Astronomy is a somewhat weird field in this in other respects. For example, astronomers frequently consider everything that isn't helium or hydrogen a "metal."

They just have a very different take on some things, because their way of looking at the universe is different from how we look at earth.

Even more bizarre is the idea of metallic hydrogen: https://astronomy.com/magazine/ask-astro/2018/06/metallic-hy...

Is this the reason Hydrogen is grouped with the alkali metals on the periodic table?

The most immediate reason is that hydrogen is a group 1 element, meaning its outer (in this case, only) shell contains just a single electron, in its s (spherically symmetric) orbital.

The electronic structure of an element greatly influences its chemical behavior, so elements in the same group tend to be chemically similar. However, hydrogen is a bit of an odd-ball as group 1 elements go.

That strikes me as sbsurd.

Sure, the table has mostly metals but there are a lot of gases.

In Astronomy STP is the exception not the rule. Not much is a gas at 3 kelvin in deep space, the other common option is stars where everything is a plasma. And that’s ignoring white dwarf’s which are mostly electron-degenerate matter, neutron stars which don’t have chemical elements, and black holes which are even stranger.

So, really hydrogen, helium, and everything else is a perfectly reasonable classification as everything else acts the same outside of fusion.

What is the typical state of matter at 3 kelvin in deep space?

Most matter in deep space is individual ions, atoms, or molecules, so has no well-defined phase. Then there's solid dust particles which are made of all sorts of stuff including silicates, organic compounds, and ices of different elements.

Solid. Everything except hydrogen and helium would be in a solid state I think.

That’s not obvious because of the ultra low pressure.

Agreed. I haven’t thought about this stuff since taking gen chem in college, so I went and looked up the phase diagram for Neon, as an example.

At low pressures, it looks like as little as 15 Kelvin is enough to make Neon a gas.

However, deep space is 2.7 Kelvin. Though of course it gets much hotter near stars and even simply being inside a galaxy is enough to hit 10-20 Kelvin.

Does this include the noble gases?

Yes, though noble gases can actually form molecules in space. https://phys.org/news/2013-12-noble-gas-molecules-space.html

The reason astronomers use “metallicity” this way is largely historical: (chemical) metals are not as prevalent in stars as non-metallic elements like carbon and nitrogen, but they are much easier to detect in spectral analysis. So historically the “metallicity” of a star really did refer to the heavy metal content (generally Fe), and considering the physical processes involved it’s not unreasonable to extend this to all non-H/He elements in the star.

I think it would be “absurd” for astronomers to change this because people who aren’t scientists get confused.

The way it was explained to me, astronomers considered them metals because they’re much more rare than H and He. They’re not made in the core of fusing stars. That might seem trivial, but a huge amount of the universe is H/He compared to the rest of the periodic table.

> nicklecompte 1 hour ago | parent [–] | on: Freshly made plutonium from outer space found on o...

The reason astronomers use “metallicity” this way is largely historical: (chemical) metals are not as prevalent in stars as non-metallic elements like carbon and nitrogen, but they are much easier to detect in spectral analysis. So historically the “metallicity” of a star really did refer to the heavy metal content (generally Fe), and considering the physical processes involved it’s not unreasonable to extend this to all non-H/He elements in the star. I think it would be “absurd” for astronomers to change this because people who aren’t scientists get confused.

My bad yo. Please accept my humble apology in not knowing the historical signifacance of something. I'll be doing my masters in 2022/23, biochemistry/microbiology.

I really should have been a bird watcher instead.

Who cares about plant proteins anyway? That's not science at all.

In math, I was taught to always be mindful of the definitions-particularly because they change on context. When studying certain systems different conventions make sense than in other studies. It’s not a judgement call as to what matters absolutely, just what helps the area of study the most. Further, the better flexible you are with working across contexts the better a grasp on the topic you have. (So further, the more ways you can explain a concept or prove a theorem the better you understand the concept.)

Universe is more than 98% hydrogen and helium by mass, with all other being less than 2%.

Maybe metal was not the best word for that, but having word for "all elements except hydrogen and helium" is useful in astronomy

see https://en.wikipedia.org/wiki/Metallicity

Ok, so, maybe they could have, you know, invented a new word? Or picked another one which isn't putting them at odds with the chemists?

Astrophysics was probably a thing before chemistry, so maybe the chemists ought to have picked a different word!

True, but at that time nobody knew about hydrogen and helium in stars.

If it works for them.

Metal vs gas is a poor choice of dichotomy: every element is a gas at the right temperature and pressure.

I would guess that they aren’t distinguishing on the basis of phase state, but rather atomic number.

Astronomers presumably mostly think about matter in the form of a star, in which presumably all the hydrogen and helium would be in some sort of plasma state, and perhaps all the “metals” would be too.

Everything's pretty much a plasma when it's burning at millions of degrees.

Sometimes it is perfectly adequate to model cows as spheres :)

This was my initial thoughts on reading the title as well. First thoughts were "what craft re-entered with that material" went to "oh, cosmologically recent".

Oceanic plates are indeed quite young compared to the continental plates. Oceanic plates are made from heavier materials and subduct below the lighter continental plates, so they only live millions of years while the material making up continental plates lives billions.

I think this is a bit misleading. It's true that the average density of oceanic crust is greater than that of continental crust, making it less buoyant, but also continental crust is way thicker and overall more massive, which is a huge factor in causing oceanic crust to subduct in addition to the buoyancy factor.

I sometimes wonder about interesting fossils and lifeforms we'll never know of, because they were subducted away before we ever had a chance to discover them.

Not just subduction but also erosion. Outside of dedicated digging efforts like deep mines or such, the fossils we find today are the ones close to the surface that are almost eroded and will be gone soon (on geologic timescales).

There is a nice curious droid video on the topic whether a humans if they went extinct tomorrow, wouldn't create a fossil record similar to dinosaurs, simply because we've been on planet earth for a too short period. https://youtu.be/8xDK2LgSeyk

> What we have to remember is that they existed for over 165 million years, 3300 times longer than modern humans have been around and the number of creatures that lived in that time frame alone, not including very small insect-sized ones must be in the trillions. However, only a tiny number of those would have been in the right conditions to be quickly covered in sediment and then fossilized. Then millions of years later they happen to be exposed on the surface at just the right time before they are eroded away for us to find them. The latest estimate is that the total number of modern humans to have ever existed over the 50,000 years we have been around is about 107 billion. But 50,000 years is a very small slice of geological time and all of our 5000 years of recorded history would be a fraction of that, and the last 200 years or so since the industrial revolution, a tiny fraction of that again, maybe a razor-thin sliver of a darkened layer somewhere in the dozens of kilometers of limestone, siltstone, and shale of the future earth's crust. Depending on where we look, there are gaps covering millions of years in the fossil records, if we were to slip into one of those, we too would be lost in time.

I sometimes wonder if life evolved on exoplanets, complete with its own billions of years of fossil record, only to be obliterated when their star reaches the red giant phase.

Or even on earth:

https://en.m.wikipedia.org/wiki/Silurian_hypothesis

Granted it is just a hypothesis but it is an interesting idea.

Hell, I'm worried about fossils on Venus buried under kilometers of lava. That's actually pretty likely IMO.

Maybe not just lifeforms, civilizations?

https://en.m.wikipedia.org/wiki/Silurian_hypothesis

What's awe inspiring to me is the fact that life on Earth was single celled for around 3.5 Billion years.

Yeah, I read "10" and my brain automatically filled in "days" until I read "millions of years"

Haha, exactly me!

Astrophysics should invent their own terminology instead of repurposing ours.

The same can (and has) been applied to every profession out there. Daemons, bugs, etc.

Jargon will always exist.