Even though it's lighter than oxygen (with one less proton and neutron), it forms in stars at a slower rate via a reaction sequence called the CNO cycle, also described on the previous page.
Most of the nitrogen in the universe was probably contributed by intermediate-mass stars (those of about 1 to 8 solar masses), where the CNO cycle predominates.
The first, which occurs in an outer shell, converts hydrogen into helium via the
CNO cycle, whereby carbon, nitrogen, and oxygen catalyze the hydrogen-to-helium reaction.