Question: How do stars form in space?

Nicolas Labrosse answered on 20 Jun 2017:

In space there are huge ‘clouds’ of gas and dust, which are cool, and not dense. Somehow, sometimes, these clouds are perturbed and there will be small pockets inside the clouds which will become slightly denser. These denser regions can become unstable: via gravity, they will attract more gas and dust in their surroundings to ‘join’ them, becoming even larger, and exerting an even stronger gravitational attraction (the more massive, the stronger the gravitational pull).
As these objects become larger and larger, and heavier and heavier, they will start collapsing under their own weight. This may sound a bit strange, but once your mass is very large, you exert a strong gravitational force on yourself, if you like. The only way to resit this ‘gravitational collapse’ is to become hotter inside, as the larger temperature will exert a pressure to resist the gravitational collapse. And as a matter of fact, when you compress a gas (for instance if the volume available to it decreases), the gas becomes hotter.
So our future stars become heavier and heavier, therefore collapsing on themselves, but becoming hotter inside (due to increased pressure), and so can resist the gravitational collapse… It’s a subtle equilibrium which is reached progressively and maintained for a long time, during which the future star continues to attract more mass (gas and dust from its surroundings), to become heavier, and to become hotter.
Eventually, the temperature may become so large at the centre of this thing, that nuclear fusion reactions will start, when hydrogen atoms combine together. This produces a lot of energy, which is then enough to stop the gravitational collapse. The energy which is produced makes the star shine brightly, and this clears the surroundings from any left over gas and dust, so there is nothing more that will ‘fall’ on the star.
At that stage, we have a new star!