Algol

Algol's name comes from the Arabic Ra's al-Ghūl — 'the head of the ogre' — and for good reason: every 2 days, 20 hours, and 49 minutes, this star dims noticeably from magnitude 2.1 to 3.4 over roughly 10 hours, then recovers. It is the prototype of the entire class of Algol-type eclipsing variable stars, and likely the first variable star ever recognised. Ancient Egyptian astronomical texts from the Cairo Calendar (~1200 BCE) appear to record its 2.85-day period, suggesting its blinking was noticed thousands of years ago.

The system is a triple: Algol Aa1, a hot B8V main-sequence star of 3.17 solar masses and 13,000 K, is periodically eclipsed by its cooler subgiant companion Algol Aa2 (0.70 solar masses, 4,500 K, 3.48 solar radii), which swollen from the main sequence passes in front of the primary from Earth's perspective. A third star, Algol Ab (1.76 solar masses), orbits the inner pair every 681 days at roughly 2.69 AU. In 2009, the CHARA interferometer produced the first resolved images of the inner binary at half-milliarcsecond resolution.

Algol presents astronomers with the classic 'Algol paradox': the less massive secondary has evolved into a subgiant while the more massive primary is still on the main sequence — the opposite of what stellar evolution predicts. The resolution is mass transfer: the secondary was originally the more massive star but shed material onto the primary, reversing their mass ratio. The system contains roughly 189 solar luminosities combined, yet from 89 light-years away it sits serenely among the brighter stars of Perseus.