Answer:
1. The temperature increases in the center of a nebula, and nuclei begin to fuse.
2. As the nuclear fusion sets off a chain reaction, the nebula gets hotter and brighter and turns into a star.
3. The star stays in the main sequence stage for a long time.
4. The star runs out of hydrogen and turns into a planetary nebula or a supernova, depending on its mass.
5. Heavy elements are created when the supernova explodes.
6. All that remains of the star now is the core. In some cases, the star turns into a black hole.
Step-by-step explanation: