Provide your best answer for the following question. Give ONLY the answer, no other words or explanation.
For example:

Answer: <most likely answer, as short as possible; not a complete sentence, just the answer!>.

The question is: The stages of evolution of a star are determined by the mass of the star at the beginning of its life cycle. Which stage of evolution is the oldest form of a massive star?
neutron star
Main sequence (for massive stars, the oldest form is not well defined as they skip the red giant stage due to their mass).