Dust, it's everywhere! From the Earth-based perspective, dust is that layer of film made up of dirt, mostly biological, that settles on things in the corners of your room, or floating in the air reflecting light. On a cosmological scale, dust is made up of a huge variety of materials, literally just about everything in the Universe. And once cosmic dust settles, it forms stars and galaxies.
Cosmic dust also has a pretty large range in size, from just a few molecules up to a couple microns (µm). There really is no upper limit, but after a certain point, things obviously start to become asteroids and comets and meteoroids and such. There is a proposed definition for a meteoroid of being between 100 µm and 10 m across. That would then classify dust any smaller then 100 µm, and starting on a molecular level. Human hair is usually around 100 µm thick.
Cosmic dust can further be distinguished by it's location.
|Intergalactic Dust Bunnies Around NGC 1316. Hubble.|
Intergalactic dust is the dust between galaxies in intergalactic space. Intergalactic dust can be free floating, but are also known to exist in clouds around galaxies, the Milky Way has at least four located nearby. The clouds are so diffuse that it would be hard to tell that you were in it though. Their grains may also be large, some would say that even mountain, or even continent or planet, sized objects could be considered dust on the large intergalactic scales. Because of the large variations in distribution, intergalactic dust may also affect measurements of quasars and supernova.
|NASA Artist Interpretation of Proto-Planetary Disk|
|Zodiacal Light. ESO.|
From our Earth, we can also see something called zodiacal light. This is the back scattering of sunlight on cosmic dust particles in our solar system. Conditions to see this light are hard to come by, as it is even fainter then the Milky Way.
|Saturn Eclipse, as seen from Cassini.|
Finally, we get to circumplanetary dust. This is the dust that orbits around planets and form planetary rings. All of the gas giants of the solar system have planetary rings, Saturn's being the most spectacular. They form from dust particles being captured by the planet from the proto-planetary disk, but to close to the planet to coalesce into moons. The stability of these rings are debated, and it may be that they will eventually dissipate or be absorbed in their planets.
So, I hoped you learned a lot about cosmic dust. It exists all over the Universe and is very important in how things work. When I first learned about it, I kind of rolled my eyes. Who would believe someone would devote their entire career to studying dust? But it turns out it is one of the most fascinating topics about our Universe.
EDIT: Also, I'd like that thank the birthday wishes from my previous post. Today (March 24) happens to be my birthday. 25, in case you are wondering.