Star Dust

The dust grains produced when big stars spill their guts via supernova or stellar wind are tiny: mostly micron or submicron scale grains. These are the stuff that planets are made of. It takes about a billion such grains to make a millimeter sized particle, and a billion of the latter to make a something of meter size. Another factor of a trillion is needed before mutual gravitational attraction can become significant.

Each of these scales, and the additional factor of a billion before you get to a real planet has its distinct and complicated physics - some of which remains poorly understood. That first factor of a billion might be the best understood. Friction with the gas in the circumstellar disk slows down those tiny particles to tiny collision velocities where they readily stick together. By the time they have fallen to the center of that disk (under the influence of the perpendicular component of the star's gravity) they have already grown to millimeter size. It take only a tens of thousands of years and a billion collisions for that to happen. Raindrops grow in similar fashion in clouds, albeit a billion times faster.

Reference: Astrophysics of Planet Formation by Philip J. Armitage


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