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We constantly read about "clouds of dust and gas" surrounding a (or being the initial state of a) star, but what elements constitute such dust and gas?

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The clouds of gas and dust that form stars are usually what are called Molecular Clouds and Giant Molecular Clouds (GMCs).

The "Molecular" means that most of the atoms are combined into molecules rather than being free atoms. So the hydrogen is mostly H2 and not H. This is really important, since a cloud comprised of molecules can radiate away the heat of collapse much easier than a cloud of atoms can. If that heat isn't radiated away, the cloud won't collapse.

As far as the contents go, clouds int he solar neighborhood today have a composition very similar to that of the outer layers of the Sun: around 90% (71%) hydrogen, 9% (27%) helium .08% (1%) oxygen, .04% (0.5%) carbon and lesser amounts of things like nitrogen, silicon, magnesium, neon, iron and sulfur. Note that these are atom% followed by (mass%). See this for a table.

Astronomers would call it 90% (71%) hydrogen, 9% (27%) helium and less than 1% (2%) "metals".

Much earlier in the universe's history the percentage of hydrogen was higher and the percentage of everything else was lower. Since the Sun is a relatively new star, the gas it condensed from was "polluted" by the gas emitted by generations of older stars which enriched it with helium and "heavy metals".

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  • $\begingroup$ Thank you. So the gas and dust are made of the same molecules? I´m trying to understand how the (cloud) reminders of the Sun formation created the rocks to later form Earth. The molecules you mention seem a little "light" to me to form a rock planet, but perhaps 1%/2% of heavy metals is enough $\endgroup$ – alvaroc Jul 23 '18 at 21:25
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    $\begingroup$ @alvaroc the Earth's mass as compared to the sun is about 1/330,000 (or approximately 0.0003%). The "less than 1%" is more than enough to make a rocky planet. $\endgroup$ – enumaris Jul 23 '18 at 21:46
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    $\begingroup$ The dust is mainly silicates, iron and carbonaceous stuff along with some ices. The gas is mainly hydrogen and helium. During the formation of planets the ultra-fine dust particles start sticking together, and those stick together (there's a "dust-bunny" stage!) and ultimately form rocks. These collect into Mars-sized planetismals. (This all happens quickly: a million years or so.) Bigger than average planetisimals in the colder parts of the solar system collect gas and form gas giants, closer to the sun, no gas and form rocky planets. (This is very oversimplified, but basically correct.) $\endgroup$ – Mark Olson Jul 23 '18 at 22:04
  • $\begingroup$ @MarkOlson I love the "dust bunny stage". I'm going to remember that one. Small point when you say the percentage of hydrogen was higher and other elements lower, I'd clarify that because the primordial helium ratio was only slightly lower, about 24-25% vs 27%. By saying lower it gives the impression that it might have been quite a bit less, but before the first stars, the universe was about 24% helium by mass. $\endgroup$ – userLTK Jul 25 '18 at 6:52

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