The Heavier Elements: A Question About Their Abundence

[Newtons Apple]

Hi,
So I've been reading and watching things recently about the origin of the Elements in the universe... And maybe it's me just over thinking..but I just feel like my simple understanding isn't really adding up...So we have 118 elements, the last few are man made, so let's remove those from this question. My main question is this, after the big bang we have the first handful created... which was enough to fuel star birth later on..Those stars may have fused up to iron and then exploded to seed more of those elements into the universe.. But how is it possible that the 100+ remaining are so well spread out through the universe...? Let's take Gold for example...The only theory I see for it's creation was big neutron star collisions...that seems so implausible that all of the gold in the universe...we have tons right here on Earth alone... was created just on the off chance that two neutron stars which are EXTREMELY small just happened to collide? With the sheer scale of space...how many times could this have even occurred? And I mean this with all of the other elements. Are we saying that this mere coincidence is the cause for the rest of the entire table? I can see if this happened and the created elements were local to the area of a neutron star collision...but the thought is that these elements are all over the unvierse?

Let's say they're not and that only our solar system is privy to these elements.. That just seems so conveinent that the entire table (minus the man mades) are all on Earth... It's hard for me to believe. And back to the neutron star theory.. Getting to a neutron star is resereved for the bigger stars, it just seems that there would be so few and that they would be randomly spread out not right next to each other that would inevitably lead them to collide.

Are there any other theories for the creation and spread of the elements?

 

[jedishrfu]

Supernova explosions can also generate heavier elements, including gold. I'm sure there are papers on the subject, including the % of elements made.

However, like so many things, our understanding is limited, and our skills in probability estimations are very weak.

We know nuclear processes are involved, and we know that nuclear fusion is insufficient to create heavier elements beyond iron, so it leaves only two choices: neutron star collisions and supernova explosions. Planet-sized chunks of material high in heavier elements are likely spewed out into space and later captured into planetary systems.

Our Earth itself is the product of one of those processes, and so the abundance of gold would be higher on Earth based on that fact.

 

[Ibix]

There are links to previous discussion in the "Similar threads" at the bottom of the page. This one seems helpful.

In particular, one of the posts in that thread notes that there's a known dwarf galaxy that's got more of the very heavy elements than others, suggesting a single really large event in that galaxy.

It's also worth noting that neutron star collisions may well be more likely than you think. It isn't just a case of a pair of tiny stars randomly colliding. A binary pair of giant stars will eventually turn into a neutron star binary which will necessarily decay via gravitational radiation into a merger unless disrupted by something.

 

[Bandersnatch]

The only theory I see for it's creation was big neutron star collisions...that seems so implausible that all of the gold in the universe...we have tons right here on Earth alone... was created just on the off chance that two neutron stars which are EXTREMELY small just happened to collide? With the sheer scale of space...how many times could this have even occurred?

It's not only the scale of space that is large here. All the numbers involved are extreme - and unintuitive. The rates of occurrence, the time scales, the amounts produced, the fraction that is present on Earth. But they do add up in the end.
Think of it as multiplying a string of coefficients, some very small, some very large. The large ones can very well compensate for the small ones, to arrive at a reasonable number in the end.

Doing a back-of-an-envelope calculation can help in seeing this.
The current expected rate of binary neutron star merger events in our Galaxy is 30 per million years. It is extremely rare.
But, when taken over the ~ 9 billion years the Milky Way had to evolve in the time before the formation of the Solar System, you end up with something on the order of ~270 thousand events.
Each explosion is estimated to eject on the order of 1% of a solar mass of heavy elements into the interstellar medium. This is thousands of Earths worth of the stuff.
(The time for the ejecta to mix with the surrounding environment is counted in tens or hundreds of thousands of years - completely negligible given the numbers we're juggling here)
This translates to the approximate total of 2.7 thousand solar masses-worth of heavy elements produced and re-seeded into the galactic gas over the time period.
The estimated present-day abundance of heavy elements in the Milky Way is on the order of 3 thousand solar masses, which is in broad agreement with the slapdash number we've come up with.
So at the very least one could say it is entirely plausible for this rare process to supply all the required heavy metals.

People do analyse this in a more rigorous manner. The estimates for the rate of mergers, ejecta mass, and the abundance of elements I took from the following two papers:
Sgalleta et al. 2023: https://academic.oup.com/mnras/article/526/2/2210/7273849
Chen et al. 2024: https://arxiv.org/abs/2402.08214
The second one in particular calculates that binary neutron star mergers, or a combination of those with neutron star-white dwarf mergers, may be sufficient to account for nearly all the heavy element abundances - with the exception of the lower-mass heavy elements that would then be supplied by core-collapse supernovae.

 

[PeroK]

that seems so implausible that all of the gold in the universe...we have tons right here on Earth alone...

Perhaps a ton doesn't amount to very much on cosmic scales? In fact, there are estimated to be about 250,000 tonnes of gold on Earth. That's a whopping 250 million kilograms. A huge amount of gold.

Is that 1% of the Earth's mass? Probably not. The mass of the Earth is $6 \times 10^{24} \ kg$. That's $2.4 \times 10^{16}$ times the mass of the gold on Earth. That means that gold has a tiny, trace amount.

If you are studying cosmology, you have to think big!

 

[Newtons Apple]

Perhaps a ton doesn't amount to very much on cosmic scales? In fact, there are estimated to be about 250,000 tonnes of gold on Earth. That's a whopping 250 million kilograms. A huge amount of gold.

Is that 1% of the Earth's mass? Probably not. The mass of the Earth is $6 \times 10^{24} \ kg$. That's $2.4 \times 10^{16}$ times the mass of the gold on Earth. That means that gold has a tiny, trace amount.

If you are studying cosmology, you have to think big!

yea I see your point...but then is it just accepted that every element just happens to be present on Earth? Like we got literally all of them here in some form or fashion just seems like such a conincidence!

 

[PeroK]

yea I see your point...but then is it just accepted that every element just happens to be present on Earth? Like we got literally all of them here in some form or fashion just seems like such a conincidence!

That's what happens in a random distribution.

 

[Nugatory]

yea I see your point...but then is it just accepted that every element just happens to be present on Earth? Like we got literally all of them here in some form or fashion just seems like such a conincidence!

Turn it around: calculate the number of atoms of each heavy element that is implied by 3000 solar masses, make a reasonable estimate of the number of atoms per planet, then consider how many atoms per planet that is. It would be really surprising if every planet didn’t get at least a few.

 

[gmax137]

every element just happens to be present on Earth?

Except for technetium. The Te present wasn't here at earth's birth.