The horizontal branch is the part of the Hertzsprung-Russell diagram of occupied by the stars in a typical globular cluster. It contains
stars with masses of 0.6 to 0.8 solar masses that are hotter and fainter
than those on the giant branch. A star
appears on the horizontal branch after it has undergone its helium
flash and begun to burn helium steadily in its core and hydrogen in
a surrounding envelope.