Here's why:
* High Luminosity: Red supergiants are incredibly luminous, meaning they radiate a lot of energy. This places them high on the luminosity axis of the HR diagram.
* Cool Temperature: Despite their high luminosity, red supergiants have relatively cool surface temperatures. This is indicated by their red color, and places them on the right side of the HR diagram, where cooler stars reside.
Therefore, the upper right corner of the HR diagram is the home of these massive, luminous, and cool stars.