I have to retype again
The differences between the non endemic Maltese Star Thistle (Centarea melitensis) and the common Southern Star Thistle (Centaurea nicaeensis) are very small, and it is difficult if you do not have the two to compare with.
The most different feature mentioned is that C. mlt has a smaller flower head from C.nica, but this is not good enough for me as it might be that you are seeing a small C.nica specimen
Another feature which differs is the phylarries of the flower (also known as involucral bracts) where in C.mlt it is said to be broader. Again we are speaking of 1mm (2mm max).
Combined together, that is a rather small flower with a relatively broad phyllaries, might be leading on the good track fro C.mlt and you should investigaet a bit more.
Take a look at the seeds, and C.mlt must have the achene part of the seed at the same length of the pappus part. If pappus is smaller, than C.nica.
Further more is another tiny detail about the leaves where C.nica do not have decurrent leaves, while C.mlt do have. Decurrent is when a leaf extends down the stem below the insertion (in the form of a scar usually)
I hope that's a good lecture Conchio...
