Review

#01

Collapse of a dense, cold cloud by gravity. The cloud fragments to make smaller cloudlets that will become individual stars. The cloudlets flatten because of angular momentum conservation. In a given cloudlet, a central core starts to heat up. The increasing gas pressure resists collapse, and a slow protostellar phase of gravitational contraction begins. Finally the core temperature gets large enough to initiate H nuclear fusion, and a star is born.

#02

Heat leads to gas pressure which resists the gravitational collapse of the gas. So heating affects how long it takes for gravity to make a star.

#03

Rotation leads to a flattened disk of gas. It resists the tendency of gravity to form a star.

#04

Charged particles can easily travel along magnetic fields, but oppose motion across magnetic fields. However, neutral particles are immune to magnetic forces. In the net some ions latch onto magnetic fields and do not participate in the collapse of the cloud. The neutral particles then have to flow past these ions, which can lead to collisions and thus some resistance to gravitational collapse. So magnetic fields make it harder to form stars at the cloud collapse stage. (Magnetic fields are probably important is helping with the spin problem though!)

#09

Brown Dwarfs are objects not massive enough to initiate core H fusion. They are rather too massive to be classified as planets.

#10

T Tauri stars are basically protostars.

#11

This is complicated. But the models and ideas for star formation make some basic predictions, like expectations for disks and jets, and clusters of stars, rapid rotation of young stars, plus the presence of magnetic fields. We expect to find young stars near dense, cold clouds. Although we never get to watch any single star ``come to life'', there are so many stars at so many different stages of formation, we can piece the circumstantial evidence together to test models and ideas for how a typical star is made and changes.

#17

When clouds collapse, they fragment. We thus expect stars to form in groups, such as clusters and associations.

#19

Old clusters are lacking luminous, massive stars along the main sequence. Young clusters can be lacking is faint, low mass stars along the main sequence.