90.00 light-years away
Algol
Why did a star get named after a demon, and what is a deaf teenager doing in the story?
Every 2.87 days this star fades to a third of its brightness and comes back, over a few hours, plainly enough to follow by eye. People noticed long before they could explain it.
A name that suggests they noticed
The name descends from the Arabic for 'the ghoul', and in the constellation Perseus it marks the head of the Gorgon. Whether ancient observers tracked the variability deliberately is disputed, but a star that visibly winks is a strange thing to place at the eye of a severed monster's head by coincidence.
Explained at nineteen
In 1783 John Goodricke — deaf since infancy, and eighteen when he began the work — measured the period precisely and proposed the explanation now accepted: a darker companion passes in front of the brighter star, eclipsing it. He was awarded the Copley Medal and died at twenty-one.
The mechanism is the reason eclipsing binaries matter so much. When one star crosses another, the timing and depth of the dip give you their sizes, their orbit and their masses directly, with a rigour that is hard to obtain any other way.
The paradox
The two stars presented a problem: the less massive one is the more evolved, which is backwards. Heavier stars burn out faster, so the heavy one should have aged first.
The resolution is that they have been trading material. The originally heavier star swelled as it aged, spilled its outer layers onto its companion, and ended up the lighter of the two. What looks like a contradiction is a record of one star feeding the other — and it is now the standard example of the process.