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On the Earliest Stars

If you’ve given some thought to the Fermi question lately — and reading Milan Ćirković’s The Great Silence, I’ve been thinking about it quite a bit — then today’s story about an ancient star is of particular note. Fermi, you’ll recall, famously wanted to know why we didn’t see other civilizations, given the apparent potential for our galaxy to produce life elsewhere. Now a paper in The Astrophysical Journal adds punch to the question by making the case that the part of the galaxy in which we reside may be older than we have thought.

Finding that our Sun is younger than many nearby stars, an issue that Charles Lineweaver (Australian National University), among others, has examined, would allow even more time for civilizations to have emerged in the galactic neighborhood. But let’s now leave Fermi behind to look at the tiny star that prompts these ruminations (and to be sure, the paper on this star makes no mention of Fermi, but does tell us something quite interesting about the early cosmos).

Discovered by Kevin Schlaufman (Johns Hopkins University), 2MASS J18082002–5104378 B is the smaller of a binary pair that orbit a common barycenter. While the primary had been previously discovered, it was up to Schlaufman and team to uncover the far more interesting companion.

Schlaufman used data from the Magellan Clay Telescope, Las Campanas Observatory and the Gemini Observatory in finding and characterizing this star. What distinguishes 2MASS J18082002–5104378 B is its size, metallicity and age. On the latter, Schlaufman believes it could be as little as a single generation removed from the Big Bang itself, and the paper pegs its age at approximately 13.5 billion years. We’ve discovered other ancient stars with low metal content, but this one is located in the Milky Way’s thin disk, that part of the galaxy in which we reside. Hence the issue of the age of local stars, as the paper recounts:

Given its thin disk orbit, the 13.535 ± 0.002 Gyr age of the 2MASS J18082002–5104378 system provides a lower limit on the age of the thin disk. Similarly old but not quite as metal-poor stars have also been seen on thin disk orbits (e.g., Casagrande et al. 2011; Bensby et al. 2014). This is somewhat older than the 8–10 Gyr age of the thin disk suggested by classical studies of field stars (Edvardsson et al. 1993; Liu & Chaboyer 2000; Sandage et al. 2003), the white dwarf luminosity function (e.g., Oswalt et al. 1996; Leggett et al. 1998; Knox et al. 1999; Kilic et al. 2017), and the ages of the oldest disk open clusters Berkeley 17 and NGC 6791 (e.g., Krusberg & Chaboyer 2006; Brogaard et al. 2012).

Image: This edge-on diagram of the Milky Way shows the thin disk in green. Credit: Wikimedia Commons (CC BY-SA 3.0).

We are talking about a star with a content of metals roughly the same as the planet Mercury. Contrast that with the Sun, whose heavy element content is equal to approximately 14 Jupiters.

2MASS J18082002–5104378 B (here’s hoping it gets a new moniker, perhaps ‘Schlaufman’s Star’) is the lowest-mass ultra metal-poor star currently known. Yet despite its extreme age and low metallicity, it is found in the thin disk, and in fact is the most metal-poor star yet found that is part of the thin disk. We would expect stars forming not long after the Big Bang to be low in metals, given that hydrogen, helium and trace lithium are all they had to work with. It would be later stellar generations that could form with the heavier elements these early stars produced in their cores, seeding the cosmos with metals through supernovae explosions.

Call that first generation Population III stars, which when first modeled by researchers produced stars far more massive than the Sun, giant objects that should form as single stars in isolation. Later models dropped the range of mass to as low as 10 solar masses but also extended it on the high end. Low-mass Population III stars only recently began to be considered when the issue of fragmentation began appearing in numerical simulations. The discovery of 2MASS J18082002–5104378 B makes the case for the emergence of such stars.

From the paper:

We use models of protostellar disks around both UMP [low-mass ultra metal-poor] and Pop III protostars plus scaling relations for the fragment mass and migration time to argue that the existence of the low-mass UMP star 2MASS J18082002–5104378 B and the extremely metal-poor (EMP) brown dwarf HE 1523–0901 B discovered by Hansen et al. (2015) implies the survival of solar-mass fragments around Pop III stars…

Thus we may be looking at a new observable that can take us back to conditions at the earliest era of star formation:

Whereas fragmentation at the molecular core scale will likely lead to massive binary stars, the emergence of gravitationally bound solar-mass clumps in protostellar disks via gravitational instability has the potential to produce low-mass Pop III stars that may be observable in the Milky Way.

Image: The new discovery is only 14% the size of the Sun and is the new record holder for the star with the smallest complement of heavy elements. It has about the same heavy element complement as Mercury, the smallest planet in our solar system. Credit: Kevin Schlaufman/JHU.

Thus far about 30 stars considered ultra metal-poor have been identified, all of roughly the Sun’s mass, but 2MASS J18082002–5104378 B is only 14 percent of the Sun’s mass. The mass, incidentally, was determined by radial velocity methods, examining the wobble of the primary star. Backing out to the wider picture, our view of the earliest stars as extremely massive objects, unobservable to us because they would have burned quickly and died, has to be modified to include low-mass stars that can, at least in some situations, emerge, as 2MASS J18082002–5104378 B did, burning for long lifetimes indeed. Says Schlaufman:

“If our inference is correct, then low-mass stars that have a composition exclusively the outcome of the Big Bang can exist. Even though we have not yet found an object like that in our galaxy, it can exist.”

The paper is Schlaufman et al., “An Ultra Metal-poor Star Near the Hydrogen-burning Limit,” Astrophysical Journal Vol. 867, No. 2 (5 November 2018). Abstract / preprint.


{ 31 comments… add one }
  • Antonio November 7, 2018, 16:33

    Very interesting. As for the age of the thin disk, how do they know that the star formed there instead of, say, inside a satellite galaxy?

    • Andrei November 8, 2018, 12:03

      It has not formed there in the disk.
      That it have a stable orbit in the disk suggest it were part in the galaxys formation when several smaller irregular galaxies merged.

      • Antonio November 8, 2018, 16:00

        Then why do they say this?

        “Given its thin disk orbit, the 13.535 ± 0.002 Gyr age of the 2MASS J18082002–5104378 system provides a lower limit on the age of the thin disk.”

        • Andrei November 9, 2018, 18:02

          Yes our galaxy is sometimes believed to 13,5 billion years, with the Universe at an total age of 13,7 billion years.
          So yes, with those numbers they do indeed consider the double star to have formed in the disk.
          Whereas I had the hypothesis that our galaxy formed a bit later ~12 billion years ago from adding irregular galaxies, and only later got the swirling disk shape.
          In the later case the star would have had to form elsewhere.
          So my reply was dependent on a different idea of galactic beginnings.

      • Antonio November 8, 2018, 16:15

        Mmm rereading your reply… Really is there no satellite galaxy that merged with the disk after disk formation? Even now there are galaxies whose orbit cross the disk, like the Sagittarius Dwarf Elliptical Galaxy.

  • Michael C. Fidler November 7, 2018, 19:04

    Something that bothered me about this is that it is a double star, the old star being a red dwarf. The problem is that the other main star has a mass of 2.44 times that of the sun, it should have been a white dwarf or neutron star eons ago!

    • Michael C. Fidler November 8, 2018, 11:16

      My apologies, should of read thru the paper first, the diameter of the primary is 2.44 times the sun. It’s actual mass is .76 of the sun which would make it an early K dwarf that has become a subgiant before going into its red giant phase after 13.5 billion years. The secondary is the M dwarf of .14 solar mass. I’m just wondering how the ultra metal-poor (UMP) stars age in relation to normal metallicity like our old Sol?

      • Bruce D. Mayfield November 8, 2018, 15:39

        Stars of the sun’s mass and metal content do age slightly faster than they would have than if they had no metals because, once the core is hot enough, carbon, nitrogen and oxygen can begin to act as fusion catalysts. (See the CNO cycle for details.) But this reaction can’t occur in M class stars no matter their metal content because the core temp never gets hot enough.

        • Michael C. Fidler November 8, 2018, 22:47

          Thank you, Bruce for the info, so the M dwarf is fully convective. The K sub giant is also metal poor ancient star from what I understand in the paper, is that correct? If so is it as metal poor? Has anybody looked to see if the what type of flaring is taking place from both stars?

          • Bruce D. Mayfield November 9, 2018, 15:40

            Binary stars forming together at the same time is extremely common, but unrelated stars passing near enough to each other so that they join up into a stable co-orbit would be very rare. Therefore the reasonable assumption would be that both of these stars are of the same age, and if the age of one is determined, the age of the other is then known also.

            In this system the K giant primary should therefore be as metal poor at the M secondary, but of course it would have converted much of its Li and H into He.

    • wdk November 8, 2018, 20:09

      Young massive stars in binaries with dwarfs. When I started looking at local binaries years ago to contemplate habitable zones, that same issue seemed to leap out with our two “Dog Stars”, Sirius and Procyon, main sequence A and F respectively but accompanied by WDs. I’ve never seen
      any paper on this, but had to wonder if close K and G binaries eventually merge and start new lives as brighter star. Alpha Geminorum or Castor has ( at least) 3 binary systems, A, B & C of descending lumnosity. The B system components are about half the mass of the sun and tightly bound. Thus, the hypothesis is that you come back in an aeon and there is only one star there. Probably some more hand waving needed, but this appears to be a recurring problem of matching theory with observation. We calibrate our evolution, energy and structural calculation of stars – but then there are all these pairs of
      short and long lived objects.

      • Bruce D. Mayfield November 9, 2018, 16:06

        It’s doubtful that there really are many pairs of stars with different ages. Gas exchange between close pairs complicates their histories.
        The WDs of Sirius and Procyon probably are the same age as their respective primaries, it’s just that they have transferred much of their mass.

        Stars can merge. The “blue stragglers” seen in globular clusters are young looking (but not young, because globulars are very old) stars that are explained by stellar mergers. But mergers would be exceedingly rare outside of areas of high stellar density.

  • AlexT November 8, 2018, 8:04

    Is low metall , but young star creation is less probable than old star that for some unknown reasons fusing hydrogen very slowly?
    Suppose there is basic mistake in the star age calculation.

  • Michael Spencer November 8, 2018, 9:52

    The paucity of observable civilizations, and our own poor understanding of the Universe, appear inextricably linked.

    Put another way: our present knowledge of physics is certainly a (very) small subset of an actual understanding. The challenges facing cosmology are indeed huge: expanding vs. contracting; baryonic matter vs. dark matter; dark energy; the disappearance of anti-matter in the first moments of the Big Bang; the reality of the Big Bag; inflation, gravity and quantum theory…the list is large, so large that a non-physicist easily conjure the issues.

    The universe is not accessible with rockets. And indeed if it IS accessible in any manner at all – if we are ever to stand on a foreign shore – it will be by means missing from any human horizon. The implications of this simple fact are obvious: we don’t see them because we don’t know how to see them.

    For now.

  • kzb November 8, 2018, 9:58

    I’ve got the same questions as the two above ! Also, is it not possible this stellar system has been deflected into a thin-disk orbit during its lifetime? It is a single example, and it could simply be a low-probability event that nevertheless happened?

  • Thomas R Mazanec November 8, 2018, 11:22

    If this is a Population III star, how much of that Mercury mass is primordial lithium? How much is post-formation contamination from the interstellar medium?

    • Bruce D. Mayfield November 8, 2018, 15:17

      It is NOT a true Pop III star, although it is about as close as has ever been found so far. This can be said because it does have some metals, but just a tiny fraction of the sun’s percentage. True Pop III stars would have no metals (beyond Li) at all.

      Since all main sequence stars produce stellar wind there shouldn’t be much contamination, except possibly from nearby supernova blasts.


      • Michael C. Fidler November 8, 2018, 23:16

        How would we know if a star is older the 13.5 billion years? The dissident Halton Arp for example would expect some very ancient stars that may have come from mergers. I prefer to look at this from a different perspective because this is what started the last great revolution of mankind’s knowledge when Galileo showed the clergy Jupiter’s moons.

        • Bruce D. Mayfield November 10, 2018, 15:00

          That interesting reply prompted much reading on my part Michael. Articles about how stellar age is estimated, the paper that Paul is herein reporting about, and even on Halton Arp.

          In the case of this ultra low metal (ULM) star system the primary star was discovered first by another group looking for ULM stars. Such stars must be very old because to be this metal poor they would have had to have been formed at a time before the interstellar medium had had much injection of metals from SNs. The authors of this paper then spent telescope time examining the spectra of this ULM star and used the radial velocity method to prove the existence of the low massed companion. Then since it’s a binary in a known orbit they could find the pairs’ masses. Then the age of the system can be better estimated by comparison with simulations of what a stars of such masses and compositions evolve into over time.

          If the primary was older, I suppose it would be brighter than it is as it transitions from H burning. (And if Arp where correct then there should be many stars even older than the Big Bang. There aren’t.)

          • Michael C. Fidler November 10, 2018, 22:17

            Without getting into a long argument over Arp’s continuous creation, let’s take a look at your last statement. I’m glad you took the time to at least to review Halton Arp’s ideas. The theory that Quasars, galaxies and even clusters of galaxies may be formed from the core of galaxies or quasars is quit foreign to most. The Armenian astrophysicists Victor Ambarzumian was the originator of that concept back in the 1950’s. https://en.wikipedia.org/wiki/Victor_Ambartsumian
            Where you assumption is that since no stars exist earlier then 13.5 billion years ago is only that the Milky Way galaxy was only ejected from another quasar or galaxy some 13.5 B years ago. That is why thru mergers or other ways, stray ancient stars would be in our galaxy. Two excellent books by Arp that are well worth reading if you are interested:
            Quasars, Redshifts and Controversies.
            Seeing Red; Redshifts, Cosmology and Academic Science.

            • Bruce D. Mayfield November 11, 2018, 17:31

              We may have to come to a friendly ‘agree to disagree’ on cosmology then, since I think the evidence is solidly behind the age of the universe being a little less than 13.8 billion years.

              But getting back to this thread’s topic, I don’t ‘assume that no stars exist that are older than 13.5By’. A major point of the paper under discussion is that now that the authors have proven that some low mass almost Pop III stars have been located it is possible that some even older true Pop III could still exist and the search should continue.

    • Gary Wilson November 8, 2018, 15:50

      I don’t understand your question Thomas. The star in question doesn’t have the mass of Mercury. It has 14 percent of our sun’s mass. I tend to agree with others that this star may have been acquired by the thin disk at some point. In other words it might be an ancient wanderer. Is this possible? Could the primary have grabbed it as it wandered by?

      • Bruce D. Mayfield November 10, 2018, 15:39

        Why is it a problem that a member of the thick disk stellar population might be found imbedded in the thin disk? I wouldn’t be surprised at all if they continue to find a few old stars like this with orbits close to our galaxy’s plain. Before our thin disk formed whould it’s area be devoid of all stars? Of course not, it would have had thick disk members scattered all though it.

        “Could the primary have grabbed it as it wandered by?”

        Note this from the paper:
        “Because it is so tightly bound to J18082002–5104378 A, 2MASS J18082002–5104378 B is extraordinarily unlikely to have been gravitationally captured outside of the birth environment of the former. They must therefore have formed in the same molecular core and thus have the same composition.”

  • Gary Wilson November 8, 2018, 15:46

    Looking for extraterrestrial intelligence is really, really hard. Give us several more centuries or even a millennium to collect more data. We just don’t have enough data and don’t know enough to necessarily even ask the right questions now. If we found simple forms of life on a few planets around nearby stars that would be a huge leap forward. Then we begin to have some idea of how common life of any kind is. I’m impatient to know if there is intelligence out there as well but I know my lifespan is just a tiny blip so my chances are remote indeed of finding out answers to any of the big questions. But we need to keep looking, keep gathering data and remain optimistic. It’s a big galaxy out there. There is a high likelihood there is intelligent life other than us in it but how often it occurs we have no idea at all.

  • Gary Wilson November 8, 2018, 15:52

    Looking for intelligence the way we do it may be the equivalent of standing in a room the size of our solar system and whispering “is anybody out there?”

    • Michael C. Fidler November 8, 2018, 22:41

      Well all we need is a Mega Heavy Metal rock band to wake everybody up!!! :-})

  • Andrew Palfreyman November 8, 2018, 16:13

    Assume the oldest metal-rich stars formed ~1 BY after BB. Our Sun formed ~8.5 BY after BB. That gives potential civilisations a maximum head start on us of ~7.5 BY.

    • Gary Wilson November 8, 2018, 18:22

      Which leads to what conclusion Andrew? Are there lots of intelligent civilizations out there? It’s a lot of time, no doubt about that but how often does life arise to allow evolution to occur? We just don’t know. If life occurs on 1 of every 10 planets the galaxy should probably be teeming with intelligent species. You know this game anyway I’m sure. The Drake equation again.

  • Gary Wilson November 8, 2018, 18:39

    Several very interesting variants of the Drake equation are out there now. I’m trying to understand them. I think the first step is clearly to find any planet anywhere with any sign of life (gaseous biosignatures for example) that we can detect and be sure is a true sign of life. Will we do this in the next 50 years? I’m not sure.

  • Michael C. Fidler November 13, 2018, 9:50

    A hidden reservoir of multi-planet systems?
    by Eckhart Spalding | Nov 13, 2018

    “The fact that compact multi-planet systems can form around stars which have a range of metallicities also stands, with an interesting twist– at the lowest tested metallicities, -0.5 < Fe/H < -0.3, there appears to be a rising increase in the likelihood. Is there a glut of undiscovered multi-planet systems that lie undiscovered at still lower metallicities? There are ancient stars in the Milky Way with metallicities of Fe/H=-3.5 and even less! Kepler didn’t have a chance to say the final word on this, and what we need now are next-generation, high-precision RV measurements of those erstwhile underappreciated metal-poor stars."


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