This is because of the Bayer designation system, a system for naming stars within a constellation. The brightest star is assigned the Greek letter "alpha" (α), followed by the constellation's genitive name.

For example:

* Alpha Centauri is the brightest star in the constellation Centaurus.

* Alpha Ursae Majoris (also known as Dubhe) is the brightest star in the constellation Ursa Major (the Great Bear).

However, it's important to note that not all the brightest stars in constellations are called "alpha". This is because the Bayer system was based on visual brightness, and sometimes a star that was originally assigned "alpha" is actually dimmer than another star in the same constellation.