About 90 percent of the stars on the Hertzsprung-Russell diagram are main sequence stars.

Here's why:

* The Main Sequence: This is the longest stage in the life of a star, where it fuses hydrogen into helium in its core. It's a diagonal band on the H-R diagram, where stars are arranged by temperature (hotter on the left) and luminosity (brighter on the top).

* Other Stages: Stars spend relatively short amounts of time in other stages, like red giants, white dwarfs, and supergiants, which are found in different regions of the H-R diagram.

So, the vast majority of stars observed are in their main sequence phase, making them the most common type of star in the universe.