How do stars form?

First, we must have a gaseous nebula or super nova remnant. this gas cloud is going to consist of hydrogen and helium mainly, with a few other things in very miniscule amounts. Virial theorem states as long as the internal energy of cloud stays above twice the gravitational force then the cloud will stay a cloud. If not, then the process for star formation begins. The cloud begins to collapse on itself and hydrogen atoms start to  clump, and this clump gets bigger and bigger as more clumps become one larger clump. as this occurs the core of this soon to be star heats up, and does not stop anytime soon. eventually, the star's core will "ignite", meaning the critical temperature needed start nuclear fusion will be reached and star will be born! *Another possible star formation event can be a supernova going off close enough to a gas cloud, compressing the gas cloud rapidly, and kick starting the star formation process by speeding up the gravitational collapse.*

Viral Theorem:

examples of star formation. 

http://en.wikipedia.org/wiki/File:Star-forming_region.jpg

http://en.wikipedia.org/wiki/File:Star-forming_region_S106_%28captured_by_the_Hubble_Space_Telescope%29.jpg

http://en.wikipedia.org/wiki/File:Orion_Nebula_-_Hubble_2006_mosaic_18000.jpg