Why are "metals" more effective at cooling molecular clouds than hydrogen and helium?

Submitted by Thomas_Bonk t3_10mtknj in space

https://www.spektrum.de/magazin/die-ersten-sterne-im-universum/828408

According to a German article about the early universe "metals" are more effective at cooling molecular clouds than hydrogen and helium, therefore the molecular clouds from which the first stars formed were much hotter than today's molecular clouds. When the first "metals" emerged, the cooling of molecular clouds became more effective and the clouds became cooler. (Although I think that the "hotter" background radiation probably also added to the first molecular clouds being hotter. Was it already in the microwave range back then?)

My question: Why are "metals" more effective at cooling molecular clouds than hydrogen and helium?

18

Comments

You must log in or register to comment.

AsherKendrell t1_j65rcag wrote

The primary way molecular clouds cool is by emitting the energy of the heat as light, which can escape the cloud.

A hot cloud of gas means there's lots of atoms and electrons zipping around, bumping into each other. If one of those bumps gives enough energy to an atom's electron, that electron gets excited to a higher energy level. When that excited electron jumps back down to a lower one, it emits the energy difference as light. Hydrogen and Helium are very simple atoms, meaning they only have 1-2 electrons to excite, few energy levels to excite to, and the minimum energy you need to make that first jump is higher. Conversely, "metals" have many more electrons and much more complex energy level structures with smaller, easier to accomplish jumps. Molecules, such as H2O, CO2, etc even more so. Therefore metals and molecules are much more likely to be able to take the energy of thermal collisions and convert it into outgoing light.

Take a look at these spectra for a visual: https://images.nagwa.com/figures/628134219594/1.svg

Each vertical line corresponds to one possible electron transition in the corresponding atom. You can imagine collisions as randomly firing along these spectra, and only when they hit one of the lines does light get emitted. So you can see, for H and He you're going have a way harder time getting hits. (And, in real life your shots aren't uniformly spread either, but skewed towards the red side at the temperatures molecular clouds are typically at. For hydrogen, you'd only have a single target to try to hit, vs the forest of lines that nitrogen has for example)

17

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.

12

Loose-Addition-5730 t1_j65oh0r wrote

Your definition of metals is interesting.

4

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.

10

Loose-Addition-5730 t1_j67m8ta wrote

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

2

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

1

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.

5

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.

3

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.

2

dude_1818 t1_j67cvpp wrote

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

2