Is there a strict difference between gas and dust? In Earthly environment most things become gaseous if heated enough. The temperature of interstellar medium seems to range mostly between 10 and 10 000 Kelvin. Is gas/dust an analog for hot/cold, or does the phase diagram of the element in question matter too? Can metals and molecules be gas in astronomical terms?
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Yes, metals and other elements and molecules can exist in gaseous form under the right conditions of temperature and pressure. A "gas" is simply one of the fundamental states of matter, as in solid, liquid, or gas (and a few other states outside the scope of this question). But as a gas, these substances exist entirely as either individual atoms, individual elemental molecules, or individual compound molecules of multiple atoms (e.g. carbon dioxide).
Dust, on the other hand, is comprised of tiny particulate matter that has undergone the stronger intermolecular bonds to create substances like ice, silicates, and carbon compounds that float around in varying densities between the stars and between the galaxies. Since these particles are still extremely small (typically a fraction of a micron across), they can appear to be a gas, but these tiny, irregularly-shaped objects still exist individually in a solid or liquid state.
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1$\begingroup$ Ofc in cosmology all matter is simply referred to as "dust" and assumed not to move at all. So one has to look at the subsection of astronomy on is dealing with. $\endgroup$ Jun 13, 2014 at 19:42
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4$\begingroup$ @AtmosphericPrisonEscape: That's not quite right. Cosmology simply borrows this from general relativity, in which "dust" means "presureless perfect fluid", and so practically any stress-energy distribution than can be adequately modeled as such. Notably, the era in which the large-scale universe could be treated as such began about $50\,\mathrm{k}$ years after the Big Bang and ended about $4\,\mathrm{G}$ years ago. $\endgroup$ Jun 15, 2014 at 7:47
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$\begingroup$ @RobertCartaino "Dust, on the other hand, is comprised of tiny particulate matter that has undergone the stronger intermolecular bonds to create substances like ice" ... Wouldn't these new bonds be weaker, only being stable at lower temperatures? $\endgroup$– BMSMar 17, 2017 at 1:28
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In astronomy, there is no formal definition of the threshold between gas and dust. Gas can be monoatomic, diatomic, or molecular (or made of photons, in principle). Molecules can be very large, and in principle, dust particles are just very large molecules. I've seen various authors use various definitions, ranging from $\sim100$ to $\sim1000$ atoms.
This is not to say that there isn't a distinct difference between molecules and dust. They have very different properties, but the transition between them is just not perfectly well-defined.
ADDED 13.6.2018: I'm currently attending a conference on cosmic dust, and 200 astronomers weren't able to answer the question of the threshold between large molecules and small dust grains. One way to define a distinction, however, is to look at the associated spectral lines: Molecules emit/absorb and specific wavelengths, whereas dust can emit/absorb over a broader region. But there isn't a specific number of atoms — for instance, the fullerene C540 is a very large carbon molecule, but if you reshuffle its 540 C atoms into amorphous carbon, it is considered a dust grain.
Gas, molecules, and dust can all be hot or cold, but if it becomes too hot, larger particles are destroyed in collisions. So while a molecular cloud typically is very cold and consists of both gas and dust, dust tends to be destroyed (though not completely) in the $\mathrm{H\,II}$ regions around hot stars through collisions with other grains, sputtering due to collisions with ions, sublimation or evaporation, or even explosions due to ultraviolet radiation (see e.g. Greenberg 1976).
To answer your final question, I haven't heard the term "gas" used for dust particles, but metals$^\dagger$ and molecules can both be referred to as gas. For instance, $\mathrm{Mg\,II}$ gas is routinely used to detect distant galaxies, and molecular clouds contain $\mathrm{H}_2$ and $\mathrm{CO}$ gas. In the interstellar medium, roughly 2/3 of the metals are in the gas phase, while 1/3 is in dust.
$^{^\dagger}$"Metals" in the astronomical sense, i.e. all other elements than hydrogen and helium.
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$\begingroup$ Now you clouded my amateurish attempt to understand this. Monatomic dust? When astronomers casually mention "dust" and "gas", are they maybe just referring to certain wavelengths in their spectrograms? Or are they just waving their hands up there to try to feel what it's like. You have pulled my legs a couple of times before here, @pela, I don't have many more legs left. This is astronomy, you can obviously tell me anything. $\endgroup$ Aug 17, 2015 at 20:14
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1$\begingroup$ @LocalFluff: :D No no, gas can be monoatomic, diatomic, etc. To call it "dust", you need many atoms, i.e. in principle molecules. If this molecule is below an ill-defined threshold, we just call it molecules. If it's too large, we call it dust. One difference between molecules and dust is its scattering properties. Whereas molecules tend to scatter light according to certain energy levels, if it becomes too large scattering depends more on the characteristic size of the conglomerate. But there is still no sharp threshold. $\endgroup$– pelaAug 18, 2015 at 8:09
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$\begingroup$ Read the second sentence again. I think it makes sense, but if not, let me know and I'll edit :) $\endgroup$– pelaAug 18, 2015 at 8:10
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$\begingroup$ The scattering properties makes sense. I've heard that there's a gray zone of "large free molecules" with absorption lines which are difficult to disentangle. In basic school and everyday chemistry gas and dust are pretty distinct, I think. But maybe not so in space? $\endgroup$ Aug 18, 2015 at 11:27
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$\begingroup$ I don't know much about molecules, but maybe you're referring to PAHs. I wouldn't say that gas and dust aren't pretty distinct. They have very different properties in many aspects. It's just that there isn't a strict boundary between them, so in a short range they aren't distinct. But on a logarithmic scale, who cares? :) $\endgroup$– pelaAug 18, 2015 at 14:05
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I may just add to the excellent answer by Robert that interstellar dust particles, very much like cigarette smoke in air, hangs in the interstellar gas and interacts with it both kinematically (is dragged along with it depending on the particle size) and energetically (exchanges heat, which can result in significant cooling of the gas). Dust particles also interact with the (stellar) radiation and can be evaporated due to high-energy radiation, but can also grow by condensing from the surrounding gas.
All larger solid astronomical objects (planets, asteroids etc, but not stellar remnants) have formed from dust, which in turn has formed from the heavier elements in the inter stellar gas.
For many astronomical purposes, dust is annoying, as it blocks the light, in particular the shorter wave lengths (reddening and darking the light of stars), hiding stars, in particular in the mid-plane of the Milky Way. As a consequence, the Galactic centre, a place of great astronomical interest, is largely invisible and can only be studied by observing other wave lengths than visible light, in particular infrared which is hardly affected by dust absorption.
