Here's a breakdown:

* Hydrogen nuclei: These are protons, the simplest form of a nucleus.

* Stars: Stars are massive balls of hot gas, primarily hydrogen.

* Combine: Due to immense pressure and temperature within the star's core, hydrogen nuclei are forced together.

* Helium and heavier elements: This combining of nuclei releases energy and forms helium, and under certain conditions, even heavier elements like carbon, oxygen, and iron.

Key takeaways:

* Nuclear fusion is the energy source of stars.

* It's the process that creates the elements heavier than hydrogen.

* It's a powerful process that releases enormous amounts of energy.

Let me know if you have any other questions!