Molecular clouds fragment as they collapse because a. the rotation of the cloud throws some mass to the outer regions. b. the density increases fastest in the center of the cloud. c. density variations from place to place grow larger as the cloud collapses. d. the interstellar wind is stronger in some places than others.

When we talk about molecular clouds in space, one of the most intriguing phenomena is gravitational collapse. This process is the driving force behind the formation of stars. Imagine a massive, cold cloud of gas and dust sitting out in space. Eventually, gravity causes the cloud to start collapsing inward on itself. As it collapses, the material within the cloud gets squeezed into a smaller and smaller volume. This squeezing increases the density and temperature in the core of the cloud.

Gravitational collapse is important because it's the first step toward star formation. Without the collapse, the gas and dust would never become dense enough to ignite nuclear fusion, the process that powers stars. Hence, gravitational collapse sets the stage for all subsequent steps in stellar formation.

Density variations are critical when understanding why molecular clouds fragment. When a molecular cloud is undergoing gravitational collapse, not all parts of the cloud are the same. Some regions have higher density, while others have lower density.
As the cloud collapses, these initial density variations become more pronounced. Imagine squeezing a sponge filled with water; some parts might compress faster than others, creating pockets where water collects. In a similar way, as the molecular cloud collapses, different regions contract at different rates due to these variations in density.
Eventually, these regions with higher density grow into separate clumps. Each clump can continue to collapse under its own gravity, potentially forming a new star or a small group of stars. This is known as fragmentation, and it explains how a single large molecular cloud can give birth to many stars.

Stellar formation is the fascinating process that transforms dense regions of molecular clouds into stars. It's a multi-stage process that begins with gravitational collapse and is often complex.
1. **Gravitational Collapse**: As previously mentioned, the process starts with the gravitational collapse of a molecular cloud.
2. **Fragmentation**: Due to density variations, the collapsing cloud fragments into smaller pieces. Each fragment can eventually form its own star.
3. **Protostar Stage**: In each fragment, the core becomes increasingly hot and dense. This core, now called a protostar, continues to gather material from its surroundings.
4. **Ignition of Nuclear Fusion**: When the core's temperature and pressure are sufficiently high, nuclear fusion ignites. At this point, the protostar becomes a main-sequence star.
5. **Stabilization**: The new star reaches a state of equilibrium, where the outward pressure from nuclear fusion balances the inward pull of gravity.
This step-by-step transformation, driven by gravitational collapse and influenced by density variations, ultimately leads to the birth of new stars and contributes to the dynamic evolution of our universe.