Arguably the most celebrated northern globular cluster, its several hundred thousand stars are crowded into a volume of space 145 light years across. The cluster members are almost all population II stars which are highly evolved low mass main sequence stars. Any star in the cluster with a mass greater than 0.8 solar masses has already left the main sequence and become a red giant. Images of M13 are dominated by Red Giants which are typically 2000 times more luminous than our sun. To get a sense of relative luminosities, if we were to look back on our sun from M13, it would not be apparent visually using even the largest telescopes. For over a century a trifurcated dark patch has been observed visually and photographically over M13. Various explanations have been given including globules of dust within the cluster, or voids of diminishing stellar densities in the dark areas. Recent investigation favors an unusually low stellar density in the dark patches. This is believed to be due to the absence of only a small number of red giants relative to the high concentration of red giants elsewhere in the cluster. The reason for this phenomenon is unknown.
M13 is typical for a globular cluster in our galaxy. Milky Way globular clusters are all uniformly old (10 to 12 billion years old). The same cannot be said for globular systems of other nearby galaxies like M31. M31 has populations of globular clusters from three separate age epochs (500 million years, 5 billion years, and 10-12 billion years old). The younger clusters are postulated to have come from different merger periods, presumably when M31 was accreting nearby satellite galaxies. There is growing evidence that many other galaxies have had similar histories and possibly the Milky Way is unique among galaxies, having a homogenous population of ancient globular clusters.