The story
From their location on the blue horizontal branch portion of the extreme horizontal branch part of the horizontal branch of stellar evolution, hooking upwards when plotted on an H-R diagram.
Etymology from Wiktionary, CC BY-SA 4.0.
blue hook star
Today it means: a type of helium-burning blue giant star that is hotter than the average star in this stage of their life, due to having a more massive helium core Full definition →