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When I was a graduate student, interstellar dust was thought to be largely icy; what one now thinks of as cometary material.

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But in fact, we found out in the meantime that dust is not that at all.

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It is more like rocks, silicates and small pieces of carbon, which you might characterize as soot.

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The other things that we discovered are that there is a wide range of particles sizes: they are not all one tenth of a micron in size.

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Some are as small as molecules; in fact, there is a borderline area in which you can think of them as very tiny dust grains or as very large molecules.

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Some of those large molecules are like small sheets of graphite with hydrogen attached to them: we call these polycyclic aromatic hydrocarbons and they can be found in terrestrial counterparts like mothballs, for example.

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Automobile soot is another good representation of a mix of these compounds.