Just a quick two cents here: supernovae, yes, but not quasars. Quasars are accreting black holes, and while there might be some production of heavy elements in their accretion disks, those elements likely do not get returned to the surrounding galaxy to form new stars. Besides supernovae, neutron star mergers (which another poster already noted) may also produce significant heavy elements, and AGB stars also produce some of the moderately-heavy elements - but with quite a different distribution. Cartoons like this one https://svs.gsfc.nasa.gov/13873 give a good summary of which routes are responsible for making each.

So would it be right to say that a binocular neuron--including one that allow for stereopsis--receives input from the same retinal position of each eye? (As opposed to it receiving input from different retinal positions) Is this right? Bc I was wondering if input from different retinal positions to the same neuron allowed for depth perception, or if it was different input from the same position

Those are two separate things so we can handle them separately:

NaCl in the galaxy as a molecule.

Nope, by mass and density it's actually going to be pretty far down.

There's a table of the elements by how frequently they get created by stars and you'll find that while their not uncommon they are pretty far down and pretty far apart.

So finding both together is even lower than that.

Now the second question:

Water being salt water.

That one is a bit more tricky to detail but in short:

Salt is really really easy to dissolve. In water.

What that means is that any body with liquid water in it that also contains masses of salt will dissolve on in the other unless luck keeps them separate.

And once it's in it's very very unlikely that chance will separate them again.

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Thank you for responding!

How about the platypus? It's the only living species in its genus, and it's also the only living genus member in its family.

The dugong is at least as lonely. It used to share a family with the Steller's sea cow, before that was hunted into extinction.

The narwhal isn't quite so lonely. It's the only species in it's genus, but it still shares a family with the beluga (which is also the only species in its genus).

In any case, Homo sapiens isn't unique in, er, being unique. It's just a question of how much diversity develops, and then how much of it survives.

Your right, I was tired. 96khz and 24 bit

There are actually some exothermic reactions that produce elements more massive than iron.

However, these are usually very short lived in the time immediately before supernovae and are limited by photodisintegration. They don't meaningfully contribute to the amount of heavier elements (Which are primarily produced during nova).

I'd like to add that this question is by far not a purely biological one.

There would be good reasons to categorize both gorillas and chimpanzees under the genus "homo" together with humans since they're genetically very close to us - a claim that has already been made by some biologists. But that also would have a lot of cultural implications: from questions about their legal and moral status and our duties towards them up to potential problems with religious groups who insist on the uniqueness of humans and their special position among all of God's creatures.

As the author of the referenced paper: I actually still don't know how common salt is in the universe. Another poster noted the relative abundance of Na and Cl - we have a pretty good sense of how much of each of these elements are out there. But we can only see NaCl, the molecule, in special locations: the disks around high-mass stars (see also https://ui.adsabs.harvard.edu/#abs/2023ApJ...942...66G/abstract) and the dissipating envelopes of dying medium-mass stars (Asymptotic Giant Branch, AGB, stars). Otherwise, we think NaCl is present, but it is probably in the solid phase and doesn't produce any easily-observable radiation. When it's in the solid phase, it is part of dust grains, and I don't think we know exactly what it does in the dust (e.g., is it mixed with water in crystals? or stuck in some silicates? or something else?).

High-mass young star disks and AGB stars are unique in being very warm and dense, which are the conditions needed to have NaCl in the gas phase and able to produce observable millimeter-wavelength radiation. We might see it in one other place, in a hot molecular cloud, but that detection is not confirmed.

There are some other cool features of molecular salt: there might be salt clouds in hot jupiters, since salt can form at higher temperatures.

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> It's fascinating that we have any kind of nontrivial amounts of those elements at all in our grasp, considering their sources.

It's easier to think of it as an extremely large number (number of stars) multiplied by an extremely small number (probability of producing those elements) which rounds out to a reasonably-sized number.

I was looking for more than just a Google answer, for relative abundance it would seem there are massive deposits where it is found but yes, absolutely, there is very little NaCl in the universe.

Yes, the luminousity of a star (which is a direct consequence of "units of matter fused per second") goes up as greater than the cube of mass, about M^(3.5). That means that even though they contain a lot more fuel, they burn through it far more quickly. So for example, a star with two solar masses has roughly twice as much fuel* as the sun, but it burns around 13 times as fast, so its lifespan is less than one sixth of the sun's, or maybe around 1.5 billion years**

So if you plug in a star with, say, 20 solar masses, all of a sudden, you're looking at a lifespan of a small fraction of a billion years.

* It gets a bit more complicated in that large and medium stars have a radiative zone at the core (high pressure supressing convection) underneath a convective zone at the surface. Small stars, smaller than the sun, are entirely convective, meaning that they can use the fuel from the entire stellar mass. Large stars have smaller convective zones which don't interface with the core, meaning that they can run out of fuel even if there's a substantial amount of hydrogen in the upper layers of the star. This is why using mass to calculate star lifetimes isn't as simple as using the entire star's mass to look at how much fuel will be fused. This is also why red dwarf stars have exceedingly long lifespans.

**These are highly handwavey numbers, don't check me on it, but you get the gist.

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You're somewhat correct — there are basically two known generations of stars, and a third hypothesized one.

The very first generation of stars would have lasted millions to tens of millions of years, were very metal-poor (being composed almost exclusively of hydrogen and helium left over from the big bang) and would almost all have gone supernova early on. None are still around today, and there is only scant evidence that they existed at all. Obtaining better evidence for this first generation of stars is one of the primary missions of the James Webb Space Telescope.

The second generation of stars that formed had a middling metallicity, as they formed from material that included the higher-mass elements formed from the first generation of stars. These were lower in mass on average and lasted much longer, hundreds of millions to billions of years.

Our Sun is a third generation star, which was likely formed from the compression of gas by second-generation stars going supernova. Third-generation stars like our Sun are much lower mass and higher metallicity, and have much longer lives on average.

All that being said, we would have obtained a mix of many elements because our Sun (and most second- and third-generation stars) and solar system were almost certainly formed out of gas clouds that had materials from numerous other exploded stars from both the current and previous generation. The second generation of stars was a lot more diverse than the first generation, and the third generation even moreso, so the diversity of elements that we seen in our solar system today comes from many different kinds of exploded stars in the two most recent generations.

Hope that helps!

This is only true for Type II supernova. Type Ia supernova occur when a white dwarf (created in the death of a star like our sun) siphons enough mass of a companion star.

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The news you hear about helium supply is not (or shouldn't be) about extractable amounts on the planet — it's about what's commercially available.

Extraordinary amounts of helium are just discarded during things like natural gas extraction because helium isn't profitable enough for those companies to extract, store, and sell. That means commercial supply goes down and, with constant or increasing demand, price goes up. At some point, it becomes profitable to separate and sell it again, at which point supply increases, price goes down, and the cycle repeats.

This process is being intensified, especially in creating lower price floors, by long-standing selloffs of nationalized helium reserves that were created when we thought dirigibles were the future of warfare.

https://medium.com/a-microbiome-scientist-at-large/science-monday-are-we-really-running-out-of-helium-c5365852cbd3

Helium is effectively a non-renewable resource (decay products are created very slowly and need to accumulate over millions of years to be harvestable, besides) and we will run out of it someday, but that day is still very far off and unlikely to happen in the lifetime of anyone alive barring major life extension advancements (yes, please!).

What we will see is continued boom/bust cycles as reserves are depleted and markets stabilize on current real extraction costs. And it'll likely be a steady increase over time as the long-term depressive effect of stockpile release dwindles.

Helium can be synthesized via nuclear reactions and, in a hypothetical situation where the Earth really "ran out," that's what we'd likely end up doing. It'd just be many orders of magnitude more expensive than today and probably make asteroid capture look very appealing. But that hypothetical day is very far off past much more prominent existential threats.

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Isn't "it's somewhere between the most common in the universe and the most rare in the universe" not particularly precise?

Yes it is! I absolutely love this stuff.

Estimates put the current count of neutron stars at one billion in our galaxy and a total of one hundred billion stars total. So around one percent of stars in our galaxy are neutron stars. Most stars are in binary orbits so taken all together it lines up with the distribution quite nicely I think. Plus remember it's by mass so one gold atom counts for as much as 79 hydrogen atoms. If we viewed it by atomic count instead of total mass heavy elements are even rarer than the graph implies.

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