dense molecular core
If a giant molecular cloud is the nursery, a dense molecular core is the individual crib: a small, especially compact knot of cold gas inside the larger cloud, the actual seed from which a single star (or a small family of stars) will grow. Picture the cloud as a turbulent fog full of clumps; the densest, calmest clumps are these cores.
A dense core is typically about a tenth to a fraction of a light-year across and holds anywhere from a fraction of a solar mass up to a few solar masses of gas. Inside it the gas is denser than the rest of the cloud, often tens of thousands of molecules per cubic centimetre or more, and just as cold, around 10 degrees above absolute zero. A core that has become dense enough that gravity is about to win over the gas's own pressure, but has not yet begun to collapse, is called a prestellar core; once collapse starts and a protostar begins to form at the centre, it becomes a protostellar core. Because the core is so cold, it glows not in visible light but in the far infrared and in millimetre radio waves from molecules like ammonia and CO.
Dense cores matter because they are the bridge between a galaxy's vast reservoir of gas and the individual stars we see. The mass of a core roughly sets the mass of the star it makes, so the distribution of core masses is a major clue to why nature builds far more small stars than big ones. Studying cores lets astronomers catch the very first moment of a star's life, before any light from fusion has switched on.
Barnard 68 is a famous nearby dark dense core, a roughly half-light-year blob of cold gas so opaque that it appears as a starless black hole punched into the rich star field behind it.
A prestellar core caught on the brink of collapse, dark because its dust hides the stars beyond.
Not every dense core collapses into a star: some are not quite heavy enough and simply disperse, and turbulence or magnetic support can hold a core stable for a long time before gravity finally wins.