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GSte2022 t1_j6551bq wrote

'Metals' (any atom with equal or more than three protons) are heavier then hydrogen or helium. So 'metals' are slower then H or He with the same kinetic energy. That means that over time the heat density becomes less along the expansion vector. This effect spreads the heat over a bigger volume and lowers the heat(energy) density, making that area cooler.

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Loose-Addition-5730 t1_j65oh0r wrote

Your definition of metals is interesting.

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swfo t1_j65p66f wrote

That is the definition of metals Astronomers use. I just learned it from Dr. Becky. It was shocking at first but makes sense from an astronomical point of view. Still, I will never be comfortable calling carbon or neon a metal.

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Loose-Addition-5730 t1_j67m8ta wrote

Coming from a chemistry background this will take some getting used to.

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TFK_001 t1_j6azo2j wrote

Our chemistry teacher would always bring up this definition in the first few weeks of class but it makes sense from an astronomical standpoint

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DarthBrooks69420 t1_j66hwx1 wrote

Welcome to astonomy, where everything that isn't H/He is a metal and what is currently going on in a galaxy 50 million light years away doesn't matter.

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dern_the_hermit t1_j662epi wrote

In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium.

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OffusMax t1_j68i0aq wrote

Chemically speaking, a metal is a substance whose outermost electrons are only loosely attracted to the nucleus and form an “electron sea” when in the solid state. Which is why metals are better conductors than non-metals.

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dude_1818 t1_j67cvpp wrote

Basically, any element that formed later than the big bang is a metal

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