An Etymological Dictionary of Astronomy and Astrophysics
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A member of a population of blue stars appearing on the → horizontal branch in the → Hertzsprung-Russell diagram of the Galactic → halo populations and → globular clusters. Belonging to → spectral types B3 to A0, they have evolved past the → red giant stage and are burning helium in their core.

→ blue; → horizontal; → branch, → star.

The hottest variety of stars on the → horizontal branch with temperatures ranging from 20,000 to 40,000 K. EHB stars are distinguished from normal horizontal branch stars by having extremely thin, inert hydrogen envelopes surrounding the helium-burning core. They are hot, dense stars with masses in a narrow range near 0.5 Msun. These stars have undergone such extreme mass loss during their first ascent up the giant branch that only a very thin hydrogen envelope survives. Stars identified as EHB stars are found in low metallicity globular clusters as an extension of the normal HB.

→ extreme; → horizontal; → branch; → star.

A → horizontal branch star with high velocity.

→ field; → horizontal; → branch; → star.

A star lying on the → horizontal branch.

→ horizontal; → branch; → star.

A star found on the red part of the → horizontal branch. According to theoretical models, these stars result from the evolution of stars with a mass around 0.8 Msun, higher than that giving rise to → BHB stars. Upon helium burning in their cores, the remnant envelope of the red giant collapses.

→ red; → horizontal; → branch; → star.

A member of a rare class of objects found in → globular clusters to lie about one magnitude above and to the blue part of the → horizontal branch. These stars are identified as post → EHB stars on their way from to the → asymptotic giant branch.

→ supra-; → horizontal; → branch; → star.

A star that has arrived on the → horizontal branch after leaving the → red giant branch. It begins → helium burning in its core.

→ zero; → age; → horizontal; → branch; → star.