Early Stars Were Whirling Dervishes


Even though some of the first stars in the early universe were massive, they probably lived fast and furious lives, as they likely rotated much faster than their present-day counterparts. A new study on stellar evolution looked at a 12-billion-year-old star cluster and found high levels of metal in the stars – a chemical signature that suggests that the first stars were fast spinners.

“We think that the first generations of massive stars were very fast rotators – that’s why we called them spinstars,” said Cristina Chiappini of the Astrophysical Institute Potsdam in Germany, who led the team of astronomers.

These first generation stars died out long ago, and our telescopes can’t look back in time far enough to actually see them, but astronomers can get a glimpse of what they were like by looking at the chemical makeup of later stars. The first stars’ chemical imprints are like fossil records that can be found in the oldest stars we can study.

The general understanding of the early universe is that soon after the Big Bang, the Universe was made of essentially just hydrogen and helium. The chemical enrichment of the Universe with other elements had to wait around 300 million years until the fireworks started with the death of the first generations of massive stars, putting new chemical elements into the primordial gas, which later were incorporated in the next generations of stars.

Using data from ESO’s Very Large Telescope (VLT), the astronomers reanalyzed spectra of a group of very old stars in the Galactic Bulge. These stars are so old that only very massive, short-living stars with masses larger than around ten times the mass of our Sun should have had time to die and to pollute the gas from which these fossil records then formed. As expected, the chemical composition of the observed stars showed elements typical for enrichment by massive stars. However, the new analysis unexpectedly also revealed elements usually thought to be produced only by stars of smaller masses. Fast-rotating massive stars on the other hand would succeed in manufacturing these elements themselves.

“Alternative scenarios cannot yet be discarded – but – we show that if the first generations of massive stars were spinstars, this would offer a very elegant explanation to this puzzle!” said Chiappini.

A star that spins more rapidly can live longer and suffer different fates than slow-spinning ones. Fast rotation also affects other properties of a star, such as its colour, and its luminosity. Spinstars would therefore also have strongly influenced the properties and appearance of the first galaxies which were formed in the Universe. The existence of spinstars is now also supported by recent hydrodynamic simulations of the formation of the first stars of the universe by an independent research group.

Chiappini and her team are currently working on extending the stellar simulations in order to further test their findings. Their work is published in a Nature article on April 28, 2011.

Source: University of Potsdam, Nature

12 Replies to “Early Stars Were Whirling Dervishes”

  1. Ah. The age old problem of angular momentum.
    Pop III might have to be fast spinners, but that means that the formation during the early stages of the universe from the progenitor nebulae would also have to be different. It would also mean that the Star Formation Rate (SFR) has more prodigious but also much more efficient; to collapse faster just to add the extra angular momentum. It would also mean the stars were ellipsoids, with most spinning on the edge of about 300 metres per second (else they’d just fly apart.)
    Nice idea, but it sacrifices too much of what we know of stellar evolution theory to be accepted point blank. “Alternative scenarios cannot yet be discarded.” I can now see why!
    Good luck selling it, Cristina Chiappini!

    1. It would also mean the stars were ellipsoids, with most spinning on the edge of about 300 metres per second (else they’d just fly apart).

      300 metres per second?! You sure about that, dude? The Earth’s rotational velocity at the equator is about 465 metres per second! 🙂

      Also, according to this paper, “Rotation Speed of the First Stars“, near break-up speeds are in the region of >1000 km/s^{-1}, as stated in the abstract:

      […] We find that there is sufficient angular momentum to yield rapidly rotating stars (> 1000 km s^-1, or near break-up speeds). This indicates that Pop III stars likely experienced strong rotational mixing, impacting their structure and nucleosynthetic yields. […]

      1. Err… My honest mistake. I certainly meant 300 kilometres per second.

        The rotational velocity of about 1000km.sec-1 is a safe theoretical number mostly for compact objects. (the fast we know are about half of this value at 500 km.sec-1)
        One of the biggest problem in forming the stars, is the limitation of maximum spin or centrifugal potential (labelled often as Φ or Φc). For stars to be able to form they must reduce to a certain rate so it can be stable objects and not literally spin apart — the so-called mass shredding (Ωshred) away from the star. (The central problem angular momentum dissipation in star formation.) Its final value highly dependant on the size of the object. (I.e. I.W. Roxburgh back in the mid-1950s did some early work on this, using uniformly rotating polytropes for his calculations. The final value is dependant on the composition (X, Y, Z), pressure, density, temperature, etc., and the overall internal structure of the star.) Needless to say it is all fairly complicated stuff and not simple.
        If M=mass and Rp= Polar radius, then the spin limit can be approximately calculated by; Ωshred = (2 /3)^3 /2 (M/Rp)^0.5

        Examples are the 1st magnitude star Achernar spins (using spectroscopic v.sin i) at some 225 km.sec-1, and is some 3.8×5.9 solar radii. Regulus is a bit faster at at 317 km.sec-1, whose rotation is merely 15.9 hours and is NFL football shaped; 30% bigger in radius at the equator than at the poles. Values for a cut off point is lightly debatable, depending on the parameters one use. About ~300 to 330 km.sec-1 is probably a pretty good guess.

        If we are talking about the first stars like Population III’s, a larger size means the value of the velocity should be slightly smaller again.

        In the end, saying >1000 km.sec-1 is a certain estimate that cannot be wrong. (Better safe than sorry, as they say.) If they can, it is as I said; “Nice idea, but it sacrifices too much of what we know of stellar evolution theory to be accepted point blank.”

        Thanks for properly pointing out the error, and I do hope that answers your question here.

      2. Thanks for taking the time to respond, and, yes, it answers my question.

        Also, your equation correction is yet another reason that we could all do with a preview/edit facility here!

      3. The equation I gave should read; Ωshedd = (2 /3)^3 /2 (M/Rp^3)^0.5
        I.e. The Rp= Polar radius is cubed.

      4. Interested people here might like to read the following paper; Aerts, C. “Maximum mass-loss rates of line-driven winds of massive stars: The effect of rotation and an application to η Carinae”; A&A., 418, 639–648 (2004) (in pdf)

        This give some information and problems of mass-loss and rotation in massive stars. As said straight up, one of the additional problems with massive stars is the influence of mass loss. I.e. They say; “Rapidly rotating massive stars may suffer mass-loss rates that are considerably higher than those of non-rotating stars. Stellar evolution calculations indeed show that rapidly rotating stars must lose a large fraction of their mass in a relatively short time ”
        It is just another problem to overcome for this idea of “spinstars” to solve the problem here. (It is the mass loss, incidentally, that enriches the galaxy and “pollute the gas”, as Nancy politely says.

    2. Also pity i don’t have the $32 to find out all about it.
      Always with “Nature”, whether it is 2011 or 1884, it is always the same old US$32. Hand over you monies, and we’ll let you learn something.
      Really. If some of these authors sent their papers to other equally respected journals, more individuals would probably read their articles and see their tax dollars in action. (I can find no other way of reading it.)
      Oh well. Probably better to read something else here. 🙁

      1. It is interesting you mentioned that Andrew. We don’t have paid access to Nature, either, which I found a little discouraging while trying to write this article. I wondered out loud on Twitter how many people actually pay money for individual articles and @NatureNews replied: “I don’t have numbers, but I think it’s quite small.” I then asked about the number of individual subscribers and they replied that yes, they have quite a few individual subscribers, though institutional site licenses are the biggest segment of their readership.

      2. Thanks for the extra comment, too. I am perhaps just a little bitter about an Nature article of the 11 July 1872 (Nature, 6, 202 (1872)) on Aurora, with the comment that I had to pay $32 for the privilege. When I went to read the abstract on the text at the ADS, the whole story was in the abstract of just 108 words!.
        Really. It was written 1872, and worst if you do buy it, you have no idea of the length of even if the article was relevant. I can, just, understand this recent evolution article being limited, even for a few month or a year, but every article ever published in Nature is a bit rich. What I find terribly amusing is even that the German version of the paper has been restricted.
        In the end, I will probable read this at the library, when the story has disappeared from the minds of everyone. Right now, the importance of the work and the paper eludes alludes all of us.
        Appreciate your efforts here, especially when your hands are so bound, so to speak.

  2. I’m just wondering if the name ‘spinstars’ is somehow a play on ‘spinsters’? If so, why, and can someone explain it? If not, then maybe I’m just looking for a punchline that doesn’t exist.

    1. … maybe I’m just looking for a punchline that doesn’t exist.

      How about this one…

      Q: What is the definition of a blunderbuss?

      A: A coach-load of spinsters on their way to the maternity hospital!

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