 – lies within our Milky Way galaxy and a mere (relatively) 6,000 light years away but is the oldest known star discovered so far, we’re announcing in a paper published in Nature.

The oldest star, marked. Stefan Keller

By studying the light from this star in detail we have, for the first time, seen the chemical fingerprint of the first stars to form in the universe.

The telltale sign that the star is so ancient is the complete absence of any detectable level of iron in the spectrum of light emerging from the star.

Starlight shines

Starlight percolates out from the hydrogen fusion reactions taking place in the star’s interior and as it passes through the outer layers, the atoms of elements present absorb light at specific wavelengths.

The spectrum of starlight has imprinted on it a unique chemical fingerprint of absorption lines that tells us what elements are present in it, and how abundant they are.

In the case of the Sun there are literally millions of absorption lines arising from a broad range of elements, from the ubiquitous hydrogen and helium to iron and rare elements such as europium.

The spectrum of the star we have discovered shows only a handful of elements – hydrogen, carbon, magnesium, and calcium. This star is made to a recipe that is remarkably different to that of our Sun, and that can tell us a lot about how and when it was formed.

Back at the Big Bang

The Big Bang gave rise to a universe filled with hydrogen, helium, and a trace of lithium. All the other elements that we see around us today – those fundamental to life – were made in stars.

As stars struggle against the inward pull of gravity they fuse hydrogen to helium, helium to carbon and oxygen. If massive enough, a star continues fusion on to iron.

At the end of a massive star’s life these products are then recycled back into the surrounding gas through a supernova explosion.

Any old iron?

The iron level of the universe increases with time as successive generations of stars form and die. We can use the iron abundance of a star as a qualitative “clock” telling us when the star was formed.

In the case of the star we have announced, the amount of iron present is less than one millionth that of the Sun, and a factor of at least 60 times less than any other star. This indicates that our star is the most ancient yet found.

Stars are like time capsules, they lock away a sample of gas from which they form. In the case of the star we have discovered, this has enabled us to study in detail a sample of gas from approximately 13.6 billion years ago.

This is so long ago that the star predates the formation of the Milky Way. It likely formed in a small cloud of gas and eventually many of such clouds fell together under gravity to form the grand spiral galaxy we call home.

This star has born silent witness to 99% of the life of the universe - it has spun impervious, slowly converting hydrogen into helium as demanded by gravity.

What are older stars made of?

The chemistry of the gas shows that our star formed in the wake of a primordial star around 60 times the mass of the Sun that died in a supernova explosion. This supernova explosion was radically different to those supernova that occur today.

The explosion was of surprisingly low energy, such that although the star disintegrated, almost all of the heavier elements, such as iron, that form near the core of the star, were swallowed by a black hole formed at the heart of the explosion.

The shockwave from the dead primordial star spread its outer layers, enriched with carbon and magnesium formed over its lifetime, into the surrounding gas. Some of this gas subsequently condensed leading to the star we have discovered.

In this way, our star is a member of the second generation of stars in the universe and is unique in that it unambiguously incorporates material from the first stellar generation.

The early stars

The first generation of stars to light up the universe are understood to be fundamentally different from the generations that followed. Formed from the pristine hydrogen and helium of the Big Bang, the first stars were massive, hundreds of times the mass of the Sun.

Without iron and molecules of carbon and oxygen, condensing stars cooled very slowly and small stars could not form. A first generation of mammoth stars lived fast and died after only a few million years (compare this to the nine-billion year lifetime of the Sun).

For this reason we don’t expect to find any member of the first star generation today. But we can use the forensic evidence left in the wake of their explosive deaths – as encapsulated in the second-generation stars such as the one we have found – to describe what they were like.

Finding the oldest stars is very much a needle in a haystack search. Our star was one of 60-million stars in our search.

The SkyMapper telescope fundamental to our discovery, seen here under the Milky Way. James Gilbert/Australian Astronomical Observatory

To cleanly separate the oldest stars from the vast bulk of pedestrian stars is made possible by the SkyMapper telescope operated by the Australian National University from our dark-sky site at Siding Spring Observatory, near Coonabarabran, NSW.

The optical filters used in SkyMapper enable us to find stars with low iron from their colour. With a large digital camera such as SkyMapper’s we can screen 100,000 stars per hour. Using SkyMapper we continue to map the southern sky every clear night.

Our best candidates are then examined in great detail using one of the twin 6.5m Magellan telescopes in Chile.

A dozen left?

We expect that there may be only as few as a dozen other ancient stars to be found. Bringing these stars to light will allow us to characterise the population of the first stars and obtain insight into an era of cosmic evolution hidden from modern telescopes: the switching on of the first stars.

This was a turning point in the history of the universe. They mark the transition from warm, dark gas to one capable of generating material for rocky planets and life.

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27 Comments sorted by

  1. Joe Gartner

    Eating Cake

    A star in our galaxy that is older than our galaxy and able to 'burn' for 13.6 billion years. That is indeed remarkable.

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  2. Peter Lang

    Retired geologist and engineer

    Great article. Thank you. And how welcome to see real science as distinct from the religious pseudo science practiced by the politically partisan, climate doomsday brigade

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  3. Andy Saunders

    Consultant

    Does that mean that this star is remarkably slow-burning? Otherwise it would have evolved into either increasing in size (giant or supernova) or shrinking to a dense dwarf?

    So it would therefore have been at exactly the right initial size to just ignite but burn very slowly?

    Just wondering (from ignorance)...

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    1. Stefan Keller

      Research Fellow at Australian National University

      In reply to Andy Saunders

      Sorry for the delay - hectic day. The star we have identified is around 80% the mass of the Sun. The lifetime of a star depends very strongly on its mass: a 100 Solar mass star lives for a few million years, a star like the Sun for 9 billion years, and the star we are speaking of for around 15 billion years. It has started to run out of hydrogen fuel in its core - we know this because it is slightly more puffed up than a young star (specifically, it has a lower surface gravity than what we would expect - logg=2.3 rather than 4.5). So it is changing - just very slowly.

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    2. Andy Saunders

      Consultant

      In reply to Stefan Keller

      Thanks Stefan. Appreciate both your reply and you taking the effort to spread the education around.

      Magic. But how on earth do you measure surface gravity from this far away? And for that matter its size (is it just a function of the main sequence so you estimate the luminosity and size from its colour?).

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    3. Stefan Keller

      Research Fellow at Australian National University

      In reply to Andy Saunders

      Hi Andy - it is only possible to measure the size for a few stars - those that are close by or extremely massive - for most stars we have to extract information directly from the point of light that a star presents. Temperature: easy - red cool, blue hot. Surface gravity: somewhat harder - it affects the wings of absorption lines. As a thought experiment imagine a ball of gas in which we could change the density. We can squeeze it tight (higher surface gravity) or let it puff out (low surface gravity). When it is squeezed tight the atoms are moving about more energetically and because of the Doppler shift due to particle motion this makes the wings of an absorption line broader. Halfway down on http://tinyurl.com/npjxlos you can read in more detail how this works. So basically we can figure out how condensed the star is.

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  4. Jeff Payne

    PhD in Political Science and Masters in Public Policy

    Great article Stefan on a fascinating subject matter. The time scales here are just unimaginable. 13.6 BILLION years. It really does make our brief appearance, indeed, the blink which is human existence itself, seem so insignificant. Yet, at the same time, we are the beings that can 'look out' at the universe and 'see' their wonders. We appear to be both insignificant and marvelous at the same time. Perhaps this is the magic of the human condition. This little planet floating in space and we rush around getting our bread and milk and worried about little things when the simple wonder of being just passes us by. You are very fortunate Stefan to see and think about such amazing things.

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    1. Stefan Keller

      Research Fellow at Australian National University

      In reply to Jeff Payne

      Thanks Jeff - it is of course near impossible to ascribe an absolute age to the star, but we can say that on the basis of the iron abundance that this star is older than others because of its record low iron abundance. There are, like many things, caveats that are associated with this. One is that if the star formed in the centre of the galaxy it would be enriched in iron faster than if it formed out in the boon docks.

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  5. Rex Ettienne

    logged in via Facebook

    Great Article, Thankyou.

    I have a question though. Is the Star still thought to be existent or are we just viewing the light that left the Star 6000 years ago? It seems incredible that it could last so long more than 13 billion years, were the first Stars different in constuction ie much larger sources of fusionable material?

    Thanks

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    1. Stefan Keller

      Research Fellow at Australian National University

      In reply to Rex Ettienne

      Hi Rex - You are right that because of the finite speed of light, we are seeing the star as it was when the light left it 6000 years ago. However, since the star has such an enormous lifetime this really doesn't matter. It is amazing that the Sun for example consumes half a billion tonnes of hydrogen every second - has done this for 4.5 billion years and will do so for another 4.5 billion years...

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  6. Cheree Corbin

    logged in via Facebook

    I don't understand how this works. I realise when we talk about fractions of a billion years, that still leaves quite a bit of time, but it looks to me like:

    • This star is 13.6 billion years old
    • The universe is 13.8 billion years old
    • Our galaxy is 13.2 billion years old

    And this isn't one of the first stars either.

    Got me scratching my head.

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    1. Stefan Keller

      Research Fellow at Australian National University

      In reply to Cheree Corbin

      Hi Cheree - the Universe came into an existence in the Big Bang (what came before is an interesting matter for philosophy) some 13.7 billion years ago. Our star is from the second generation of stars to form (not the first stars - they were very massive and only lived for a few million years each). Our star would have formed before the Milky Way was anything like what we see today. It probably formed in a clump of gas and many of these clumps fell together under gravity to produce the Milky Way (http://tinyurl.com/ncedghd). We can't give an exact age for the star, rather we can say that it older than any other we have looked at due to the low amount of iron.

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    2. Cheree Corbin

      logged in via Facebook

      In reply to Stefan Keller

      Cool. Thanks for the response and thanks for that link. Obviously you've discounted the possibility that its low in iron because it's almost finished burning up those heavier elements.

      Don't worry about what came before the Big Bang - I've got that sorted. Think of the Universe as consciousness breathing in and then out. We're living during the in-breath at the moment - hence the rapid expansion. Then it will breath out again and maybe in another 14 billion years, take another breath.

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    3. Stefan Keller

      Research Fellow at Australian National University

      In reply to Cheree Corbin

      Hi Cheree - a star starts out turning the simple stuff (hydrogen) into more and more complex stuff: Hydrogen fuses to helium, helium to carbon & oxygen ... all the way up to iron. Then fusion can't extract any more energy (a star can then collapse and go supernova if it gets too big an iron core). So iron doesn't get used up inside a star.

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  7. Karen Leemen

    logged in via Facebook

    Hi Stefan - great article. Thanks! Mind blowing that at star has been burning for that whole period of time. What a find.
    I have a question: if 'your' star is about 80% of solar mass, are you able to estimate what mass it started out? Presumably it's been losing mass through solar (stellar?) wind.. and other radiation (?).. for 13.6 billion years - how much difference does that make to a star's mass?
    Also, is there any way of knowing part of what generation of stars our sun is?

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    1. Stefan Keller

      Research Fellow at Australian National University

      In reply to Karen Leemen

      Hi Karen - it is unlikely that the mass of the star we have found has changed much over its vast lifetime. Although every star has a out flow of energetic gas from it, this gas is extremely tenuous and doesn't amount to much over most of the star's life. When the star puffs up to become a red giant it will eventually loose a significant amount of mass to stellar winds. This material is then recycled into the next generation.

      Judging by the levels of the heavier elements such as iron in the Sun we understand that it is the result of 1000-odd stellar generations. But hard to give a precise number here as it depends on how much mixing the supernova ejecta undergoes.

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    2. Peter Lang

      Retired geologist and engineer

      In reply to Stefan Keller

      Great answers to everyones questions. Thanks very much for taking the time.

      What do you mean by "Judging by the levels of the heavier elements such as iron in the Sun we understand that it is the result of 1000-odd stellar generations. "?

      And you mentioned our Sun is second generation, whereas I learnt long ago it is third generation. What's changed to cut back a generation?

      Or perhaps I am getting confused and it is the rocky and heavy elements beyond iron that make up Earth and the inner planets that are from third generation stars. Can you please clarify for me?

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    3. Stefan Keller

      Research Fellow at Australian National University

      In reply to Peter Lang

      Hi Peter - The star we have found incorporates the material ejected from a single first generation star's supernova. It is a second generation star. The Sun on the other hand, has at least 10^7 times more iron than the star we have found. That iron resulted from many supernovae perhaps as many as a thousand. Hence our Sun is a member of the 1000th-odd (approx!) generation of stars to have formed in the Milky Way. So there are many generations of stars to have formed in the Milky Way - unlike human…

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    4. Peter Lang

      Retired geologist and engineer

      In reply to Stefan Keller

      Thank you. Very interesting. Excellent enlightening responses. You have an ability to answer questions directly and explain succinctly in a way the generalist can understand. I may have to attend more of the Amateur Astronomer meetings at Stromlo. Will you be speaking at one of their sessions some time?

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  8. Ulises

    logged in via Twitter

    Along with this cosmic evolution, which is the probability that within a young galaxy like the milky way, from the dust of its exploded stars, the living being who uses a computer was formed - computer included? A favourable case among infinite unfavourable possibilities? Fifty-fifty? To be or not to be, is that the question? Or is it a zero followed by a radix point and an infinite amount of zeros behind, but finishing with a one emerging from error or compassion when rounding up? Are calculations…

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    1. Stefan Keller

      Research Fellow at Australian National University

      In reply to Ulises

      There are a lot of open questions. How life comes about may not be the overwhelming improbability that you imagine I would contend. Take raw ingredients, add time in quantities beyond our comprehension, and energy from a benign star (the Sun)...
      For you I end with a question: why is all life based on molecules that are of the same chirality? http://en.wikipedia.org/wiki/Homochirality

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