- They all have main sequence turn-off points in a similar position and a giant branch joining the
main sequence at that point.
- They have a horizontal branch running from near the top of the giant branch to the main
sequence above the turn-off point.
- In many clusters, there is a region of the horizontal branch which is populated only by stars
of variable luminosity, known as RR Lyrae stars.
- There is considerable variation in the position of the main sequence turn-off point, with
the lowest being in about the same position as those of the globular clusters.
- In many clusters there is a gap between the main sequence and the giant branch, known
as the Hertzsprung gap.