[meteorite-list] Chondrule formation mechanism (Info Please)

From: Sterling K. Webb <sterling_k_webb_at_meteoritecentral.com>
Date: Fri Oct 27 02:31:14 2006
Message-ID: <00c701c6f991$74460d80$a5714b44_at_ATARIENGINE>

Hi, Rob, Pete, Ed, List,

Rob wrote:
> The iron is formed in the cores of all stars.
> Nuclearly speaking it is the stablest of all elements
> (lowest binding energy per neucleon...or is it the
> highest, can't remember)

    I hate it when I have to dive into thick books more
suited for anchors than reading but here goes...

    Not all stars form iron. The one thing that determines
the entire life of a star is how fat it is. An anorexic "star"
is just another Jupiter or Super-Jupiter. At somewhere
around 12-13 times the mass of Jupiter, a star starts to
burn deuterium and we can really call it a star.

    Stars "burn" hydrogen. Deuterium is just regular
hydrogen toting a neutron in its backpack. Slap two
of them together and you get helium (and a lot of excess
energy). All stars, regardless of size, start out as hydrogen
burners. The D-D chain is the easiest reaction to get
started but there are lots of routes from hydrogen to
helium that use other elements for their intermediate
stages (called proton-proton reactions) and I'm not
going to type them all out. So there.

    Fast forward a few billion years. A star will use up
all of its hydrogen. About the time it's running on fumes,
the helium "ash" left over from burning up all your hydrogen
like there was no tomorrow has sunk to the core and is
getting hotter and denser. Eventually, that helium in the
core starts to burn. Now, the star is a helium-burner.

    This nuclear heat generated in the helium-burning core
causes the star to expand and expand and expand into
a big gasball many times its original size: a red giant.
A star has to be at least half the mass of our Sun to do
this. Our Sun will do this... in another 4-5 billion years.
Goodbye, Solar System.

    A helium burner this big will evolve carbon12-burning.
Again there are many possible reactions, but most of
the carbon is turned directly into oxygen16. As things get
hotter, we get neon20, magnesium24, silicon28, each one
is produced by slapping ("fusing") a helium nucleus into
the last one, hence the jump by 4, 4, 4, 4...

    Now, a nice little star like our Sun will just end up as
a bright superdense carbon12 diamond a few thousand
miles across, called a white dwarf. But if the mass of a
star is 1.4 times the mass of the Sun or greater, it will
just go crazy with this fusion stuff. The end result is a
star with an "onion" structure: an outer shell of hydrogen
burning surrounding a shell of helium burning, surrounding
a shell of carbon burning, surrounding a shell of neon
burning, surrounding a shell of oxygen burning, surrounding
a shell of silicon burning, surrounding a core where the
really weird stuff goes on.

    Silicon burning should proceed until iron is built, but
it doesn't happen. By this time the heat, pressure and
energies involved are so great that the LIGHT produced
by the fusion becomes more powerful and energetic than
all the other players! As soon as a nuclei heavier than silicon
is produced, a photon on steroids knocks it apart, slaps
it down, and kicks it around until it gives up those extra
nucleons and crawls off in all its silicon shabbiness. Iron
may get formed but it doesn't last.

    And, yes, iron has the HIGHEST binding energy per
nucleon and a high electric charge barrier, but the real
problem is that the photons produced by creating it are
energetic enough to rip it apart. If you want to picture the
true violence of a stellar interior, try imagining a beam of
light powerful enough to smash atoms... OK, they're
super-gamma rays, but they're still just light.

    The iron (and nickel) core forms inside the silicon
burning shell as some of the iron continually being formed
escapes from the cycle of birth and instant photo-death
by "dripping" down out of sight in the core as it forms.
But the iron core is doomed. Eventually the mass and pressure
of the star's outer layers collapse the core and compress
it so much that the star crushes its own core hard enough
to "squeeze" the electrons in its atoms into the protons in
those atoms and turn them into neutrons. The body of the
star is blown away and there's a 20-mile lump of neutrons
left behind, almost pure "neutronium" with a thin crust of
hot diamond and some silicon, perhaps.

    This is how a universe that started out as nothing but
hydrogen, helium, and a smidge of deuterium got so
interesting, because core collapse triggers those nasty
supernovae. Supernovae come in two varieties: Regular
Really Nasty, and Extra Special Super Nasty, sometimes
boringly called Type I and Type II. The supernova
explosion is triggered by the collapse of the iron core ("I
just can't take it any more!")

    Oddly, core collapse is very, very orderly*, not the mad
chaos you'd think would happen, not at all. Entropy is constant
and the core is in perfect equilibrium the whole milliseconds long
duration of its collapse. Collapse starts when the core reaches a
density of about 1,000,000,000 gm/cm3. Nothing can stop the
collapse until the density reaches 270,000,000,000,000 gm/cm3
when the core is now one immense elemental particle, a single nucleon
miles across, at zero internal pressure, having achieved its true
happiness as a particle. Unfortunately, momentum wants it
to try to collapse further; it fails, and the core bounces back.

    The mild slap of its rebound will knock 10 or 100 solar masses
of star away at a goodly fraction of the speed of light, releasing
100,000,000,000,000,000,000,000,000,000,000,000,000 ergs of
energy, a mere 1% of the core's rebound force. The Regular
Really Nasty supernova core remains behind as a neutron star,
and the Extra Special Super Nasty supernova core retreats
from the universe altogether, becoming a Black Hole. It all
depends on the mass of the star.
    ( * New study on "orderly" supernova:
http://www.space.com/scienceastronomy/061026_exploding_star.html)

    So how do we get all the elements heavier than iron? Not
directly by fusion of lighter elements; that can't happen. No,
it's by two processes that go on in the exotic environment of
big energetic stars. There's always plenty of neutrons hanging
around in such places. A nucleus swallows a neutron, spits up
an electron ("beta-decay"), and advances one square to the
next isotope on the game board.

    But all nuclei are different; some will transform rapidly
("r-process"); some will only do it slowly ("s-process").
There are 27 isotopes that the r-process can't produce, but
the s-process can, blah, blah. Somehow they all get made,
most of them in the final short time before the star's death.
Oddly enough for something so huge, stars are remarkably
alike, and the mixture of elements in most stars is very much
like our Sun's (the "cosmic abundances").

    Ed raised the question of dating the age of the actual
elements themselves. Originally, hydrogen is the great
grand-daddy of elements, along with some of the helium.
Everything else is made in stars. If the universe started out
with only hydrogen and helium, the earliest stars would not
have had anything else in them. No stellar process makes hydrogen
atoms, so almost all the hydrogen you meet (like the hydrogen
walking around in its disguise as a fellow human being) is as
old as the Universe itself!

    The lifetime of a star depends entirely on its mass and
almost nothing else. The heaviest known star is HD 93250
at 120 solar masses. It'll be gone in a few years (its lifetime
is about 600,000 to 700,000 years). On the other hand, a tiny
0.10 solar mass red dwarf will last about 10 trillion years!
(Lifetime = ( 1 / SolMasses^(K-1) ) * 10^10 years, where
K is 3 for big stars and 4 for small stars)

    When a star novasplodes, it blows a mix of elements
out as the gas and dust from which new stars will be made.
Later stars will start out as a mix of elements and make
more. The newest stars should have a richer mix of elements
than our old Sun. And this is what we find: old red dwarves
that are mostly hydrogen and wildly metal-rich new stars
and everything inbetween.

    The age of the actual elements? Why, they're still being
made today! And some are nearly as old as the universe.
Which, BTW, is 13.7 +/- 0.5 billion years old. Judging the age
and mix of all the elements, our Milky Way Galaxy calculates
as 10.16 billion years old, our solar system (planets) as 4.6
billion (with the Sun about 160 million years older). Using
other measures, globular clusters, the oldest groupings of
stars, seem to be 11 billion years old based on their mix
of age and size of stars.

    But when the researchers teased a few atoms of iron60
out of the Pacific mud from only 2.5 million years ago, those
atoms were "made" from scratch in some supernova less than
5 million years ago. So, while you and your hydrogen atoms
are venerable and ancient, your car is made of younger stuff...
Even if it is 5 billion years old. (Time to trade it in!)

    As for chondrules (at last!), the theories are many, the
facts are few. The theories of their origin are: by impact
melting from very early planetesimals, the product of a very
hot inner solar nebula, by ablation of small objects, by an
energetic outburst of the Sun, by bipolar solar outflows,
by magnetic flares, by nebular lightening, by shock waves
in accretion or some other nebular process, or by shock from
a nearby supernova. Lots of theories to choose from (limit
three to a customer).

    I think Derek Sears' theory is clever and well-thought-out
and ingenious and probably wrong. He supposes that the
resonant orbits from which the Earth receives its many
chondrites are wall-to-wall with condrite parent bodies, that
these bodies are the ONLY chondrite bodies there are, that
they are few and rare, that Earth's meteorite population is
specific and unique, that chondrules and their accreted
chondrites were a rare and unique by-product of the early
solar system and not representative of early solar materials
at all. In other words, aren't we special...?

    Very narrow zones of unique chondrite parent bodies
implies both an early solar system and a present asteroid
belt that is very tightly zoned. In other words, the Earthly
prevalence of chondrites would just be a coincidence.
The evidence is that the asteroid belt is a gumbo, though,
full of all sorts of things that "don't belong" there. The
failure to find obvious sources for chondrites in the asteroid
belt is one of the great nagging problems that has never
been answered well, so he may have something. I'm just
not sure what.

    Sears says one advantage of the theory is that otherwise
the energy required to flash melt a solar system full of
chondrules is a major fraction of the total energy available.
Of course a precursor supernova that melted them would
take care of that problem, too. Supernovae have a way of
making short work of both problems and non-problems alike!
The nearest short-term supernova candidate is HR8210 or
IK Pegasi, which is incomfortably close at 150 light years.
http://www.eso.org/outreach/eduoff/edu-prog/catchastar/casreports-2004/rep-310/
and
http://www.newscientist.com/article.ns?id=dn2311
Of course, it could take millions of years to go super,
or it could happen in 10,000 years, or it could start up
tomorrow.

    That's what makes life so interesting.


Sterling K. Webb
-----------------------------------------------------------------------
----- Original Message -----
From: "Rob McCafferty" <rob_mccafferty_at_yahoo.com>
To: "Pete Pete" <rsvp321_at_hotmail.com>; <meteorite-list@meteoritecentral.com>
Sent: Wednesday, October 25, 2006 2:52 PM
Subject: RE: Re : [meteorite-list] Chondrule formation mechanism (Info
Please)


>I suppose you are correct. I suspect the iron flecks
> in chondrites must be stellar relics.
>
> The iron is formed in the cores of all stars.
> Nuclearly speaking it is the stablest of all elements
> (lowest binding energy per neucleon...or is it the
> highest, can't remember)
> So as a consequence it is the final fusion product in
> the cores of all stars which are heavy enough to get
> that far (red dwarf stars aren't considered massive
> enough to get beyond the helium burning phase).
> However, only supernovae spread their innards out at
> the end so every atom of iron was created by a
> supernova as indeed was every atom that isn't
> hydrogen, helium or lithium. All others are created in
> stars. However, the atoms higher in the periodic table
> cannot be made in stars as they require a net input of
> energy to fuse whereas the lighter ones relase energy.
> Only in a huge energy surplus can you manufacture
> these higher elements. This is where the supernova
> comes in. In that brief period where the star
> aoutshines an entire galaxy, there is enough excess
> energy to create quantities of elements up to Uranium
> (and possibly beyond but non of these are stable).
> This is a most wonderful process which not only
> creates all the elements needed for life but also
> seeds the universe with them.
> And not a crackpot creationist theory involving
> venting asteroids into space in sight.
>
> As for the ages of the iron/nickel. I'm not sure if
> ages are measured or if they can be. That'd be
> interesting if they could. It's probable that our sun
> and solar system are not even second or third
> generation. The big stars last only a short period and
> there's been a long time for the cycle to repeat a few
> times.
>
> Rob McC
>
> --- Pete Pete <rsvp321_at_hotmail.com> wrote:
>
>> Hi, all,
>>
>> This discussion about chondrules is fascinating!
>>
>> Hoping not to digress off this topic too much, but a
>> question I have is
>> about the metal flecks (not the later-formed iron
>> meteorites) in any of the
>> stonies.
>>
>> Have they ever been given an estimated age?
>>
>> If the heavy elements, such as nickel and iron, are
>> created by a supernova,
>> and the chondrules are in theory formed much later
>> during the future
>> dynamics of our solar system's nebula, would it be
>> fair to say that the
>> metal flecks would be billions and billions
>> (apologies, Carl) of years OLDER
>> than chondrules?
>>
>> And that they came from a distance much further than
>> our solar system's
>> vicinity?
>>
>> Considering that the supernova is exploding outward
>> and the new elements'
>> density is thinning out very quickly, wouldn't it be
>> more likely that these
>> iron and nickel flecks that eventually found a new
>> home in our solar nebula
>> and meteorites have come from more than one,
>> probably a lot more, supernova?
>>
>> If so, why don't we see any remnants of any
>> supernova explosion in our
>> relative proximity? The Helix Nebula is the closest
>> to us, at 450
>> light-years!
>>
> http://images.google.ca/images?q=helix+nebula&hl=en&lr=&sa=X&oi=images&ct=title
>>
>> Not even a wisp left...
>> Are tiny, but very dense, nebulas even possible? I
>> can't imagine dust-bunny
>> nebulae.
>>
>> If not, would it be unreasonable to expect that our
>> planetary nebula could
>> have extended out to Centauri, where our closest
>> star neighbours are?
>> When I dwell on the "Pillars of Creation" photos
>> (Orion stellar-formation nebula,
>>
> http://hubblesite.org/newscenter/newsdesk/archive/releases/1995/44/image/a)
>>
>> that describes a small point being comparable to the
>> breadth of our solar
>> system, ~4.3 light-years to Centauri isn't that
>> far...
>>
>> Maybe the seldom-discussed/appreciated metal flecks
>> are the real gems in the
>> meteorites?
>>
>> Or, is the nebula in my head too dense that am I
>> just missing something
>> obvious?
>> How is my logic flawed?
>>
>> Cheers,
>> Pete
>>
Received on Fri 27 Oct 2006 02:30:51 AM PDT