I just saw this YouTube video made by PBS Studios. There they explain that $H_2$ can reduce the traslational kinetic energy of regular $H$ atoms when it is formed since the energy can also be stored in vibrational and rotational modes after each collision. They state that the gas was so hot that regular atomic $H$ generated enought pressure to impede gravitational collapse and that, only because of $H_2$, it was possible to lower that pressure and stars could form.

My question is, if $H_2$ were impossible to form (for some reason) wouldn't gravitational collapse eventually happen as the universe expanded and the gas cooled down? Or the expansion itself would be enought to prevent this by lowering the density of the gas before any star could be formed?

Would a universe without $H_2$ had formed any stars in its entire evolution?


1 Answer 1


The role of H$_2$ is to allow primordial gas to cool down sufficiently to allow the collapse to start and then to hold the gas at a relatively low temperature as it gets much more dense. The formation of H$_2$ is essential because atomic hydrogen simply has no way of cooling itself below temperatures that are capable of exciting the $n=2$ level and then emitting Lyman alpha photons. This means that without H$_2$ the gas can't cool effectively once it gets below about $10^4$ K and it will never reach the densities required to form stars.

Without molecular H$_2$ you get stuck with large ($\sim 10^4 -10^6M_{\odot}$) pressure-supported "protogalaxies" that won't collapse any further. These are formed at redshifts of around 20-40 from growing primordial density perturbations and have initial temperatures of a few thousand K. The ongoing formation of H$_2$ in primordial gas phase reactions (no dust required) allows a collapsing cloud to cool down to around $\sim 200$ K.

The importance of H$_2$ cooling is outlined in reviews by Glover 2005 and reiterated more recently by Klessen 2018. A telling quote is from the review by Bromm 2013, who says

If the gas were unable to cool, there would be no further collapse, and consequently no gas fragmentation and star formation. The gas would simply persist in hydrostatic equilibrium, roughly tracing the density profile of the Dark Matter. Early on, it was realized that cooling in the low-temperature primordial gas had to rely on molecular hydrogen (H2) instead (Saslaw and Zipoy 1967).

Conversely, in gas that is enriched with heavier elements, H$_2$ cooling is totally unimportant because there are more efficient routes for a collapsing cloud to radiate away energy (e.g. Glover & Clark 2012).

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    $\begingroup$ Very interesting! So we can conclude that the fact that Hydrogen can bond is key for the existence of all the elements in the Universe as stars would have never formed otherwise (except from small traces from Big Bang nucleosynthesis I guess). $\endgroup$
    – Swike
    Sep 24, 2020 at 19:25

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