Gravity and pressure
Gravity is what triggers the formation of stars and galaxies. Gravity is a "long range force" since it acts at great distance; the force between two masses m and M at a distance is inversely proportional to the distance squared (~ G m M/ r2, G = gravitational constant = 6.67 10-11 Newtons/m2 Kg, ). It is always attractive and is experienced by all particles that have a mass. Should gravity be the only force, then all the dust and gases of the Universe would coalesce in smaller and smaller clumps (recall that the force becomes bigger as the distance gets shorter!) until only black-holes remained. In reality, what happens is the following. Suppose that in a cloud of dust and gas there is a region that is denser than the others. Then nearby gas starts to follow into that region; as it does, the density increases as well as the thermal pressure, due to the fact that the gas gets hotter due to friction (there are many more encounters among atoms than in the lesser dense regions). Thus, the infall of gases towards denser regions is balanced by the pressure of gases. If the dense gas is massive enough, the center core might reach tempereratures high enough that thermonuclear reactions begin.In the Sun, or other "normal stars", a conditions of "hydrostatic equilibrium" between the inward push of gravity and the outward push of pressure is achieved. How stars form from gas and dust is still a poorly understood problem.
The Orion Nebula is a place where star formation is occuring. The word nebula comes from Latin and means cloud. The clouds shown in the picture are illluminated by stars embedded in the cloud or just behind it. Te average density of the Orion Nebula is around 1,000 hydrogen (H) atoms per cm3. In general, clouds reach sizes of parsecs and their density goes from 10 H atoms for cm3 to 106, or 10-19 to 10-13 of an atmosphere!
Internally related links:
Java applets about gravitation
Big Bang Theory