The brightest star on a Hertzsprung-Russell (HR) diagram isn't necessarily a single star, but rather the stars that lie along the top right corner. This region represents red supergiants and hypergiants.

Here's why:

* Luminosity: The HR diagram plots stars based on their luminosity (brightness) and temperature. The vertical axis represents luminosity, with higher values indicating brighter stars.

* Temperature: The horizontal axis represents surface temperature, with hotter stars on the left and cooler stars on the right.

* Red Supergiants/Hypergiants: Red supergiants and hypergiants are extremely large and luminous stars, making them the brightest stars on the diagram.

Important Note: While red supergiants and hypergiants are the brightest stars on the HR diagram, their apparent brightness as seen from Earth depends on their distance from us. A star closer to Earth can appear brighter than a more distant, but intrinsically brighter star.