Stars produce heat by the process of 1. combustion. 2. implosion. 3. nuclear fission. 4. nuclear fusion.

4. nuclear fusion.

Explanation:

The process that helps stars generate heat energy from atomic nuclei is called nuclear fusion. Due to gravity, pressure on the hydrogen gas in the center or core of the stars are enormous causing temperature to reach about 27 million°F (15 million°C).  This hotness makes hydrogen atoms to fuse together to form helium atoms. This process is called nuclear fusion. Vast amount of energy is released which enables the star to shine.