How is a colour magnitude diagram (CMD) actually created?
Here, HST data of the core of NGC 3201 (HST/ACS catalogue, Sarajedini+, 2007) is shown.
The colour represents the visual appearance to the human eye and the stellar brightness is given by the circle size:
Do you see any pattern? Maybe that most stars share the same white colour and faint magnitude.
When the x-axis is sorted by colour ...
... and the y-axis sorted by brightness (magnitude) ...
we end up with the colour magnitude diagram (CMD of NGC 3201) which is fundamental in astrophysics:
The CMD with stars that could be explained by stellar evolution of single stars:
The so-called 'binary main sequence' (yellow region):
And blue straggler stars (blue region):