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Kebsis
2004-Jul-07, 07:09 PM
It's my understanding that a massive star can only fuse elements up to a certain Iron isotope, at which point it becomes impossible for the star to fuse any further and it collapses into either a neutron star or a blackhole. Correct me if I'm wrong about this.

But if that's the case then where do elements heavier than iron come from?

ToSeek
2004-Jul-07, 07:14 PM
It can fuse further, but it actually costs energy rather than creating it. I think most of the heavy elements are created as a side effect of supernovas, when the star is destroying itself, anyway.

beskeptical
2004-Jul-07, 07:20 PM
Supernovas (http://www.dailycal.org/article.php?id=985)

Eta C
2004-Jul-07, 07:20 PM
Supernovae. Fusion beyond iron is possible, but it requires energy to drive it (endothermic rather than exothermic fusion). Once fusion to iron is complete, the radiation pressure holding the star up against gravity ends and the star collapses (note, this requires a star about 3X the mass of the sun. Lighter stars don't becom supernovae; after red giant expansion, a white dwarf star remains). During this collapse, endothermic fusion creates the heavy elements which are then blown into space together with the rest of the supernova debris. These elements eventually get incorporated into new stellar systems. Hence Carl Sagan's rather poetic comment that all of us here on Earth are made of "star stuff."

beskeptical
2004-Jul-07, 07:24 PM
Supernovae. .....Darn, I beat you but used the wrong plural spelling. :lol:

Eta C
2004-Jul-07, 07:27 PM
That's OK, I left the "e" off become. :oops:

Grand_Lunar
2004-Jul-11, 08:38 AM
Supernovas (http://www.dailycal.org/article.php?id=985)

I like the article's use of the trampoline as an analogy to what makes a star go supernova. I assume that the explosion acts like a particle accelerator, smashing nucli together, thus making elements heavier that iron, right? To think that we contain material from inside a star!

Normandy6644
2004-Jul-11, 02:08 PM
Yeah, as has been said, Iron 56 is the most stable isotope in the universe, and that's where stars generally stop fusing their interior elements. Because there is no added energy from this process, the star's gravity begins to take over, collapsing it. Once the radiating pressure builds up again, depending on the mass of the star you get either a planetary nebula/white dwarf, supernova/neutron star, or supernova/black hole. The heaviest elements found in nature probably form as a byproduct of the explosions. Keep in mind that there isn't much in the way of heavy elements in the universe, so it is conceivable (and probable) that they all formed this way over billions of years.

Crimson
2004-Jul-11, 03:37 PM
Actually, supernovae are not the whole story. Many heavy elements, such as strontium, yttrium, and zirconium, arise primarily through the s-process, which occurs in red giants and supergiants. In fact, historically, the s-process ("s" for "slow") was discovered BEFORE the r-process ("r" for "rapid"), which is thought to occur in supernovae.

For more on the origin of the heavy elements, especially the s- and r-processes, see the title chapter of Ken Croswell's book The Alchemy of the Heavens. (http://KenCroswell.com)

Tobin Dax
2004-Jul-11, 04:55 PM
Yeah, as has been said, Iron 56 is the most stable isotope in the universe, and that's where stars generally stop fusing their interior elements. Because there is no added energy from this process, the star's gravity begins to take over, collapsing it. Once the radiating pressure builds up again, depending on the mass of the star you get either a planetary nebula/white dwarf, supernova/neutron star, or supernova/black hole. The heaviest elements found in nature probably form as a byproduct of the explosions. Keep in mind that there isn't much in the way of heavy elements in the universe, so it is conceivable (and probable) that they all formed this way over billions of years.

Only stars that go supernova produce iron (and only when they involve core collapse). A lower mass star that will become a supernova will not create elements as massive as iron. They will stop fusion earlier than that.

Normandy6644
2004-Jul-11, 05:36 PM
Yeah, as has been said, Iron 56 is the most stable isotope in the universe, and that's where stars generally stop fusing their interior elements. Because there is no added energy from this process, the star's gravity begins to take over, collapsing it. Once the radiating pressure builds up again, depending on the mass of the star you get either a planetary nebula/white dwarf, supernova/neutron star, or supernova/black hole. The heaviest elements found in nature probably form as a byproduct of the explosions. Keep in mind that there isn't much in the way of heavy elements in the universe, so it is conceivable (and probable) that they all formed this way over billions of years.

Only stars that go supernova produce iron (and only when they involve core collapse). A lower mass star that will become a supernova will not create elements as massive as iron. They will stop fusion earlier than that.

I know, I was just trying to give an overall picture. Sorry for the lack of clarity. :oops:

Brady Yoon
2004-Jul-11, 06:07 PM
Is there any way of creating heavier elements naturally that doesn't involve nuclear fusion, the s-process in red supergiants, or the r-process in exploding supernovae?

Crimson
2004-Jul-12, 03:15 PM
The s-process also occurs in red giants, which never explode. Thus, it is not true that massive stars produce all the heavy elements in nature. From page 101 of Ken Croswell's book The Alchemy of the Heavens (http://KenCroswell.com) (emphasis added):

"Burbidge, Burbidge, Fowler, Hoyle
Took the stars and made them toil:
Carbon, copper, gold, and lead
Formed in stars, is what they said.

"The 1957 research paper opens with words from Shakespeare, runs a hundred and four pages long, and reveals where gold, carbon, platinum, silver, oxygen, iron, and nearly every other element came from. It will tell you, for example, that the gold in the world's treasuries and the iodine added to salt were forged in the fiery explosion of a supernova, while most barium and zirconium on Earth slowly blossomed within the heart of a red giant star. By setting an audacious goal--to explain how and where in the universe every element from hydrogen to uranium had originated--and largely achieving that goal, the four authors of the paper created one of the greatest monuments in the history of science.

"`Originally,' said British astronomer Fred Hoyle, `we didn't intend anything more than a normal paper. But it grew and grew as the months slipped by, and eventually it was of such enormous length that only the Reviews of Modern Physics could handle it.' In the 1940s, when Hoyle began advocating the idea that the stars had created the elements, most scientists thought the idea preposterous, but by the mid-1950s the discovery that old stars had lower metallicities than young stars had convinced many astronomers that he was right--that the Milky Way, through its stars, was the true creator of the elements."

Another process that can produce heavy elements is the p-process, (http://nedwww.ipac.caltech.edu/level5/Sept01/Meyer/Meyer5_2.html) in which protons rather than neutrons bombard nuclei, presumably during a supernova. This is a much harder way to make elements, because protons are naturally repulsed by the positive charge of the nuclei; however, the p-process probably does account for a decent fraction of molybdenum (atomic number 42), according to Anders and Grevesse (1989). (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1989GeCoA..53..197A)

tofu
2004-Jul-12, 08:40 PM
is there any possible way to guess how many stars had to die to give our solar system its diversity of elements?

Russ
2004-Jul-12, 09:05 PM
But if that's the case then where do elements heavier than iron come from?
Dunkin Donuts. :)

Crimson
2004-Jul-13, 12:41 AM
is there any possible way to guess how many stars had to die to give our solar system its diversity of elements?

Crimson would guess lots--as in millions, possibly even billions.

How many supernovae exploded in the Milky Way during the approximately 10 billion years that elapsed from its birth to the birth of the Earth? If three supernovae explode in the Galaxy every century, then that's 300 million supernovae.

How many planetary nebulae shed material into the Milky Way during the same 10 billion years? If one planetary nebula is born each year, which this reference (http://aa.springer.de/papers/9342002/2300426/sc7.htm), the result of a Google search on "PN birth rate," cites, then that's 10 billion planetary nebulae.

So we're up to 10 billion planetary nebulae and 300 million supernovae that contributed elements to the Milky Way before the Sun was born and that may have contributed to the solar system.

Now the hard part. What fraction of those planetary nebulae got mixed into the cloud of gas and dust that formed the Sun and the Earth? And what fraction of those supernova remnants got mixed into that cloud of gas and dust? Beats Crimson, but a supernova explodes with such force that some of its debris may enrich a substantial fraction of the Galaxy. We are, right now, in a small interstellar cloud hurled our way by a supernova that exploded around Antares, which is 600 light-years away. Shall we guess 1%? Planetary nebulae have less oomph, so each one presumably enriches a much smaller part of the Galaxy. Shall we guess 0.1%? Then we get 3 million supernovae and 10 million planetary nebulae, for a total of 13 million stars that contributed to your body.

And if you don't like those fractions, you can increase them up or down and get different answers. I mean, it's just like the Drake equation! :)

Gullible Jones
2004-Jul-13, 01:23 AM
Hmm... This s-process is one I've never heard of... The B^2FH team came up with it? I'll have to look this one up...

Crimson
2004-Jul-15, 02:25 PM
Here are heavy elements that are made more by the s-process (in red giants and supergiants) than by the r-process. This is based on Crimson's interpretation of the somewhat cryptic page 200 of Anders and Grevesse (1989). Asterisks denote elements that are made almost entirely by the s-process.

Gallium (atomic number 31)
Germanium (32)*
Krypton (36)
Strontium (38 )*
Yttrium (39)*
Zirconium (40)*
Niobium (41)*
Cadmium (48 )
Tin (50)
Barium (56)*
Lanthanum (57)
Cerium (58 )
Neodymium (60)
Mercury (80)
Thallium (81).

tofu
2004-Jul-15, 03:08 PM
Thanks Crimson. The question of how many stars contributed to us sounds like a good topic for a phd thesis.

Is there a website somewhere that lists all of the elements and exactly how each is made? Some of them are thought to be created in some really exotic process. I remember reading somewhere that platinum is created when two neutron stars collide.

Crimson
2004-Jul-15, 10:11 PM
Is there a website somewhere that lists all of the elements and exactly how each is made? Some of them are thought to be created in some really exotic process. I remember reading somewhere that platinum is created when two neutron stars collide.

No website to my knowledge has this information. The two best sources are, at a technical level, page 200 of the Anders and Grevesse (1989) (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1989GeCoA..53..197A) paper, and at a more layperson-friendly level, The Alchemy of the Heavens (http://KenCroswell.com). A good university library should have the Anders and Grevesse paper, and both Amazon (http://www.amazon.com/exec/obidos/ASIN/0385472145) and any good library should have The Alchemy of the Heavens.

For the specific case of platinum: according to page 200 of Anders and Grevesse and page 111 of The Alchemy of the Heavens, it is made almost entirely by the r-process. The r-process is often assumed to occur in supernovae, but the site of the r-process is not as well-known as it should be. The s-process is a lot easier to observe, because it's a slow process.

Gold is also made by the r-process--in fact, entirely by the r-process.

Even today, the origin of some elements is unknown. A good example is fluorine. (http://KenCroswell.com/fluorine.html)

Morrolan
2004-Jul-16, 04:08 AM
when you sit down and contemplate on this for a bit (as i just have) this really is one of the most stupendous processes imaginable... just thinking about the scales of energy and time involved in these elements ultimately ending up in our planet and (in the case of some of these elements) on your finger, makes my head spin. i'll never look at my wedding ring in the same way again... :o

thanks for the info, Crimson.

ngc3314
2004-Jul-16, 03:36 PM
when you sit down and contemplate on this for a bit (as i just have) this really is one of the most stupendous processes imaginable... just thinking about the scales of energy and time involved in these elements ultimately ending up in our planet and (in the case of some of these elements) on your finger, makes my head spin. i'll never look at my wedding ring in the same way again... :o

thanks for the info, Crimson.

It gets wilder on that ring. The difference between the colors of silver and gold, which have similar outer electron configurations and thus would normally have analogous optical properties, is attributed to the inner electrons in gold having velocities which are not trivial compared to c and thus different energy levels than a classical extrapolation from lighter elements would have indicated. [Nitpick - colloquial definition of velocity used here, feel free to substitute "binding energy a nontrivial fraction of mc^2]. Relativity on your finger (if not fingertips). I first encountered this tidbit in some fiction by Stephen Baxter and found it confirmed by actual physical chemists. The silver/gold pair is the best one to see such a difference because we're familiar with both in a fairly pure atomic form and both have outer energy levels such that they're reflective across good slices of the optical spectrum (so the shift is obvious to our eyes). The cadium/mercury comparison is mseed up by the whole liquid issue, and lead doesn't have wonderful optical properties. Heavier than that isotopes tend to glow.

Crimson
2004-Jul-19, 05:56 PM
Table 2 of Arlandini et al. (1999) (http://www.journals.uchicago.edu/ApJ/journal/issues/ApJ/v525n2/40275/40275.html) states how much the s-process contributes to the heavy elements. According to this table, silver, gold, and platinum arose mostly by the r-process, as follows:

Silver: 79% r-process, 21% s-process
Gold: 95% r-process, 5% s-process
Platinum: 95% r-process, 5% s-process.

As a reminder, the r-process means the formation of elements via rapid bombardment of nuclei by neutrons, as presumably happens in supernovae. The s-process means the formation of elements via slow bombardment of nuclei by neutrons, as happens in red giants and supergiants.