Is the sun a main sequence star?

Answer 1

A "Main Sequence" star does not "really exist".

The term "main sequence" refers to a period in a stars life when it is basically converting hydrogen into helium. Once it has consumed all of the hydrogen, a star will evolve out of main sequence into stellar remnants.

The main sequence is a continuous and distinctive band that appear on plots of stellar colour versus brightness on the Hertzsprung Russell diagram.

Our Sun is smaller and a little more yellow than most other main sequence stars, which are white. It will gradually run out of hydrogen fuel in another 5 billion years, at which point it will expand to become a red giant. After that it will dim, cool down, and fade into a white dwarf.

See related question on the life of a star.