And so I went home from that conference and in the next several weeks I looked at other elements in the periodic table going just systematically to helium. Well, that wouldn't work, helium and hydrogen don't give you anything; helium and helium don't give you anything, because neither lithium-5 nor beryllium-8 are stable nuclei. So you had to go on and when you took lithium, beryllium, boron, they could give you a very big energy production together with hydrogen but they would then be totally exhausted immediately... until I came to carbon and carbon was totally different, namely when you start with carbon and add a hydrogen you get nitrogen-13 which disintegrates spontaneously into carbon-13. You can add another proton, make nitrogen-14, and then another proton making oxygen-15 and nitrogen-15, and finally when you have nitrogen-15 and add a proton, you don't get oxygen-16 as you might believe, but instead it disintegrates into carbon-12 and helium-4. And so you get your initial nucleus back, the carbon only acts as a... as a catalyst to bring four protons together to form a helium nucleus; from that you get all the energy release which is involved in that reaction, and you get it without having to invoke weak interactions like the proton-proton reaction. It is therefore generally accepted now that it is the proton-proton reaction for the sun, but it is the carbon cycle - carbon citrogen cycle - for all massive stars beginning a little below Sirius and going up all the way in the Stellar Table to the most luminous stars.