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“Impossible” Star Exists in Cosmic Forbidden Zone

This ancient star, in the constellation of Leo (The Lion), is called SDSS J102915+172927 and has been found to have the lowest amount of elements heavier than helium of all stars yet studied. It has a mass smaller than that of the Sun and is probably more than 13 billion years old. Credit: ESO/Digitized Sky Survey 2

Astronomers say a newly found star should not exist and is in the “forbidden zone” of a widely accepted theory of star formation. The star, called SDSS J102915+172927, is composed almost entirely of hydrogen and helium, with only remarkably small amounts of other chemical elements in it. And how should this star be classified? We suggest either ‘easy listening’ or ‘jazz,’ as this star is certainly not heavy metal! But it may be one of the oldest stars ever found.

This faint star is located in the constellation of Leo (The Lion), and has the lowest amount of elements heavier than helium (what astronomers call “metals”) of all stars yet studied. It has a mass smaller than that of the Sun and is probably more than 13 billion years old.

“A widely accepted theory predicts that stars like this, with low mass and extremely low quantities of metals, shouldn’t exist because the clouds of material from which they formed could never have condensed,” said Elisabetta Caffau (Zentrum für Astronomie der Universität Heidelberg, Germany and Observatoire de Paris, France), lead author of the paper appearing in this week’s edition of Nature. “It was surprising to find, for the first time, a star in this ‘forbidden zone’, and it means we may have to revisit some of the star formation models.”

The team found the star with the X-shooter and UVES instruments on the Very Large Telescope. This allowed them to measure how abundant the various chemical elements were in the star. They found that the proportion of metals in SDSS J102915+172927 is more than 20,000 times smaller than that of the Sun.

“The star is faint, and so metal-poor that we could only detect the signature of one element heavier than helium — calcium — in our first observations,” said Piercarlo Bonifacio (Observatoire de Paris, France), who supervised the project. “We had to ask for additional telescope time from ESO’s Director General to study the star’s light in even more detail, and with a long exposure time, to try to find other metals.”

This picture shows the distribution of the light of different colours coming from the remarkable star SDSS J102915+172927 after it has been split up by the X-Shooter instrument on the ESO VLT. Credit: ESO/E. Caffau

The prevailing theory is that hydrogen and helium were created shortly after the Big Bang, together with some lithium, while almost all other elements were formed later in stars. Supernova explosions spread the stellar material into the interstellar medium, making it richer in metals. New stars form from this enriched medium so they have higher amounts of metals in their composition than the older stars. Therefore, the proportion of metals in a star tells us how old it is.

“The star we have studied is extremely metal-poor, meaning it is very primitive. It could be one of the oldest stars ever found,” adds Lorenzo Monaco from ESO, also involved in the study.

Also very surprising was the lack of lithium in SDSS J102915+172927. Such an old star should have a composition similar to that of the Universe shortly after the Big Bang, with a few more metals in it. But the team found that the proportion of lithium in the star was at least fifty times less than expected in the material produced by the Big Bang.

“It is a mystery how the lithium that formed just after the beginning of the Universe was destroyed in this star.” Bonifacio added.

Is this star one-of-a-kind? Probably not, the researchers say. “We have identified several more candidate stars that might have metal levels similar to, or even lower than, those in SDSS J102915+172927. We are now planning to observe them with the VLT to see if this is the case,” said Caffau.

Read the team’s paper (pdf file)

Source: ESO

About 

Nancy Atkinson is Universe Today's Senior Editor. She also works with Astronomy Cast, and is a NASA/JPL Solar System Ambassador.

Comments on this entry are closed.

  • Anonymous September 1, 2011, 3:01 AM

    This has me slightly confused. The PopIII stars are strange in of themselves, and this star almost qualifies as some low mass PopIII star. A gas with just H and He has a low opacity and forms stars poorly. The PopIII stars are thought to be rather large objects which transiently fuse elements rapidly and explode. Yet here is a stable star which harkens from around that time period, with the chemical make up expected of a PopIII star.

    LC

    • Anonymous September 1, 2011, 3:29 AM

      I don’t understand. How is it that a possible population III star could still exist? Could our models for early star formation be wrong?

      • Anonymous September 1, 2011, 3:37 AM

        Agreed, this is strange. Unfortunately I am not much on stellar astrophysics, so I can’t say a lot with confidence. H and He has low opacity and a modest accumulation of the gas is not likely to hold photons enough to sustain a reaction. PopIII stars are then generally thought to have been very large. This is a modest star with the chemical or low metal make up expected of a PopIII star. I guess this is why this star is “impossible.”

        LC

        • HeadAroundU September 2, 2011, 12:53 AM

          http://www.newscientist.com/article/dn20850-astrophile-the-impossibly-modern-star.html

          “It’s also possible that low-mass, low-metal stars like this one could be detritus from giant stars’ birth, suggests Abraham Loeb of Harvard University, who first suggested that carbon and oxygen were required for forming low-mass stars but was not involved in the new study.

          Some recent simulations have shown that giant stars form a disk of gas around them, and that disk can split up and scatter, like an out-of-control merry-go-round. Such a small star would not need the cooling effect of carbon and oxygen to clump into separate pieces. “It’s quite possible that one could make low-mass stars out of pristine gas,” he says.”

          • Anonymous September 2, 2011, 2:52 AM

            The problem is in getting a star made entirely of H and He to function. They have very low opacity and do not hold thermalized photons from fusion in their cores. If it is supersized it can more or less work. The question is how can a modest sized star of this sort function at all.

            LC

          • Christopher September 4, 2011, 6:38 AM

            Could it be possible this star was a binary pair during the accretion process, and the denser metals fell towards the more massive companion? The greater mass star would have dissappeared much sooner leaving this star made up of less dense material. Could it also be possible that this star had more mass at one point but then after fusion began the more massive companion continued to steal mass from it? That would also explain the stars low mass. I dont know, I’m not the most knowledgeable on stellar formation, but I think it could work.

          • Anonymous September 4, 2011, 3:31 PM

            Remember that gravity accelerates all masses equally; think of Galileo and the famous if not apocryphal story of dropping balls off the leaning tower of Pisa. The difference in motion between different species has to do with transport properties, whether that be with sediment in water or ions and electrons in a plasma.

            When I saw the Tower of Pisa it occurred to me there seems to be almost nothing else to do with it than to drop things. I rather imagine people dropped different things repeatedly by folks before Galileo made his observation.
            .
            LC

  • Anonymous September 1, 2011, 4:25 AM

    Your comment here

  • FTLFactor September 1, 2011, 6:04 AM

    If I remember correctly, lithium fuses easily so Pop I stars should burn it off in a pre-Main Sequence “lithium flash”. I also understand that low-mass Pop I stars tend to be fully convective. However, with low opacity this star would presumably have less convection. How then would stellar envelope lithium get mixed down into the stellar core to get burned away? The missing lithium is indeed a puzzle.

  • FTLFactor September 1, 2011, 6:04 AM

    If I remember correctly, lithium fuses easily so Pop I stars should burn it off in a pre-Main Sequence “lithium flash”. I also understand that low-mass Pop I stars tend to be fully convective. However, with low opacity this star would presumably have less convection. How then would stellar envelope lithium get mixed down into the stellar core to get burned away? The missing lithium is indeed a puzzle.

  • julanna hennessy September 1, 2011, 6:21 AM

    Reminds of a short story I read where astronomers were out of work because it turns out all those wonderful things we see were all works of art by aliens.

  • julanna hennessy September 1, 2011, 6:21 AM

    Reminds of a short story I read where astronomers were out of work because it turns out all those wonderful things we see were all works of art by aliens.

    • Anthony Patino September 4, 2011, 12:17 AM

      What’s the name of that story?

      • julanna hennessy September 4, 2011, 2:43 AM

        I looked for it but I can’t find it, and I can’t remember where I read it. I was a few years ago. Very entertaining.

  • Anonymous September 1, 2011, 6:52 AM

    As we have recently learned from the Kepler mission with regards to planetary system formation, there are very few things that are impossible. You simply have to surmise that there were conditions present in the early universe that are not yet known. We know that brown dwarfs or “failed stars” can form from the normal star formation process. Gas giants can form from gravitational instability in an accretion disc. So why couldn’t a runt of a star form in an eddy in the shadow of a massive Pop III star? A more far fetched explanation could be that the star lost mass via piracy by a small black hole or white dwarf companion which it later separated from.

  • The Red Pill September 1, 2011, 4:20 PM

    Couldn’t a star collision mess with the contents of the star? The more dense, heavier bits would get knocked harder than the less connected gas – perhaps this could knock out the denser core as well as supply the ‘older star’ with additional hydrogen. This is just a hypothesis of course but it seems that simulations could investigate this possibility.

  • Anonymous September 1, 2011, 8:02 PM

    Is the universe homogeneous? Are heavy metals present everywhere? Is it possible that there are anomalous areas rich in hydrogen and helium that could coalesce into a star? Just asking. I don’t know.

  • Anonymous September 2, 2011, 12:39 AM

    Given it is13 billion years old, it has had plenty of time to burn off mass.

    • Anonymous September 2, 2011, 2:49 AM

      Doesn’t quite work like that. In a way, I wish it did, it would make this mystery less puzzling.

  • Anonymous September 2, 2011, 3:01 AM

    Very interesting object… Gotta love when something throws a spanner in the works!

  • Anonymous September 2, 2011, 5:30 AM

    Just for a bit of fun…
    Signs of a Type II civilization?

    ‘Star lifting is a process where an advanced civilisation could remove a substantial portion of a star’s matter in a controlled manner for other uses’.

  • Anonymous September 2, 2011, 5:30 AM

    Just for a bit of fun…
    Signs of a Type II civilization?

    ‘Star lifting is a process where an advanced civilisation could remove a substantial portion of a star’s matter in a controlled manner for other uses’.

  • Anonymous September 2, 2011, 5:30 AM

    Just for a bit of fun…
    Signs of a Type II civilization?

    ‘Star lifting is a process where an advanced civilisation could remove a substantial portion of a star’s matter in a controlled manner for other uses’.

  • Anonymous September 3, 2011, 1:22 PM

    I find it interesting how often the ‘experts’ are baffled by something new they never expected.

    • Andres Mora September 4, 2011, 3:44 AM

      I don’t think they are baffled by simply finding something new they are baffled by the mystery this new thing reveals.

      • Christopher September 4, 2011, 6:08 AM

        I think experts become experts because they are always looking for the opportunity to learn something new. That’s how they obtain so much knowledge in the first place.

  • Jimmy September 4, 2011, 12:49 AM

    Is it possible for a solar system to have formed around this star? If so, would the planets have formed like say our solar system or would they lack because of their being nearly no heavy metals?

  • Htos1 September 4, 2011, 8:20 AM

    Very cool.

  • Aurelio Czargb Ramos September 6, 2011, 9:49 PM

    I’m glad it’s been brought up already. Is it time we start to consider the possibility that intelligent life has altered that star? It would sure be a shame if we modified our theories to include data from this star, but it was in fact the result of influence from a type II civilization.

  • Mike September 6, 2011, 10:52 PM

    I’m not very familiar with stellar formation processes so someone please educate me. It was my understanding that at the time this star formed it would be made almost entirely of hydrogen and helium and the heavier metals would only form later as those elements were continuously fused into heavier and heavier elements. That’s what I thought was confusing and “impossible” about this star, i.e. that after so much time (13 billion years), very little heavy elements were present even though all of that fusion should have taken place. I ask because it seems like many people are commenting based on the idea that the star would form with these heavy elements (metals) being present at the time of it’s formation. So did I just show how ignorant I am? :)

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