BHB stars are more evolved and have helium burning cores, although they still have an extensive hydrogen envelope. They also have moderate masses around so they are often much older than more massive blue giants. The BHB takes its name from the prominent horizontal grouping of stars seen on colour-magnitude diagrams for older clusters, where core helium burning stars of the same age are found at a variety of temperatures with roughly the same luminosity. These stars also evolve through the core helium burning stage at constant luminosity, first increasing in temperature then decreasing again as they move toward the AGB. However, at the blue end of the horizontal branch, it forms a "blue tail" of stars with lower luminosity, and occasionally a "blue hook" of even hotter stars.