Variable Stars in M15
From the author of Sizing Up The Universe
Variable stars are shown as pairs of points connected by a line-segment
Stars above the red line are presumed to be variable stars.
Variable stars between magnitude 14 and 15 are the RR-Lyrae stars.
Here are the two image files (in "fits" format):
m15_2013-LRGB.fit
m15_2015-LRGB.fit
Here is the Matlab function used to make the HR-diagram:
makeHRdiagRRlyrae.m
Click here for webpage showing individual images of M15