Caption: It is thought usually some TRIGGERING EVENTS are need to increase the density of dark, dusty molecular clouds for the molecular clouds to become dense enough to initiate gravitational collapse under self-gravity and then fragmentation to dense cores that subsequently collapse to become protostar and stars on zero-age main sequence (ZAMS) undergoing hydrogen burning in their inner cores (Se-221).

Four TRIGGERING EVENTS are:

1. Cloud-cloud collisions that great regions of high density.

2. Radiation pressure and stellar winds from hot young stars (mainly OB stars). Such stars have typically formed in an earlier generation of star formation in the same molecular clouds.

3. The shock pressure from supernova ejecta. Supernova progenitors are the O stars and and the more massive B stars. These stars typically formed in an earlier generation of star formation in the same molecular clouds. The stars go through their lives quickly within a few to ten million years and explode. This TRIGGERING EVENT is closely related to TRIGGERING EVENT 2.

4. The compression of a spiral density wave which is the cause spiral arms in grand design spiral galaxies, but NOT in flocculent spiral galaxies.

EXCEPT flocculent spiral galaxies do NOT have spiral density waves as a trigger. But nevertheless, most star formation happens in flocculent spiral arms which are wound-up giant molecular clouds.

In fact, in all kinds of spiral galaxies, one expects almost all star formation to happen in the spiral arms.

Star formation usually does NOT happen in elliptical galaxies which are usually quenched galaxies.

Star formation does happen in irregular galaxies which, in fact, are a lot like spiral galaxies with one or more fragmentary spiral arms.

Credit/Permission: © David Jeffery, 2003 / Own work.
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