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Q: Astronomy ( Answered,   0 Comments )
Subject: Astronomy
Category: Science > Astronomy
Asked by: ceowang-ga
List Price: $5.00
Posted: 03 Dec 2002 21:43 PST
Expires: 02 Jan 2003 21:43 PST
Question ID: 118903
How were the chemical elements past Iron on the periodic table formed
in the universe?
Subject: Re: Astronomy
Answered By: kutsavi-ga on 03 Dec 2002 22:30 PST
Hey there Ceowang, 

I have found three excellent, verifiable sources for you on elemental
formation, one from the National Optical Astronomy Observatories, the
second from NASA, and the third from the University of Dublin, Trinity

First, here's a very readable article on elemental formation from the
National Optical Astronomy Observatories in Tucson, AZ.
(Quote follows)

Ancient Stars in Milky Way Reveal Colorful Epochs of Heavy Element
Formation, by Douglas Isbell and Kathi Etherton

Astronomers studying how elements heavier than iron were produced in
the early Milky Way have identified a distinct series of epochs of
galaxy-wide chemical formation.

This evolutionary timeline, stretching from the Big Bang onward for
several billion years, has the potential to serve as a cosmic
"fingerprint manual" that could help astronomers categorize some of
the quirky high-redshift galaxies seen in recent samples of the
ancient Universe, such as the Hubble Deep Field, and in deeper sky
surveys to come.

The research team looked at nearly 100 stars in the halo around the
Milky Way, carefully selecting them to be relatively nearby, old and
at least 10 to 100 times depleted of metals as compared to our Sun.
Subsequent plots of elemental ratios in the stars revealed obvious
trends over time that allow a larger chronology to be developed.

"This is one of the largest studies yet of the abundances of heavy
elements in galactic halo stars," says Debra Burris of Oklahoma City
Community College, lead author of a paper scheduled for the November
20 issue of The Astrophysical Journal. "The behavior of the trends in
abundances of these elements give us major clues about the conditions
and populations of stars that existed early in the Milky Way's

"Our results tell us that the history of the galaxy is tied very
closely to the ways that stars change from generation to generation,"
explains co-author Catherine Pilachowski of the National Optical
Astronomy Observatory (NOAO) in Tucson, AZ. "Certain chemical elements
don't form until the stars that make them have had time to evolve.
Therefore, we can read the history of star formation in the
compositions of the oldest stars."

"It's extremely difficult to accurately age-date a star. The chemical
signatures give us an effective chronometer that we can use to probe
the earliest epochs," says co- author Taft Armandroff of NOAO. "We
probably would not have found these trends if we did not have such a
large sample"

The research team developed this evocative timeline to explain their

The Pre-Stellar Epoch 
The Big Bang jumpstarts the initial large-scale production of
hydrogen, deuterium, helium and lithium.

The Epoch of Very Massive Stars 
The earliest stages of heavy element formation in the Galaxy were
dominated by stars with masses ten times that of the Sun or more, and
lifetimes of a few million years or less. These supermassive stars
produced small amounts of all the elements, but their presence can be
identified most clearly by excesses of elements like strontium,
yttrium and zirconium. Released by supernovae and absorbed by new
star-forming clouds, these elements were incorporated into the next
generation of stars

The Europium Epoch 
For the next 30-100 million years, element formation was dominated by
supernovae from stars with about 8-10 times the mass of the Sun. These
longer-lived stars enriched the Milky Way in heavier elements like
barium, europium, and other lanthanide elements in the Periodic Table,
such as cerium.

The Double Shell Epoch 
A major shift from previous epochs, lasting from about 100 million to
a billion years after the Galaxy formed, it featured stars with
perhaps 3-7 times the mass of the Sun. These stars produced more
strontium, barium, and some particular lanthanides from
nuclear-burning interior shells during the later stages of their
evolution, not by supernovae. Their products are characterized by more
solar-like distribution of heavy elements.

The Iron Epoch 
From one billion to three billion years after the Galaxy formed,
supernovae from white dwarf stars a bit larger than the Sun produced
large amounts of iron. The addition of large amounts of iron to the
Milky Way's chemical stew can be deduced by the relative decrease of
heavier metals within stars which hold about 1/100th of the Sun's
overall metal abundance.

Since this epoch, which ended roughly 10 billion years ago, the major
[addition] to the Galaxy's inventory of heavy elements has been
lithium, but the exact source is unknown.

The measurements used to develop these epochs were obtained with
spectrographic instruments on the National Science Foundation's Mayall
4-meter telescope and Coude-Feed telescope operated by Kitt Peak
National Observatory near Tucson.

Because even the largest telescopes with the latest cutting-edge
technology do not have the ability to resolve individual stars in
ancient, high-red shift galaxies, detailed spectrographic measurements
of the oldest stars in the Milky Way and other local group galaxies
may be the only way we can study element formation in the very early

"The fossil structure of the Milky Way tells us about a time even
earlier than the most distant galaxies yet discovered," Pilachowski
notes. "But as we find galaxies at higher and higher redshifts, we
will eventually be able to investigate galaxies similar to what the
Milky Way must have looked like during these early epochs."

In the future, team members may also study the relative motions of the
same sample of stars to try and deduce whether they originate from any
common groups or "streams" of stars. Such information could help
inform debates about whether the Milky Way began as a loose cloud of
gas that formed stars and then spun down into the spiral disk we know
today, or whether a more clumpy structure analogous to dwarf galaxies
came first. In this model, the dwarf-like galaxies were torn apart and
then merged by tidal forces into the current disk.

"Ultimately, we hope to answer questions like 'How long did the
initial burst of star formation in the Galaxy last, 100 million years
or five billion years?'" Armandroff explains. "There are a lot of
interesting clues in the local population."

Other co-authors of the paper are Christopher Sneden from McDonald
Observatory and the University of Texas, John Cowan from the
University of Oklahoma and Henry Roe from the University of California
at Berkeley.

A sample spectrum used in this research, with labels pointing to key
elements, is available electronically on the Internet at:

A PDF version of the Periodic Table of the Elements is available at:

NOAO is operated by the Association of Universities for Research in
Astronomy (AURA), Inc., under cooperative agreement with the National
Science Foundation. The NSF also supported this research through
grants to several co-authors.


Another source, NASA's web site on the ACE Mission, (Advanced
Composition Explorer, launched 1997), has much more technical
information on the different formational environments and processes. 
The PP and CNO cycles referred to below are mathematical explanations
of different formational processes:  PPO, (Proton-Proton Cycle) refers
to the sequence of reactions for nucleosynthesis utilizing hydrogen. 
This is the model used by small and medium stars of comparatively low
mass, and a series of equations is given outlining the sequence.  CNO
refers to the Carbon-Nitrogen-Oxygen Cycle, occurring on more massive
stars at temperatures above 15 million degrees C.  You'll have to go
to the web site in order to make sense of all the equations and such.
(Quote follows)

The PP cycle and the CNO cycle compete with each other for the purpose
of producing energy on stars. The two factors that determine which
process will occur are the availability of the fuels, and the
temperature. The CNO cycle cannot occur if those isotopes are not
available on the star. Since only a very small amount of these
isotopes is required, this condition is often fulfilled and
temperature becomes the determining factor. Stars with masses five to
ten times the mass of our Sun possess temperatures high enough for the
CNO process to dominate. Most of the heavy elements from oxygen up
through iron and nickel are thought to have come from fusion on stars
of this type.

Both the PP cycle and the CNO cycle occur without the influx of any
additional energy. The already high temperatures and pressures found
on these stars is sufficient for the reactions to occur. To produce
all of the heavier elements (the elements above iron and nickel on the
periodic table), energy must be put in for the fusion to occur. For
that reason these reactions are referred to as endothermic reactions.
Temperatures are sufficient for this to occur in the explosion of a
large star, a supernova, and most of the heavier elements are produced
there. But, this is a relatively rare occurrence, there is a
comparatively low percentage of the heavy elements found in the

Another source, "Earth Wind and Fire; A Catastrophic View of Geology"
by Philip Allen and Alex Densmore at University of Dublin, Trinity
College, has this to say in a more simplistic way:

In the largest stars (> 6 solar masses), this chain of reactions
continues until Fe is formed (at temperatures of about 1 billion K).
The pressure and temperature within the star can never get high enough
to burn Fe, so at this point the chain stops. The star again
collapses, but this time there is nothing to slow the collapse. The
temperature rapidly rises to as high as 10 billion K. The extremely
high pressures and temperatures force protons (p) and electrons (e)

[image of calculation]

forming neutrons and liberating lots of energy (n), all on a timescale
of a few seconds. This huge energy release blows off the outer layers
of the star at velocities up to 3e7 m/s (0.1c, or 1/10th the speed of
light) - a supernova. In the turbulent eddies created by the supernova
shock wave, P and T can get high enough to form all the elements of
the periodic table. Thus all elements above Fe were formed during
supernovae. The sun is neither large enough nor old enough to have
produced many heavy elements; the fact that it does is evidence that
the sun is itself composed of the remains of previous, long-dead stars
- cosmic recycling!

formation universe elements above iron periodic table

Thanks for a very fun question!
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