Ok I was wrong (very wrong) as long as we're arguing with unrealistic semantics because Wikipedia defines a molecule as "the smallest particle in a chemical element or compound that has the chemical properties of that element or compound" which means that atomic helium is a molecule. Hence the "element or compound" bit.

Assuming your definition holds: Do you have an actual (peer reviewed) citation that demonstrates that H2O is the second most prominent molecule in the universe, for any real definition of molecule? If not, do you have a citation showing that diatomic oxygen is less likely to form than water in the interstellar medium (considering the momentum of hydrogen atoms will be significantly greater than that of oxygen atoms and thus impact the chemical reactions drastically)? If not, do you have evidence to show that our capabilities to detect polar molecules like water are able to account for the absurd abundance of nonpolar molecules (for which electromagnetic detection is practicaly impossible) based solely on first principles of astro or even quantum chemistry?

I'm not trying to play devil's advocate or prove you wrong. These are questions I'd love to have the definitive answers to but I'd rather have no answers at all than a random HN post which only cites a Wikipedia article on the prevalence of atoms (not molecules, missing the entire point of chemistry) and another unreviewed HN post on astrochemistry from first principles without a single significant citation.

The purpose of [citation needed] is not to take the wind out of your statement but to force you to provide the necessary evidence to back up your assertion.

> Wikipedia defines a molecule as "the smallest particle in a chemical element or compound that has the chemical properties of that element or compound" which means that atomic helium is a molecule.

No it doesn't; https://en.wikipedia.org/wiki/Molecule cites that as an earlier, less precise definition than the one that's commonly used today. The definition right at the top of the page is quite clear that by the usual definition, a single atom by itself does not constitute a molecule.

> The purpose of [citation needed] is not to take the wind out of your statement but to force you to provide the necessary evidence to back up your assertion.

In polite discussions, attempting to "force" your conversational partner to do anything is kind of discourteous. If you're honestly seeking information, then by all means feel free to ask questions. But whether you intended to or not, your responses are coming across as very adversarial and dismissive.

Ok so I'm very, very wrong along another semantic axis but in this context it's absolutely irrelevant, using the definition given in the second paragraph:

> In the kinetic theory of gases, the term molecule is often used for any gaseous particle regardless of its composition. According to this definition, noble gas atoms are considered molecules despite being composed of a single non-bonded atom.

Unless you're saying that water (or any other molecule for that matter) can form inside of a star, which would open an entirely new field of chemistry, this is the definition to use when talking about astrophysics.

Regardless of what Wikipedia says, chemists distinguish between molecules and atoms.

Diatomic oxygen is extremely rare in the universe, because it's extremely unstable. Oxygen would rather be bonded to pretty much anything rather than itself, including hydrogen. [citation: go burn some hydrogen]. It's common round these parts due to some rather bizarre nonequilibrium chemistry [aka life].

What we know about the prevalence of various molecules is a combination of what we know about the prevalence of atoms combined with what is thermodynamically stable. It goes hydrogen, water, methane, ammonia, carbon monoxide, I think? At this point it depends on local conditions, temperature and so forth, but the stability of water combined with the prevalence of oxygen makes it basically unassailable as the second most common molecule after H2.

Can you see why instead of "forcing someone to provide references" it would be better to ask someone nicely for references?

It was a casual answer to an astronomy question buried in a comment thread on a message board for startup enthusiasts. The level of rigor you're demanding is unnecessary. [Citation not needed], truly. I'm absolutely sure that I won't run into trouble with HN's moderators or community for having commented despite having no "definitive answers with better-than-wikipedia citations" to give you about the chemical make-up of the universe. Nor do I think an HN where questions go unanswered lest the leading expert in current theory happens to respond in person with a bibliography in hand is a better one.

We're not debating the multiverse theory or alternatives to the standard model, we're literally literally talking about the stoichiometric ratios and chemistry of the universe, a field of knowledge that is as fundamental to astrophysics as the concept of fusion itself.

If you can't provide a single citation to support such fundamental statements as "H20 is the second most common molecule in the universe" outside of irrelevant Wikipedia articles, they are worthless, HN moderator and community standards notwithstanding.

Nice to have somebody on my side. From what I could find so far CO seems to be the second most common molecule in the interstellar space[0], and water does not seem to take part in a nuclear fusion[1], so I think that the discussed statement may not even necessarily be true.

[0] - https://en.wikipedia.org/wiki/Carbon_monoxide

[1] - https://en.wikipedia.org/wiki/Nuclear_fusion#Nuclear_fusion_...

H₂ is pretty clearly, it seems, the most common molecule in the universe (the sources that claim something else is seem to be fairly casual sources, and seem to neglect single-element molecules entirely), but from a quick skim of results, it seems that superficially decent sources split between saying that H₂O is second most common and those saying that it is third, behind CO.