Here's why:

* Molecular: These clouds are primarily composed of molecules, mostly hydrogen (H2) and helium, but also containing traces of other molecules like carbon monoxide (CO) and water (H2O).

* Cold and Dense: They are extremely cold (around -260 degrees Celsius) and dense, allowing the molecules to stick together and form larger clumps.

* Gravitational Collapse: The dense clumps of molecules within the cloud collapse under their own gravity, pulling more material towards the center and increasing its density even further.

* Star Birth: As the core of the collapsing clump becomes increasingly hot and dense, eventually, nuclear fusion ignites, and a star is born.

So, while we often talk about "star nurseries," the more accurate scientific term is a molecular cloud.