Thread by @anderssandberg

#FridayPhysicsFun – coming home after 6 months of absence implied a lot of dusting. Still almost nothing compared to the dust in Messier 64, the "Black Eye Galaxy". What and why is dust? en.wikipedia.org/wiki/Black_Eye…

“Dust” refers to fine particles of solid matter. It can be almost anything. But the smallness makes it behave different from larger pieces.

Household dust is dead skin cells, various fibres, pollen, mites, soil particles etc. The composition depends a lot on where you are and who you are: the bacterial content is about half determined by humans.
ehp.niehs.nih.gov/doi/10.1289/eh…
linkinghub.elsevier.com/retrieve/pii/S…

You can of course make art out of the dust, like Allison Cortson's portraits: phaidon.com/agenda/art/art…

Outdoors it shows up as wind-borne dust, formed by erosion as sand-grains rub against each other in deserts or when fine soil particles are dislodged. Important factor in local & global weather & ecology (iron seeding in oceans, rainfall in Amazonia). en.wikipedia.org/wiki/Saltation…

The dust I care most about is in space (as evidenced by the build-up of domestic dust at my home).
en.wikipedia.org/wiki/Cosmic_du…

There is fine dust in the solar system producing the Zodiacal light as it scatters sunlight. Much is only mildly scattered, making up the triangular glow around the sun. Some backscatters straight back, making up the gegenschein opposite to the sun.
en.wikipedia.org/wiki/Zodiacal_…

A small dust particle (<1 μm) is pushed out of the solar system since the light pressure dominates over the Sun’s gravity. Bigger particles (up to 1 mm) experience Poynting–Robertson drag that slow them, making them spiral inwards. en.wikipedia.org/wiki/Poynting%…

It looks like the main source is comets but ejection from Mars also looks important. Comets mostly shed organic, soot-like dust but recently iron and nickel atoms has been observed too – it is yet unclear how that works. nature.com/articles/d4158…

There is a lot of interstellar dust as one can see in the dark bands of the Milky Way. If you send a light ray towards the galactic centre the probability of it hitting a dust grain is almost 1. Still, it is about 1% of the mass of the interstellar medium. en.wikipedia.org/wiki/Cosmic_du…

The average density is about 500 grains per cubic kilometre. A semiconductor factory clean room would be proud to be so empty! (Dark clouds would actually not comply with the standard) en.wikipedia.org/wiki/Cleanroom

The opaque dust is very fine: the area of grains scale as r^2 while volume and mass go as r^3, so a given amount of matter has more area the more finely it is ground. Hence, it obscures more, down to a hundred nanometre limit set by light wavelengths.

The dust around the Pleiades form a reflection nebula: most light is forward scattered because of ≈1 μm dust, and blue because of Rayleigh scattering. Iron and nickel aligns the grains with magnetic fields, producing the “combed” appearance.
en.wikipedia.org/wiki/Pleiades#…

Dust grains can also acquire electric charges on their surface, making them spin and produce microwaves.
en.wikipedia.org/wiki/Spinning_…

This dust tends to form in the stellar wind of carbon stars or red giants with some contribution from supernovas. The carbon and silicate grains get covered by ices in cold interstellar clouds. It also evaporates due to UV light. academic.oup.com/mnras/article/… earth-planets-space.springeropen.com/articles/10.50…

The reason I care about interstellar dust is the problem of fast interstellar travel: you do not want to run into a large grain when travelling close to the speed of light. iopscience.iop.org/article/10.384…

We do not know the density of bigger grains since astronomical observations can mainly see the finer dust. It is generally believed that such grains are rare, but how rare remains uncertain. osti.gov/biblio/1525731

The dust density sets a travel distance limit at any given speed, equal to when the volume swept by the craft is big enough to contain a dangerous particle. I think we can make narrow craft and the density isn't so bad, but we need to find out.