A science post
Dec. 4th, 2008 10:58 am![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
beccastareyesI haven't done one of these for a while, and with Dr. Salpeter's death, I feel like I should talk a bit about some things he worked on.
So, it's story time! Today I tell the story about how we found out where the elements come from. Some astronomy, some nuclear physics. It'll be fun!
Okay, so let's start with the elements. We classify elements by the number of protons they have in their nucleus -- hydrogen has one, helium has two, and so on. There are 90 elements that occur regularly in nature, with nuclei that contain anywhere from 1 to 92 protons. Each element can have several different forms (isotopes), depending on how many neutrons it has. For example, hydrogen has hydrogen-1, which has no neutrons, hydrogen-2 (deuterium) which has one, and hydrogen-3 (tritium), which has two. The number is the total number of nucleons (protons and neutrons).
Now, isotopes can be stable or radioactive. Radioactive means that they will change into another element by giving off particles -- hydrogen-3 is radioactive and will change into helium-3 if you let it sit. Stable means they won't. Generally, radioactive elements decay in two ways -- they give off an alpha particle (a helium nucleus) if they are too heavy and need to lose mass, or they will convert a neutron to a proton (and give off an electron) if they have too many neutrons. (They can also give off a positron (an antimatter electron), and change a proton to neutron if they have too many protons, but this is a lot less common.) Generally, nuclei want to be somewhat light (there's a reason all the heavy elements aren't that stable), and about even in their proton-neutron number (though the big ones need more neutrons) -- remember that protons are positive and want to be as far from one another as possible. There's also another force (the weak nuclear force) that regulates the proton-neutron interaction that prevents you just cramming crazy numbers of neutrons into a nuclei. A good analogy is that a radioactive nucleus is a pencil balanced on its eraser, while a stable nucleus is one on its side. The radioactive nucleus isn't stable and is eventually going to 'fall back' to a more stable state.
One of the consequences of the fact atoms can convert protons to neutrons (and vice versa) through beta decay is that generally there's only one or two stable atoms with a certain mass. An atom with one nucleon could be a hydrogen-1 or a neutron, but the neutron will decay into hydrogen-1. An atom with two nucleons could be helium-2, hydrogen-2, or two neutrons, but both will decay into hydrogen-2. You can have sulphur-36 and argon-36, but all heavier elements will decay to argon, and lighter ones to sulphur. (Chlorine-36 can decay to either, being between them -- heads it's argon, tails it's sulphur).
Now, the reason I mention this is that it has consequences for making elements. The universe started as a hot mass of stuff. When things finally (a millionth of a second post-Bang) cooled down enough to make protons and neutrons, and then they won out over antiprotons and antineutrons*, the protons and neutrons could hit each other hard enough to stick and make heavier elements (this happened for about 20 minutes starting three minutes after the Bang). Thing have to be hot for this to happen, since protons repel each other, and neutrons aren't stable. So, you have to shove them hard enough to overcome the fact the protons are repelling each other, until you get them close enough that they stick and make a hydrogen-2 nucleus by turning a proton to a neutron. You can then either shove another proton at the hydrogen-2 to make helium-3, or shove two hydrogen nuclei at each other to make helium-4. Our Sun is doing this right now in its core -- since a helium-4 nucleus is lower energy than the four protons that make it up, this releases energy, which eventually gets to the surface, giving us light and heat and wonderful things.
* No, I don't know why they did. Everything tells us that there should have been exactly equal numbers made. If you have a scientific reason that we can test, please tell me so I can take the nice trip to Sweden to meet the king too.
The problem with this, is that helium-4 is very, very stable. It does not want more nucleons. You can get it to accept a a deuterium nucleus or a helium-3 nucleus to make lithium, but there isn't many of those floating around, since they are likely to get hit by protons to make more helium-4. If you hit it with a proton, it'll just spit the proton back out. If you hit it with another helium nucleus, it'll spit it back out. As a result, when you only have twenty minutes to make elements, this is a bit of a problem. The Big Bang only made hydrogen, helium and a tiny bit of lithium and beryllium. (The fact that the Big Bang predicts the hydrogen/helium ratio in old stuff is a big proof of the theory -- the paper for this was authored by then-grad student Ralph Alpher and his advisor George Gamov. Gamov then decided to add his friend Hans Bethe to the author list so it could be the Alpher-Bethe-Gamov paper. This got Bethe interested in the field, and he ended up doing some major stuff in working out the details.)
So, how do you make heavier elements? Fred Hoyle suggested that it was something to do with the Carbon-12 nucleus, since it was so darn common in the universe and Ed Salpeter was the one who wrote down the equation. What happens is that you smash two helium nuclei together, then before they get away, you smash a third into it -- hence, this is called the 'triple-alpha process'. Hoyle's suggestion was that carbon was just the right energy for this to actually be likely. It's still a pretty slow reaction, but it happens in the hearts of red giant stars, once they are done making helium from hydrogen.
Once you get carbon, it's just a matter of adding helium or hydrogen to get everything up to iron, which the biggest stars do as they try to stave off death, or blasting neutrons into everything, which happens in a supernova. There's still some debate about how much of the carbon, nitrogen and oxygen is released from big stars versus small stars, but that's details.
So, consider this. Carbohydrates and fats are made of carbon, hydrogen and oxygen. Proteins add in nitrogen, and DNA adds in phosphorus. Sulphur, a part of many proteins, is added in to complete the big six. Five of those elements were made in the hearts of stars, as was the calcium and magnesium in your bones. The iron in your blood, and a number of your trace elements were made in the violent death of a star.
We are starstuff, to quote another famous Cornellian.
So, it's story time! Today I tell the story about how we found out where the elements come from. Some astronomy, some nuclear physics. It'll be fun!
Okay, so let's start with the elements. We classify elements by the number of protons they have in their nucleus -- hydrogen has one, helium has two, and so on. There are 90 elements that occur regularly in nature, with nuclei that contain anywhere from 1 to 92 protons. Each element can have several different forms (isotopes), depending on how many neutrons it has. For example, hydrogen has hydrogen-1, which has no neutrons, hydrogen-2 (deuterium) which has one, and hydrogen-3 (tritium), which has two. The number is the total number of nucleons (protons and neutrons).
Now, isotopes can be stable or radioactive. Radioactive means that they will change into another element by giving off particles -- hydrogen-3 is radioactive and will change into helium-3 if you let it sit. Stable means they won't. Generally, radioactive elements decay in two ways -- they give off an alpha particle (a helium nucleus) if they are too heavy and need to lose mass, or they will convert a neutron to a proton (and give off an electron) if they have too many neutrons. (They can also give off a positron (an antimatter electron), and change a proton to neutron if they have too many protons, but this is a lot less common.) Generally, nuclei want to be somewhat light (there's a reason all the heavy elements aren't that stable), and about even in their proton-neutron number (though the big ones need more neutrons) -- remember that protons are positive and want to be as far from one another as possible. There's also another force (the weak nuclear force) that regulates the proton-neutron interaction that prevents you just cramming crazy numbers of neutrons into a nuclei. A good analogy is that a radioactive nucleus is a pencil balanced on its eraser, while a stable nucleus is one on its side. The radioactive nucleus isn't stable and is eventually going to 'fall back' to a more stable state.
One of the consequences of the fact atoms can convert protons to neutrons (and vice versa) through beta decay is that generally there's only one or two stable atoms with a certain mass. An atom with one nucleon could be a hydrogen-1 or a neutron, but the neutron will decay into hydrogen-1. An atom with two nucleons could be helium-2, hydrogen-2, or two neutrons, but both will decay into hydrogen-2. You can have sulphur-36 and argon-36, but all heavier elements will decay to argon, and lighter ones to sulphur. (Chlorine-36 can decay to either, being between them -- heads it's argon, tails it's sulphur).
Now, the reason I mention this is that it has consequences for making elements. The universe started as a hot mass of stuff. When things finally (a millionth of a second post-Bang) cooled down enough to make protons and neutrons, and then they won out over antiprotons and antineutrons*, the protons and neutrons could hit each other hard enough to stick and make heavier elements (this happened for about 20 minutes starting three minutes after the Bang). Thing have to be hot for this to happen, since protons repel each other, and neutrons aren't stable. So, you have to shove them hard enough to overcome the fact the protons are repelling each other, until you get them close enough that they stick and make a hydrogen-2 nucleus by turning a proton to a neutron. You can then either shove another proton at the hydrogen-2 to make helium-3, or shove two hydrogen nuclei at each other to make helium-4. Our Sun is doing this right now in its core -- since a helium-4 nucleus is lower energy than the four protons that make it up, this releases energy, which eventually gets to the surface, giving us light and heat and wonderful things.
* No, I don't know why they did. Everything tells us that there should have been exactly equal numbers made. If you have a scientific reason that we can test, please tell me so I can take the nice trip to Sweden to meet the king too.
The problem with this, is that helium-4 is very, very stable. It does not want more nucleons. You can get it to accept a a deuterium nucleus or a helium-3 nucleus to make lithium, but there isn't many of those floating around, since they are likely to get hit by protons to make more helium-4. If you hit it with a proton, it'll just spit the proton back out. If you hit it with another helium nucleus, it'll spit it back out. As a result, when you only have twenty minutes to make elements, this is a bit of a problem. The Big Bang only made hydrogen, helium and a tiny bit of lithium and beryllium. (The fact that the Big Bang predicts the hydrogen/helium ratio in old stuff is a big proof of the theory -- the paper for this was authored by then-grad student Ralph Alpher and his advisor George Gamov. Gamov then decided to add his friend Hans Bethe to the author list so it could be the Alpher-Bethe-Gamov paper. This got Bethe interested in the field, and he ended up doing some major stuff in working out the details.)
So, how do you make heavier elements? Fred Hoyle suggested that it was something to do with the Carbon-12 nucleus, since it was so darn common in the universe and Ed Salpeter was the one who wrote down the equation. What happens is that you smash two helium nuclei together, then before they get away, you smash a third into it -- hence, this is called the 'triple-alpha process'. Hoyle's suggestion was that carbon was just the right energy for this to actually be likely. It's still a pretty slow reaction, but it happens in the hearts of red giant stars, once they are done making helium from hydrogen.
Once you get carbon, it's just a matter of adding helium or hydrogen to get everything up to iron, which the biggest stars do as they try to stave off death, or blasting neutrons into everything, which happens in a supernova. There's still some debate about how much of the carbon, nitrogen and oxygen is released from big stars versus small stars, but that's details.
So, consider this. Carbohydrates and fats are made of carbon, hydrogen and oxygen. Proteins add in nitrogen, and DNA adds in phosphorus. Sulphur, a part of many proteins, is added in to complete the big six. Five of those elements were made in the hearts of stars, as was the calcium and magnesium in your bones. The iron in your blood, and a number of your trace elements were made in the violent death of a star.
We are starstuff, to quote another famous Cornellian.
