Sternfusionen : Physik auf der Suche nach dem Ursprung der Erde

All die „schweren“ Elemente, aus denen die Erde und letztendlich auch wir bestehen, hängen an den Fusionsprozessen, die in Sternen stattfinden. Wir, beziehungsweise unsere Atome, stammen alle aus dem Inneren von Sternen oder aus Supernovae. Damit haben wir in gewisser Weise schon alle eine weite Reise hinter uns. Beginnen wird der Beitrag aber „etwas“ später, als die Elemente, die man benötigt, um ein Planetensystem zu basteln, bereits existierten und in großen kalten Gebilden als interstellare Molekülwolken in einer Galaxie zu finden waren.
Planet formation and star formation are intimately linked to each other. As a star forms, it is surrounded by a protoplanetary disk consisting of gas, dust and ice particles. Collisions between the particles dominate the early evolution of solids, as particles stick together by surface forces and grow to larger aggregates. Contact forces are an important ingredient and are not well known down to particle sizes of nm and especially not for ice particles. In novel laboratory experiments with a thermal gradient force microscope, these data on sticking, rolling and twisting between particles are collected. Initial aggregation processes can numerically be simulated in an N-body approach considering the motion of every dust grain in an aggregate. However, already in mm-particles the number of grains is too large for such simulations of collisions and other approaches are necessary. Laboratory experiments provide means in a size range up to decimeters to study the outcome of energetic collisions. They show that bouncing dominates at low velocities for compact aggregates and that fragmentation of decimeter bodies occurs at velocities well below velocities expected in protoplanetary disks. They also show, however, that high energy collisions of a small projectile can lead to mass gain of a larger target, as impact energy is dissipated by a large degree of projectile fragmentation. This allows the formation of larger seed particles to planetesimals in protoplanetary disks feeding on the smaller particles which are numerous as they cannot grow through the bouncing barrier.
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