Canis Major
Canis Major is a prominent constellation because of its many bright stars. These include Sirius (Alpha Canis Majoris), the brightest star in the night sky, as well as three other stars above magnitude 2.0. Furthermore, two other stars are thought to have previously outshone all others in the night sky—Adhara (Epsilon Canis Majoris) shone at -3.99 around 4.7 million years ago, and Mirzam (Beta Canis Majoris) peaked at −3.65 around 4.42 million years ago. Another, NR Canis Majoris, will be brightest at magnitude −0.88 in about 2.87 million years' time.
Bayer designation
Although the brightest star in Draco is Eltanin (Gamma Draconis), Thuban was assigned "alpha" (α) by Bayer because, due to precession, Thuban was the north pole star 4,000 years ago. Sometimes there is no apparent order, as exemplified by the stars in Sagittarius, where Bayer's designations appear almost random to the modern eye. Alpha and Beta Sagittarii are perhaps the most anomalously designated stars in the sky. They are more than two magnitudes fainter than the brightest star (designated Epsilon), they lie several degrees south of the main pattern (the "teapot" asterism), they are more than 20 degrees off the ecliptic in a Zodiacal constellation, and they do not even rise from Bayer's native Germany (while Epsilon and several other brighter stars do). The order of the letters assigned in Sagittarius does correspond to the magnitudes as illustrated on Bayer's chart; but the latter do not agree with modern determinations of the magnitudes.