Can amateur observers discriminate the core of dense globular clusters like M3 and M15? Rodney Howe AAVSO SID Analyst Strikis Iakovos - Marios Hellenic.
Video: Globular Cluster M3. Note the variable stars scattered around the cluster!
The period-luminosity relation was discovered by Henrietta Leavitt in 1912. The longer a Cepheid's pulsation period, the more luminous the star. Since measuring a Cepheid's period is easy, the period-luminosity relation allows astronomers to determine the Cepheid's intrinsic brightness and hence its distance. If the Cepheid is in another galaxy, the Cepheid's distance gives the distance to the entire galaxy. luminositiesCepheid variablesLeavitt period-luminosity relation http://www.daviddarling.info/encyclopedia/P/period-luminosity_relation.html
One approach for better identification of the 'core' area vs. the other outer region areas can be done by finding any potential correlation between the angular size of the target - and its standard deviation of the stars, VS. their distance from the core.