When fusion reactions occur in the core of the protostar, lots of energy is released. Here are the steps involved:

1. Energy release heats the protostar. As energy is released in the core of the protostar, it becomes hotter. The core heats up and brightens, eventually forming a star.

2. Heat generated forms helium. The heat from fusion reactions causes hydrogen nuclei in the core to fuse and form helium nuclei. These reactions release more energy, allowing further helium production.

3. Two key forces act on the star. During fusion, two forces keep the star in balance, making it a main sequence star:

• Gravity pulls the star's matter inward, causing gravitational collapse.
• Pressure from fusion pushes the star's matter outward, causing expansion.