Algol, the Demon Star of Perseus, is an eclipsing binary that dims sharply every few days as a fainter companion passes in front of its blue-white star.
At a glance
| type | blue-white star (class B) |
|---|---|
| distance from the Sun | 89.9 light-years (27.6 pc) |
| its light left it | 89.9 years ago |
| luminosity | 178 × the Sun |
| radius | 3.19 × the Sun |
| surface temperature | 11,800 K |
| constellation | Perseus (Hero) |
Read off the piece’s own catalogue, the same values it draws with.
About Algol
Algol looks like a steady star most of the time. Then, every few days, it fades noticeably over a few hours and recovers: a dimmer, cooler companion is passing in front of the bright blue-white star, blocking part of its light. The orbit is seen almost edge-on from Earth, which is why the eclipses happen at all. A third star orbits the pair farther out.
Its fading was timed carefully in the age of the early variable-star observers, and the explanation by an eclipsing companion was proposed then, long before the companion could be detected. Algol became the prototype of eclipsing binaries.
The name comes from the Arabic for the head of the demon, and in the Greek figure it marks the head of Medusa, held in the hand of Perseus. The pair holds a puzzle once called the Algol paradox: the less massive star is the more evolved, explained by gas flowing from one star to the other over their lives.
What you see in the piece
In the piece Algol is drawn as its blue-white primary, at its catalogue brightness. Its eclipses are not shown: the companion is too close to separate at the scale of the stars around it.
Questions
Why is Algol called the Demon Star?
Its name comes from the Arabic for the head of the demon, and in the Greek figure it is the head of Medusa held by Perseus. Its regular fading may have added to the reputation.
How far away is Algol?
Algol is 89.9 light-years (27.6 pc) from the Sun.