Where are the main sequence stars?
Most stars fall on a rather wide line (highlighted in red) that passes from the lower right to the upper left of the page. This line is called the main sequence. It tells us interesting things about how stars work. You can't pick just any brightness and temperature and expect to find a star with these qualities.
The brightness of the stars on the main sequence decreases as one moves toward cooler temperatures (moving from blue to red). This is not surprising because stars are black bodies and cooler black bodies give off less light than hotter ones.
But the brightness drops off faster than it should for blackbodies of the same size. From this we can tell that the stars decrease in size as you move toward red on the main sequence.