 In more massive stars , helium is produced in a cycle of reactions catalyzed by carbon called the carbon @-@ nitrogen @-@ oxygen cycle . 
 In more massive stars , helium is produced in a cycle of reactions catalyzed by carbon called the carbon @-@ nitrogen @-@ oxygen cycle . 
In more massive stars , helium is produced in a cycle of reactions catalyzed by carbon called da carbon @-@ nitrogen @-@ oxygen cycle .