“NEUTRONS in the core. You end up with things like helium-4 which is a helium atom which is number 2 on the periodic table. That 4 means that it has four bits, two protons and two neutrons sitting in its nucleus. Helium-4 is like one of the most stable things that you get.”
http://www.astronomycast.com/transcripts/AstroCast-080922_transcript.pdf