A phase of stellar evolution where intermediate-mass stars burn helium in their cores while maintaining relatively constant luminosity. These stars appear as a horizontal band on the Hertzsprung-Russell diagram of globular clusters.
Named for the horizontal appearance of these stars on color-magnitude diagrams of star clusters, first identified in the 1950s. The term 'branch' refers to the branching paths of stellar evolution tracks on the H-R diagram.
Horizontal branch stars are stellar time machines that help us determine the ages of the oldest star clusters in our galaxy! The precise temperature and color distribution of these helium-burning stars can reveal whether a globular cluster is 12 or 13 billion years old - crucial for understanding when the first stars formed.
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