Hertzsprung–Russell diagram
MY Cephei is a very luminous, cool and large extreme supergiant star, with a luminosity more than 100,000 times that of the Sun () and a radius in excess of a thousand times the Sun's radius (). It is likely the most luminous, coolest, and the largest supergiant star in its open cluster, and occupies the upper-right hand corner of the Hertzsprung–Russell diagram.