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 :)