if you say God I'm gonna be very disappointed

August 16, 2005 12:01 AM

Where do heavy elements come from?

Everything in our planet comes from the center of a star, right? But don't stars generally cease fusing new elements around the time they hit iron? Where do elements like uranium come from?

posted by clockzero to science & nature (13 comments total)

Supernovas is where the heavier elements come from.
posted by stoneegg21 at 12:07 AM on August 16, 2005

Supernovas are. Gaah.
posted by stoneegg21 at 12:07 AM on August 16, 2005

What you're looking for is r-process, or rapid neutron capture. Both slow and rapid neutron capture can occur in normal stars but its a rare occurence and many of the intermediate elements will quickly decay before additional neutrons are captured.

So, some uranium, very little, is formed in normal stars but the bulk of it is formed in environments with large numbers of free neutrons - i.e. supernovas. Preceding a supernova event, the overheated iron core actually starts disintegrating into alpha particles and neutrons. Many of these neutrons are captured by some of the other iron atoms to form a rapid buildup into elements like uranium.

Then, of course, the whole thing explodes. And becomes part of the next generation of normal stars which will already have uranium they inherited from the interstellar material from which they formed. And fragments will spin out into planets...
posted by vacapinta at 12:52 AM on August 16, 2005

Synthetic elements, like Neptunium and Americium, are generally created by artificially creating the same process. Americium, for example, is created by bombarding Plutonium-239 with neutrons inside a nuclear reactor. It becomes Pu-240, then Pu-241, which then decays into Am-241. It's then sliced up into teensy pieces and put into smoke detectors.
posted by Plutor at 5:50 AM on August 16, 2005

vacapinta spake.
For further info there's an excellent and fairly light (sub-200-page) book by John Gribben called "stardust" which goes through all this and a good deal more.
(BTW, He's an established and widely respected writer of explanatory popular science, so though he knows his stuff, he's also practised in the art of not bewildering his readers too much.)
posted by NinjaPirate at 6:09 AM on August 16, 2005

Love the title, by the way.
posted by Civil_Disobedient at 6:46 AM on August 16, 2005

Simon Singh's book Big Bang has a big section explaining how hard it was to even explain the formation of anything larger than helium. I was surprised by that — apparently it's quite difficult to get the maths to work out.
posted by smackfu at 6:48 AM on August 16, 2005

Does the star-based generation of elements do anything to confirm or muddy up the various scientific estimates of the age of the universe? I've often wondered how many generations of stars had to live and die for our solar system to form. I take this page to imply that the early universe must have had lots and lots of stars that would have absolutely dwarfed (no pun intended) ours; is that the overall consensus?
posted by kimota at 6:50 AM on August 16, 2005

Years ago I read that there is so much energy in these stars that when they explode, they give off a hundred times more energy in ten seconds, from an area the size of a small city, than our sun has given off in thirteen billion years. That amout of energy is difficult to comprehend.
posted by weapons-grade pandemonium at 10:00 AM on August 16, 2005

There was a great table in some Larry Niven book with scales of energy, from "fly taking off" to "Big Bang". Supernovae were pretty far down on the list, along with "quasar, lifetime output". Alas, I can't find it on the Web.
posted by Aknaton at 10:12 AM on August 16, 2005

And at the tiny end, (as Carl Sagan said) all the astronomical information we have gathered from all radio telescopes combined was gleaned from an amount of energy equivalent to a snowflake hitting the ground.
posted by weapons-grade pandemonium at 11:11 AM on August 16, 2005

When it comes to understanding the origin of elements much heaver than iron, however, scientists can reconstruct much of what must have happened, but the astrophysical factory has not been clearly identified. Intermediate-mass elements are made in a neutron-rich environment in which successive neutron captures occur slowly, and neutron-rich nuclei undergo beta decay back to more stable elements. Still heavier nuclei must have been made by a succession of rapid neutron captures, referred to as the r-process. A dense, highly neutron-rich environment must exist for the r-process to occur. Also seen in the abundances are the traces of other mechanisms, including possible evidence of nucleosynthesis induced by neutrinos. The element fluorine, for example, can be made by neutrinos interacting with supernova debris. In fact, it is strongly suspected that supernovae, once again, must be the place where the remaining elements up to uranium are built, but there is no detailed understanding of how the process occurs. Resolving this problem requires observational data from supernova remnants, experimental data from both nuclear physics and neutrino physics, and the ability to make detailed, fully three-dimensional, theoretical calculations of supernova explosions.[Source]
posted by kickingtheground at 11:43 AM on August 16, 2005

Thanks, everyone. Especially vacapinta and Civil_Disobedient.
posted by clockzero at 11:57 AM on August 16, 2005