Here's why:

* Main Sequence Stars: These stars are fusing hydrogen into helium in their cores. They are stable and represent the longest phase of a star's life.

* Sirius: Sirius is a binary star system consisting of Sirius A (the brighter one we see) and Sirius B (a white dwarf). Sirius A is a main sequence star of spectral type A1V. This classification means it's fusing hydrogen in its core and is in the main sequence phase.

Sirius B, on the other hand, is a white dwarf. It's a remnant of a star that has exhausted its nuclear fuel and has collapsed. White dwarfs are not considered main sequence stars.

In summary: While Sirius is a binary system with one component (Sirius B) that is not a main sequence star, the primary star, Sirius A, is a main sequence star.