1. Gravity overcomes internal pressure:

* Sufficient mass: The cloud needs to have enough mass for its own gravity to overcome the outward pressure from the gas particles within it.

* Low temperature: Cold gas particles have less kinetic energy, meaning they move slower and are more easily pulled together by gravity.

* High density: A denser cloud has more particles packed together, increasing the gravitational force between them.

2. Triggering event:

* Supernova shock waves: The shock wave from a nearby exploding star can compress the gas cloud, increasing its density and triggering collapse.

* Collision with other clouds: Two gas clouds colliding can cause a compression that leads to gravitational instability.

* Spiral arms of galaxies: The density waves in spiral galaxies can trigger the collapse of gas clouds within them.

In summary, the collapse of interstellar gas clouds to form stars occurs when their gravity overwhelms the internal pressure, a process often triggered by external factors like shock waves or collisions.