Here's why:

* Main sequence: This is a stage in a star's life, not a separate category. Our Sun is currently a main sequence star.

* White dwarf: This is the final stage of a star's life after it has exhausted its fuel.

* Red giant: This is a stage in a star's life before it becomes a white dwarf.

* Neutron star: This is the incredibly dense remnant of a massive star's core after a supernova explosion.

All of these objects are types of stars at different points in their evolution.