by ems57fcva » Mon Apr 16, 2018 8:30 pm
jajohnson51 wrote: ↑Thu Oct 26, 2017 6:26 pm
ems57fcva wrote:Ann wrote:
I remember seeing a Hubble (or Chandra?) picture of a supernova remnant, and the caption said that we can be sure that this is the remnant of a massive star and its core collapse. And the reason why we can be sure of that is that the remnant contains so much oxygen, and oxygen is produced in core-collapse supernovas. And indeed, today's chart says that oxygen is produced almost exclusively by such supernovas.
I also remember reading about red giants and all the chemical processes that go on in them before they shed their outer layers altogether and turn into planetary nebulas and white dwarfs. The text where I read about that said that many elements are created here because neutrons are incorporated into the nuclei of other elements.
If you check out the chart, the "green" elements (green because they were created by low-mass stars) seem kind of weird. There are few well-known elements among them, apart from lithium, carbon and nitrogen, and the highly poisonous elements mercury and lead.
Fascinatingly, though, some silver and even some gold is apparently made by dying red giants!
Ann
As I understand it, dying small stars lack the needed temperatures and pressures to create anything even as heavy as iron; and their planetary nebulas will be full of oxygen (which is not to say that supernovae do not produce a lot of oxygen themselves). In fact, in that other APOD that I referenced, Sulfur was as heavy an element as small stars could create. However, this arrangement seems to come from the LIGO-VIRGO collaboration or someone close to it. See
http://growth.caltech.edu/images/gw1708 ... -table.jpg, which is associated with
http://growth.caltech.edu/news-gw170817.html. If this is a mistake, it was made by someone who should have known better.
One of the people who put the graphic together here. The heavy elements (=beyond the iron peak) are made by capturing neutrons onto "seed" nuclei, such as iron. Neutron capture is needed because the electric repulsion between a nucleus like iron (26 protons) and an additional proton is very strong, but between a neutron and an iron nucleus there is none. Dying low-mass stars can make heavier elements not because they reach very high temperatures, like those needed to fuse silicon towards iron, but because nuclear reactions happen in these stars that create free neutrons. These neutrons can then be captured by iron (for example) nuclei that are in the star not because they were formed in the star but because those seed nuclei were in the gas out of which the star was born. The Sun has iron nuclei in it for this reason.
These neutron-capture reactions are not important for creating energy in the star, so when we discuss nuclear fusion in stars, this process is rarely mentioned. For the origin of the elements it matters! And it has been spectacularly demonstrated to be correct, as we have seen newly-minted technetium in these dying low-mass stars. Because the longest-lived technetium isotope has a half-life of 4.2 million years, much, much shorter than the lives of these stars, any technetium in these stars must be made there, rather than being in the natal gas.
I just stumbled across this, albeit months later. The response make a certain degree of sense, except for one thing: Small stars cannot make iron via nuclear fusion! And yet iron is being cited as the "seed" nuclei for the s-process. So this response, as-is, does not resolve my concern.
However, I also see an "out" for this: The primary neutron-creating processes involve a Helium nucleus (or alpha particle) and either
13C or
22Ne. And there are paths for the s-process to create iron starting even with carbon. (In fact, an s-process path to and beyond iron exists for
9Be.) So now I have a new concern about the chart: It seems that all of the elements between carbon and iron are subject to nucleosynthesis via the s-process, but that dying low-mass stars can do this is not being accounted for. Maybe this is just more of my ignorance, but if so I would like to know what the right answers are for this.
[quote=jajohnson51 post_id=276310 time=1509042360]
[quote="ems57fcva"][quote="Ann"]
I remember seeing a Hubble (or Chandra?) picture of a supernova remnant, and the caption said that we can be sure that this is the remnant of a massive star and its core collapse. And the reason why we can be sure of that is that the remnant contains so much oxygen, and oxygen is produced in core-collapse supernovas. And indeed, today's chart says that oxygen is produced almost exclusively by such supernovas.
I also remember reading about red giants and all the chemical processes that go on in them before they shed their outer layers altogether and turn into planetary nebulas and white dwarfs. The text where I read about that said that many elements are created here because neutrons are incorporated into the nuclei of other elements.
If you check out the chart, the "green" elements (green because they were created by low-mass stars) seem kind of weird. There are few well-known elements among them, apart from lithium, carbon and nitrogen, and the highly poisonous elements mercury and lead.
Fascinatingly, though, some silver and even some gold is apparently made by dying red giants!
Ann[/quote]
As I understand it, dying small stars lack the needed temperatures and pressures to create anything even as heavy as iron; and their planetary nebulas will be full of oxygen (which is not to say that supernovae do not produce a lot of oxygen themselves). In fact, in that other APOD that I referenced, Sulfur was as heavy an element as small stars could create. However, this arrangement seems to come from the LIGO-VIRGO collaboration or someone close to it. See [url]http://growth.caltech.edu/images/gw170817/periodic-table.jpg[/url], which is associated with [url]http://growth.caltech.edu/news-gw170817.html[/url]. If this is a mistake, it was made by someone who should have known better.[/quote]
One of the people who put the graphic together here. The heavy elements (=beyond the iron peak) are made by capturing neutrons onto "seed" nuclei, such as iron. Neutron capture is needed because the electric repulsion between a nucleus like iron (26 protons) and an additional proton is very strong, but between a neutron and an iron nucleus there is none. Dying low-mass stars can make heavier elements not because they reach very high temperatures, like those needed to fuse silicon towards iron, but because nuclear reactions happen in these stars that create free neutrons. These neutrons can then be captured by iron (for example) nuclei that are in the star not because they were formed in the star but because those seed nuclei were in the gas out of which the star was born. The Sun has iron nuclei in it for this reason.
These neutron-capture reactions are not important for creating energy in the star, so when we discuss nuclear fusion in stars, this process is rarely mentioned. For the origin of the elements it matters! And it has been spectacularly demonstrated to be correct, as we have seen newly-minted technetium in these dying low-mass stars. Because the longest-lived technetium isotope has a half-life of 4.2 million years, much, much shorter than the lives of these stars, any technetium in these stars must be made there, rather than being in the natal gas.
[/quote]
I just stumbled across this, albeit months later. The response make a certain degree of sense, except for one thing: Small stars cannot make iron via nuclear fusion! And yet iron is being cited as the "seed" nuclei for the s-process. So this response, as-is, does not resolve my concern.
However, I also see an "out" for this: The primary neutron-creating processes involve a Helium nucleus (or alpha particle) and either [sup]13[/sup]C or [sup]22[/sup]Ne. And there are paths for the s-process to create iron starting even with carbon. (In fact, an s-process path to and beyond iron exists for [sup]9[/sup]Be.) So now I have a new concern about the chart: It seems that all of the elements between carbon and iron are subject to nucleosynthesis via the s-process, but that dying low-mass stars can do this is not being accounted for. Maybe this is just more of my ignorance, but if so I would like to know what the right answers are for this.