Four types of stars reside within the center of the globular cluster NGC 6397. They have been identified in the accompanying image as: 1) helium white dwarfs; 2) blue stragglers; 3) normal white dwarfs; and 4) cataclysmic variables.
1) Helium white dwarfs (blue squares): Three faint blue stars that do not vary in brightness can be seen near the center of the cluster. These stars may be very-low-mass white dwarfs, formed in the cores of giant stars whose evolution is somehow interrupted by a stellar collision or an interaction with a binary companion. When a giant star interacts with another star, it can lose its outer layers prematurely, exposing its hot, blue core.
2) Blue stragglers (blue circles): A blue straggler is a hot, bright, young star that is the result of a direct collision between two stars that have merged together to form a new star. They stand out among the old stars that make up the vast majority of stars in a globular cluster.
3) Normal white dwarf (purple circle): These stars appear throughout the cluster and form through the normal stellar evolution processes, when outer layers have been burned off of the star. Since they don't involve any stellar interactions, which occur predominantly near the cluster center, there are very few visible in this close-up image of NGC 6397. Nearly 100 such burned-out stars were identified in the entire image.
4) Cataclysmic variables (red triangles): A cataclysmic variable is a pairing of a normal, hydrogen-burning star and a burned-out star, or white dwarf. In this binary system, material pulled off the surface of the normal star by the white dwarf will encircle the white dwarf in an "accretion disk," and eventually falls onto it. The result of this accretion process is that cataclysmic variables vary in brightness.