1. Explain how stars are formed in a nebula.

Stars are born within enormous clouds of gas and dust called nebulae. Here's a breakdown of the star formation process:<br /><br />1. **Gravity's Role:** Nebulae aren't uniform. Areas within a nebula might be slightly denser than others. These denser regions exert a slightly stronger gravitational pull on the surrounding gas and dust, causing more material to be drawn in.<br /><br />2. **The Collapse:** As more material accumulates in these dense regions, the gravitational pull intensifies, accelerating the inward collapse. This process can be triggered by external events like a nearby supernova explosion sending shockwaves through the nebula.<br /><br />3. **Fragmentation:** As the nebula collapses, it often fragments into smaller clumps. Each of these clumps can eventually become a star.<br /><br />4. **Protostar Formation:** The collapsing clump continues to draw in material, increasing its density and temperature. At this stage, it's called a protostar. It's not yet a true star because nuclear fusion hasn't begun. The protostar is surrounded by a rotating disk of gas and dust called a protoplanetary disk, from which planets may eventually form.<br /><br />5. **Nuclear Fusion Ignites:** As the protostar continues to collapse under its own gravity, the core temperature and pressure increase dramatically. When the core reaches a critical temperature of about 10 million Kelvin (18 million degrees Fahrenheit), nuclear fusion begins. Hydrogen atoms fuse together to form helium, releasing enormous amounts of energy in the process.<br /><br />6. **A Star is Born:** This release of energy marks the birth of a star. The outward pressure from nuclear fusion balances the inward pull of gravity, stabilizing the star. The star now enters what's called the main sequence, where it will spend most of its life fusing hydrogen into helium.<br /><br />The size and lifespan of a star depend on the initial mass of the collapsing clump. More massive stars burn brighter and hotter but have shorter lifespans than less massive stars.<br />