Algol
What is Algol?
Stars· Distance 90 ly · the prototype eclipsing binary. The 'Demon Star' of Perseus. Two stars eclipse each other, so every 2.87 days Algol fades to a third of its brightness for about ten hours and then recovers — the most famous variable you can follow with the naked eye.
📍 Visible from where you are right now — computed in your browser
bearing · altitude · rise time · best time tonight
Where Algol is in your sky right now: https://nowsky.org/solar/sky?sel=algol — location and time set automatically, no login.
Distance and spectral type
Algol is about 90 light-years away, a blue-white main-sequence star (spectral type B8V), in the constellation Perseus. Its apparent magnitude is 2.1, visible to the naked eye.
Stellar figures
| Distance | 90 ly |
|---|---|
| Magnitude | 2.1 dipping to 3.4 |
| Period | 2 d 20 h 49 m |
| Constellation | Perseus |
Figures are representative approximations. Source: compiled from NASA and IAU public data.
Behind the name
Arabic "the demon's head" — Medusa's eye. An eclipsing binary that dims every 2.87 days, long deemed ill-omened.
Frequently asked
- How far is Algol from Earth?
- 90 ly.
- How bright is Algol (apparent magnitude)?
- 2.1 dipping to 3.4.
- How long is the orbital period of Algol?
- 2 d 20 h 49 m.
- Which constellation is Algol in?
- Perseus.
- Where is Algol in the sky right now?
- Open the sky view for your location and time and it shows the position in the sky right now.
The Algol paradox — why the lighter star is older
Algol's two stars defy common sense. Heavier stars evolve faster, so the heavier A should have aged first — yet the lighter B (0.7 solar masses) has already swollen into a subgiant while the heavier A (3.2 solar masses) is still a young main-sequence star. Dubbed the "Algol paradox" in the 1950s, the contradiction was resolved by mass transfer: B was originally the heavier star, aged and swelled first, and handed its outer layers to A, leaving itself the lighter of the two.
Most of A's mass, then, came from B, and B is an old star stripped to its core. The two lie only a few solar diameters apart, and ultraviolet observations still catch material streaming from B to A. Algol was the first case to show that evolution in a close binary can swap the fates of two stars, and the same principle explains X-ray binaries and the precursors of Type Ia supernovae.
The star in depth
Algol is a second-magnitude star in Perseus, placed where the hero holds the severed head of Medusa. Its Arabic name Ras al-Ghul means 'the demon's head', and its habit of dimming on schedule made it an ill omen for centuries. It lies about 90 light-years away and is in fact a triple system.
John Goodricke explained the dimming in 1782: a dim companion, B, passes in front of the bright primary, A, every 2.87 days and blocks part of its light. The very idea of an eclipsing binary was born here. At minimum Algol drops from magnitude 2.1 to 3.4 for roughly ten hours, so the change is visible within a single night — the first physical explanation ever given for a variable star.
On autumn and winter evenings look below Cassiopeia, near Perseus's brightest star Mirfak. Select Algol in the Sky view to highlight the constellation's shape, note how it compares with neighbouring stars, then check again two nights later to catch it in eclipse.
Related
Figures are representative approximations. Simulated positions use J2000 mean elements.