Instructions: Given a scientific passage and an answer, generate a question for the given answer.
Input: Passage: For most of a star's life, hydrogen atoms fuse to form helium atoms. A star like this is a main sequence star . The hotter a main sequence star is, the brighter it is. A star remains on the main sequence as long as it is fusing hydrogen to form helium. Answer: helium atoms
Output:
For most of a star's life, hydrogen atoms fuse to form what?