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The Sun in a Crowded Sky

We’re so used to thinking of our Sun as a solitary object that having two Suns in the sky inspires the imagination of artist and writer alike. But what about whole clusters of stars? Evidence is mounting that the Sun was actually born in such a cluster. That’s quite a jump from the era, not so long ago, when astronomers assumed stars like ours formed without companions, but cosmochemists like the wonderfully named Martin Bizzarro (University of Copenhagen) think they have the data to prove it.

So here’s the new notion: Most single stars like the Sun evolve in multiple systems, clusters of stars that also contain massive stars that burn their hydrogen and explode while the cluster is still producing young stars. If this is the case, then we should expect the early history of the surviving younger stars to be affected by the nearby fireworks. Bizzarro’s team studied short-lived isotopes like aluminum-26 (26Al) and iron-60 (60Fe) as found in meteorites to see whether stellar debris from such explosions left its mark.

The trick, as discussed in a fine summary by G. Jeffrey Taylor in Planetary Science Research Discoveries, is that these short-lived isotopes no longer exist in most meteorites, their half lives ranging from 0.1 to 100 million years. But they have left decay products including 60Ni (from the decay of 60Fe and 26Mg (from the decay of 26Al). Taylor’s article goes through the process of isotope measurement to determine the ratio of decay products to common isotopes.

This, in fact, is where Bizzarro’s work will receive the most scrutiny, since not all such studies agree. But working with terrestrial samples, Martian meteorites, chondritic meteorites and differentiated meteorites — from asteroids that melted at an early stage of protoplanetary development — the team comes up with its result. One thing stands out: The lack of 60Fe in the differentiated meteorites, which represent the oldest planetesimals to form in the Solar System. Let Taylor summarize:

Formation of the Sun might have involved the formation and rapid life span (only 4 million years) of a massive star, 30 times more massive than the Sun. Astronomical observations indicate that such stars pass through a stage in which they lose mass–up to an Earth mass per day!– rapidly by blowing it into space at a couple of thousand kilometers per hour. These stellar winds contain 26Al, but 60Fe is still ensconced in the interior. At some point the star blows up, sending the products of nuclear fusion into interstellar space, including 60Fe. Note the sequence here: 26Al leaves with the strong stellar winds, which possibly triggered the collapse of a cloud of gas and dust to form a new star, our Sun. There is good evidence that 26Al was uniformly distributed throughout the Solar System… The 60Fe comes about a million years later when the star explodes, but also after many planetesimals differentiated. They did not contain any 60Fe, but had their full complement of 26Al. In fact, they had enough 26Al to heat up internally and melt.

Was the exploding star that left its mark on these meteorites a Wolf-Rayet star? Such stars blow off heavier elements on their way to eventual supernova explosion. It’s an interesting hypothesis that it was just such a star that helped give birth to our Sun and its accompanying system. What should emerge next is a series of further analyses of such meteorites to pin down the data on 60Fe deficiency. These investigations are nowhere near conclusion, but we are at least developing a working theory on stellar formation that may depict our Sun’s earliest era.

The paper is Bizzarro et al., “Evidence for a later supernova injection of 60Fe into the protoplanetary disk,” Science, Vol. 316 (2007), pp. 1178-1181 (abstract). The Taylor article, called “The Sun’s Crowded Delivery Room,” is from the July 6, 2007 issue of Planetary Science Research Discoveries, available online.

Comments on this entry are closed.

  • Steve Pease July 18, 2007, 16:18

    Is there enough data about the proper motion of stars to turn back the clock to see which stars associate with the Sun – and thereby identify possible elements of the Sun’s original cluster?

  • philw July 18, 2007, 17:07

    Too much time has passed. Over 16 revolutions around the galaxy with literally billions of gravitational influences. Plus, any stars more massive than the sun have burned out.

  • ljk September 21, 2007, 11:14

    Is our Sun a Singleton?

    Authors: D. Malmberg, M. B. Davies, J. E. Chambers, F. De Angeli, R. P. Church, D. Mackey, M. I. Wilkinson

    (Submitted on 19 Sep 2007)

    Abstract: Most stars are formed in a cluster or association, where the number density of stars can be high. This means that a large fraction of initially-single stars will undergo close encounters with other stars and/or exchange into binaries. We describe how such close encounters and exchange encounters can affect the properties of a planetary system around a single star.

    We define a singleton as a single star which has never suffered close encounters with other stars or spent time within a binary system. It may be that planetary systems similar to our own solar system can only survive around singletons. Close encounters or the presence of a stellar companion will perturb the planetary system, often leaving planets on tighter and more eccentric orbits. Thus planetary systems which initially resembled our own solar system may later more closely resemble some of the observed exoplanet systems.

    Comments: 2 pages, 1 figure. To be published in the proceedings of IAUS246 “Dynamical Evolution of Dense Stellar Systems”. Editors: E. Vesperini (Chief Editor), M. Giersz, A. Sills

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0709.3101v1 [astro-ph]

    Submission history

    From: Daniel Malmberg [view email]

    [v1] Wed, 19 Sep 2007 20:17:54 GMT (18kb)

    http://arxiv.org/abs/0709.3101

  • ljk December 21, 2007, 13:28

    UV Radiation Fields Produced by Young Embedded Star Clusters

    Authors: M. Fatuzzo, F. C. Adams

    (Submitted on 20 Dec 2007)

    Abstract: A large fraction of stars form within young embedded clusters, and these environments produce a substantial ultraviolet (UV) background radiation field, which can provide feedback on the star formation process. To assess the possible effects of young stellar clusters on the formation of their constituent stars and planets, this paper constructs the expected radiation fields produced by these clusters. We include both the observed distribution of cluster sizes $N$ in the solar neighborhood and an extended distribution that includes clusters with larger $N$. The paper presents distributions of the FUV and EUV luminosities for clusters with given stellar membership $N$, distributions of FUV and EUV luminosity convolved over the expected distribution of cluster sizes $N$, and the corresponding distributions of FUV and EUV fluxes. These flux distributions are calculated both with and without the effects of extinction. Finally, we consider the effects of variations in the stellar initial mass function on these radiation fields. Taken together, these results specify the distributions of radiation environments that forming solar systems are expected to experience.

    Comments: Accepted for publication in ApJ

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0712.3487v1 [astro-ph]

    Submission history

    From: Marco Fatuzzo [view email]

    [v1] Thu, 20 Dec 2007 17:50:57 GMT (54kb)

    http://arxiv.org/abs/0712.3487

  • ljk August 21, 2008, 0:21

    Telescopes versus Microscopes: the puzzle of Iron-60

    Authors: Jonathan P. Williams

    (Submitted on 19 Aug 2008)

    Abstract: The discovery that the short-lived radionucleide iron-60 was present in the oldest meteorites suggests that the formation of the Earth closely followed the death of a massive star.

    I discuss three astrophysical origins: winds from an AGB star, injection of supernova ejecta into circumstellar disks, and induced star formation on the boundaries of HII regions. I show that the first two fail to match the solar system iron-60 abundance in the vast majority of star forming systems. The cores and pillars on the edges of HII regions are spectacular but rare sites of star formation and larger clumps with masses 1e3-1e4 solar masses at tens of parsec from a supernova are a more likely birth environment for our Sun.

    I also examine gamma-ray observations of iron-60 decay and show that the Galactic background could account for the low end of the range of meteoritic measurements if the massive star formation rate was at least a factor of 2 higher 4.6 Gyr ago.

    Comments: to appear in the proceedings of the Les Houches Winter School “Physics and Astrophysics of Planetary Systems”, (EDP Sciences: EAS Publications Series)

    Subjects: Astrophysics (astro-ph)

    Cite as: arXiv:0808.2506v1 [astro-ph]

    Submission history

    From: Jonathan Williams [view email]

    [v1] Tue, 19 Aug 2008 00:16:20 GMT (2250kb)

    http://arxiv.org/abs/0808.2506

  • gavin immerson November 20, 2008, 7:06

    Absolutely, wonderful name, (Dr.?) Martin Bizzaro! One yearns for a colloquim incl. that other renown space scientist Dr. Bunsen and his top tech Beaker.