Here's why:

* Nebulas are giant clouds of gas and dust. These clouds are primarily composed of hydrogen and helium, the building blocks of stars.

* Gravity plays a key role. Over time, gravity pulls the gas and dust particles in a nebula closer together.

* Density increases. As the particles gather, the density of the cloud increases, and the temperature rises due to friction.

* Nuclear fusion ignites. Eventually, the core of the collapsing cloud becomes so dense and hot that nuclear fusion begins. This is the process where hydrogen atoms fuse to form helium, releasing immense energy and marking the birth of a star.

Therefore, nebulas are the "stellar nurseries" where new stars are born. Some famous examples of star-forming nebulas include the Orion Nebula, the Eagle Nebula, and the Carina Nebula.