A bright star in the constellation Perseus, notable for its variable brightness; also refers to pain-inducing substances in Arabic alchemy.
From Arabic al-ghul 'the ghoul' or 'the demon,' reflecting ancient beliefs that the star's dimming represented a supernatural event. The name entered European astronomy through medieval Arabic scientific texts.
Algol was the first 'eclipsing binary star' discovered—ancient observers saw it mysteriously dim every few days without knowing that an invisible companion star was passing in front of it, making it seem like a malevolent object.
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