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Lecture6 Formation Stars &amp; Galaxies

# Lecture6 Formation Stars &amp; Galaxies - Formation of...

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2/27/09 Astronomy 309R - Fall 2008 1 Formation of Stars and Galaxies

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2/27/09 Astronomy 309R - Fall 2008 2 Formation of Stars Main Concepts Turbulence : violent, unsteady, and random movement of gas in space, where it collides and mixes with itself. Gravitational fragmentation : physical process in which a larger accumulation of gas in space, bound together by gravity, splits into smaller accumulations, each separately bound by gravity. Accretion : an object bound by its own gravity attracts and incorporates the surrounding material (interstellar gas) by gravity. Transport of angular momentum : physical process in which angular momentum gets redistributed from one part of a system to another part. The part that loses angular momentum can collapse under its own gravity. Stellar initial mass function : the number of stars that form with a given mass (e.g., the number of stars that are 0.5, 1.0, 1.5, 2.0 etc. times the mass of the Sun).
2/27/09 Astronomy 309R - Fall 2008 3 Gravity Makes Turbulence Consider a self-gravitating ball of gas surrounded by some more gas. The ball’s gravity pulls the surrounding gas. The surrounding gas falls toward the center of the ball. The falling gas hits the surface of the ball unevenly from different directions. The interior of the ball is pushed around by the falling gas. If the surrounding gas keeps falling, the interior of the ball remains perturbed and in the state of motion. The interior of the ball is turbulent . Fig. Wise & Abel 2007

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2/27/09 Astronomy 309R - Fall 2008 4 Gravitational Fragmentation Gravity of individual parts of a cosmic accumulation of material wins over the gravity of the whole accumulation. This process continues hierarchically , forming ever smaller pieces. Gravitational fragmentation is a form of instability : a small perturbation makes the system to transform itself into an entirely new state. In gravitational fragmentation, small parts of the whole are initially not self-gravitating, but they become self-gravitating as the gravity of individual pieces strengthens.
2/27/09 Astronomy 309R - Fall 2008 5 accretion without angular momentum: material falls straight toward the center accretion with angular momentum: material cannot fall toward the center, but instead forms a disk around the center uncommon outcome, because in general, material always has angular momentum common outcome

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