We study the dense core of the globular cluster Messier 13 ( NGC 6205 ) using pre-refurbishment Planetary Camera-I images obtained with the Hubble Space Telescope .
Short exposures ( 60 s ) through the F555W and F785LP filters ( similar to Johnson V and I , respectively ) have been used to obtain V and I photometry of 2877 stars brighter than V \sim 20 in a 1.25 arcmin ^ { 2 } region of the cluster including its core and extending out to r \sim 66 ^ { \prime \prime } ( 2.3 pc ) from its center .
The sample is complete to V \simeq 18.3 ( the main sequence turnoff ) and the 1 \sigma photometric error is about 0.1 mag .
We find 15 blue straggler star candidates and 10 other possible blue stragglers in this region of M13 .
Their specific frequency is in the range F _ { BSS } = 0.04 –0.07 , comparable to what is observed near the centers of other dense clusters .
A comparison between M13 ’ s observed V band stellar luminosity function and a theoretical model ( Bergbusch & Vandenberg 1992 ) for the luminosity function of an old , metal-poor cluster shows that the model predicts too few of the brightest red giants ( V \sim 12.5 –15 ) by a factor of two relative to subgiants/turnoff stars ( > 6 \sigma effect ) .
The radial distributions of the red giants , blue stragglers , and subgiants are consistent with one another , and are well fit by a King profile of core radius r _ { core } = 38 ^ { \prime \prime } \pm 6 ^ { \prime \prime } ( 90 % confidence limits ) or 1.3 pc .
Stars in the blue horizontal branch of M13 , however , appear to be centrally depleted relative to other stellar types .

We combine data from three dense ‘ King model clusters ’ , M13 , M3 , and 47 Tuc , and two post core collapse clusters , M30 and M15 , and compare the distributions of various stellar types as a function of ( r / r _ { half~ { } light } ) and ( r / r _ { core } ) .
The horizontal branch stars in the combined sample appear to be centrally depleted relative to the giants ( 97 % significance ) —this depletion is only a 1 \sigma – 2 \sigma effect in each of the clusters taken individually .
The blue stragglers in the combined sample are centrally concentrated relative to the giants .