The way I understand fusion

 I'm not a nuclear physicist but this is how I presently understand fusion.

The way I see it the gravity of something as large as a sun causes various chain reactions regarding the energy within that sphere of influence. This then causes Hydrogen (one proton and one electron) per atom to heat up and combine together to form helium which has (2 protons and 2 electrons) and over time within a sun or star it forms more complex atoms over time.

This is why humans can call themselves literally stardust simply because all planets are made up of the stardust that stars create by starting from hydrogen and then through fusion creating more and more complex atoms over time.

So, not only are humans Stardust the planets all are too.

However, through this fusion of hydrogen becoming Helium and then becomes next lithium which has 3 protons and 3 electrons. This continues on down through all the elements over time through fusion.

The fusion process creates both heat and light which lights the whole universe with stars and galaxies. So, the fusion within stars literally creates all the light (at first) in the universe that you can see at night looking up into the milky way at night.