In all stars found on the MAIN SEQUENCE, hydrogen is currently being fused to form Helium. This is their source of energy against gravitational collapse. As we learned already, stars like the Sun use the Proton-Proton Chain to fuse hydrogen. A very small amount of energy is also created through what is called:

Starting at the top left and moving clockwise:

1) ¹²C, Carbon with 6 protons and 6 neutrons, collides and sticks with Hydrogen, p. A gamma ray is also released. This creates Nitrogen (¹³N).

2) ¹³N decays (a proton changes to a neutron and releases the beta+ particles) to an isotope of Carbon, ¹³C.

3) Another proton is added to ¹³C), creating Nitrogen again, but a more massive isotope, ¹⁴N.

4) Yet another proton is added and ¹⁴N becomes ¹⁵O.

5) ¹⁵O decays (proton --> neutron, releasing beta+ particles) to create ¹⁵N.

6) Finally, another Hydrogen (p) is added to ¹⁵N, but now it's too big! An alpha particle (the Nucleus of a Helium Atom) is released and we are back to our ¹²C

Let's count up what went in and what came out. We started with ¹²C, and ended with ¹²C. All we did was keep adding Hydrogen atoms until a Helium atom broke off!

Just like the P-P Chain, four Hydrogen atoms (the same as bare protons) are converted to one Helium atom. The energy difference (E=mc²) creates the thermal pressure inside the star.

For very massive stars, with very hot central cores, the CNO Cycle is the predominant way in which they fuse Hydrogen to Helium.