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

Comments

Post a Comment

Popular posts from this blog

Anti-Libertarian: re-post

The most pointed critique of social security and liberalism in general is the libertarian critique. The problem with libertarians, for a liberal, is that we have too much in common. We both believe in individual rights, tolerance of individual differences, and dislike government prescription of religion. The basic difference, it seems to me, is the different answers we give to Cain's (with thanks to the Captain - my original version had the corpse asking the question!) famous question: "Am I my brother's keeper?" That's not really my basic bitch against the libertarians though. My real complaint is the same as my complaint against most religion - its premise is a fraud. For those who can't stand to wait for the punchline, I believe that trying to implement libertarian principles leads to tyranny or social disintegration. Demonstrating that takes some historical (and pre-historical) context. For all but the last 15,000 or so of the 100,000 years the human spec...
Read more

Uneasy Lies The Head

Uneasy lies the head that wears a crown ........... Shakespeare Henry IV, Part II. So it is with us and our slightly distant primate cousins , so it seems. A 9-year study of wild baboons in Kenya by Princeton University and the Institute of Primate Research may turn conventional assumptions of alphas males upside down. The study found that alpha males have high testosterone levels, which allows them to dominate other baboons, get access to more food, and attract better mates. This is expected. Surprisingly, though, the alpha males also have high levels of stress hormone glucocorticoid, which scientists measured from their feces samples. In fact, the stress of alpha baboons was on par with low-ranking baboons. The convention wisdom is that access to food and females provides life security and thus less stress for dominant males. Conversely, weak males who are constantly threatened by starvation and physical violence from stronger males are thought to be more stressed. The st...
Read more

We Call it Soccer

The US played its best, and maybe only good game of World Cup, but lost thanks to brilliant goalkeeping by the Swedish keeper and the bizarre method soccer uses to settle ties when three US stars could not put the ball on frame from the 12 yard spot.  At the end it came down to a blocked shot that spun in by maybe a couple of centimeters. Well, that's football - too much like life.   Which brings me to a modest proposal.  Replace the thirty minute overtimes with as many 7-player 15 minute periods as needed to settle it.  Scoring should be much easier with fewer players.
Read more