evolution of elements

[sedric]

The Cosmos series made me think about how dense elements are formed from the life of a star. Wouldn't there then be an evolution of elements over time as new stars are formed from the remnants of supernovas? If that's the case incredible new elements would be formed. I'm sure I'm missing something here maybe it's that a star can only be born of hydrogen therefore there might be a limit on the atomic makeup of elements. I don't know. Just had that question stick in my mind.

Edited May 25, 2017 by sedric

[Sensei]

The Cosmos series made me think about how dense elements are formed from the life of a star. Wouldn't there then be an evolution of elements over time as new stars are formed from the remnants of supernovas? If that's the case incredible new elements would be formed. I'm sure I'm missing something here maybe it's that a star can only be born of hydrogen therefore there might be a limit on the atomic makeup of elements. I don't know. Just had that question stick in my mind.

 

Elements with more than 82 protons in nucleus are unstable or extremely unstable.

[KipIngram]

Once you get above a certain atomic number none of the elements are stable. The ones with longer half lives stick around long enough for us to find them lying around, but I think instability mounts as you go up in number. So beyond a certain point (probably exactly what we see now) heavier elements that got synthesized wouldn't stick around long enough to matter.

[Strange]

Some lighter elements are formed by fusion in normal stars. The heavier elements are made in supernovas.

[Delta1212]

You're sort of halfway correct on both counts.

 

Later generations of stars are "born" with higher concentrations of elements other than hydrogen and helium than the earlier generations of stars. (And I guess some trace amounts of lithium and beryllium, too).

 

However, new elements naturally accumulate in the star as byproducts of the fusion process of the course of its life. What elements a star is capable of producing depends on the mass of the star. Heavier elements require more energy to fuse, so more massive stars have the energy to fuse heavier elements before they run out of viable fuel and the fusion process breaks down.

 

The hard limit on this is iron. Fusing elements lighter than iron releases more energy than is consumed in the fusion process, which is what drives the stellar engine. At iron and above, this is no longer the case, and fusing it robs the star of energy rather than generating more. So once a star starts accumulating iron in its core, it is running out of fuel, no matter how large it is.

 

As a result, other than some small trace amounts created during the course of normal fusion processes, pretty much all elements heavier than iron are only created as byproducts of supernovae once the star runs out of enough fusable material to maintain the reactions that keep it stable.

 

So there is an evolution of stars based on the previous production of elements, but since the elements generated by a star have more to do with its size than its starting composition, this doesn't have much impact on what elements it produces, and stars can be born with elements other than hydrogen (although they're still mostly hydrogen because matter is in general is still mostly hydrogen by a considerable amount) but there is a hard limit restricting them to using lighter elements as a source of fuel even if it isn't exclusively hydrogen.

[Strange]

My (v. limited) understanding is that first generation (pure hydrogen) stars tend to be much bigger and shorter-lived than later ones. (Because physics that I know nothing about.)

 

Trivia that may help when researching this: astronomers call all elements above hydrogen "metals" so 2nd and later generation stars have higher metallicity.

[Eise]

Trivia that may help when researching this: astronomers call all elements above hydrogen "metals" so 2nd and later generation stars have higher metallicity.

 

Minor correction: all elements above helium.

 

There’s an old joke that the astronomer’s periodic table consists of three elements: hydrogen, helium, and metal. It’s a nice joke, but when you understand how little of matter in the universe is “metal”, you can understand why astronomers focus on hydrogen and helium.

 

From here.

[Strange]

Minor correction: all elements above helium.

Thank you. (And it makes sense now!)

[John Cuthber]

It's perfectly possible- perhaps even probable- that a supernova can make very heavy elements.
It's just that those elements can't leave the ashes of the nova faster than roughly the speed of light.

So they only get so far before they decay (even helped out by relativity).

You can only find them "close" to the bang.

Anything "close" to the nova will not be in a position to report the existence of super-heavy elements

[MigL]

University was a long time ago and my recollection of Nuclear Physics perhaps a bit foggy...

But weren't there 'islands of stability' at much higher atomic numbers than 82 ?

[Delta1212]

University was a long time ago and my recollection of Nuclear Physics perhaps a bit foggy...

But weren't there 'islands of stability' at much higher atomic numbers than 82 ?

There are elements with extremely long half-lives above 82 to the point that they might as well be considered stable for most purposes.

 

"Islands of stability" is a term generally applied to elements with much higher atomic numbers that are relatively more stable than those around them. These would still most likely have half lives measured in days at best, not be actually stable elements. They are also thus far theoretical and have not been physically produced or synthesized as yet.

[John Cuthber]

For the Islands, "stability" is a relative term.