Here's a breakdown:

1. Molecular Cloud: These are vast, cold, and dense regions of space primarily composed of hydrogen and helium, along with trace amounts of other elements.

2. Gravitational Collapse: Within a molecular cloud, gravity pulls the gas and dust particles closer together. Over time, the density and temperature of the core increase.

3. Protostar: As the core continues to collapse, it forms a hot, dense object called a protostar. This is not yet a true star because nuclear fusion hasn't started yet.

4. Main Sequence Star: Eventually, the core of the protostar becomes hot and dense enough for nuclear fusion to begin. Hydrogen atoms fuse together to form helium, releasing immense amounts of energy and marking the birth of a main sequence star.

So, while the term "star" is often used to encompass all stages, the true beginning is within a molecular cloud.