Stars are born in large, cold clouds of gas and dust called molecular clouds or nebulae, which can span hundreds of light-years and contain from thousands to millions of times the mass of the Sun

. These molecular clouds are dense and cold enough for gas and dust to clump together under gravity, forming high-density pockets that collapse to create protostars, the earliest stage of star formation

. These star-forming regions are often called stellar nurseries or dense cores within molecular clouds. The dense cores are egg-shaped cocoons of gas and dust that are opaque to visible light but can be studied using radio and infrared telescopes because these wavelengths penetrate the dust

. Famous examples include the Eagle Nebula, where visible light shows dark pillars of dust, but infrared imaging reveals newborn stars hidden within

. Additionally, recent research suggests that nearly all stars form in pairs within these dense molecular clouds, implying that binary star formation is common in these stellar nurseries

. In summary, all stars are born inside dense molecular clouds, often in clusters, where gravity causes gas and dust to collapse into protostars that eventually ignite nuclear fusion to become stars