# Algol (Beta Persei)

Algol (Beta Persei) is a star in the constellation Perseus, magnitude 2.1.

**The Demon Star that winks every three days — two suns eclipsing each other in plain sight.**

Every few days the Demon Star loses about two-thirds of its light. Algol is two stars in a tight orbit seen edge-on: when the larger, cooler, fainter partner passes in front of the hot main star, Algol fades for about ten hours, then recovers. A third star circles the pair every 680 days. It is the namesake of a whole class of [eclipsing variables](https://astropilot.space/glossary/variable-star).

Its name, from the Arabic raʾs al-ghūl, means “head of the ogre”, and it marks the severed head of Medusa in Perseus’s hand. Whether ancient stargazers knew it winked is unproven. Geminiano Montanari noted its changes in 1667, and in 1783 the British amateur John Goodricke found the period and proposed a dark body passing in front of it, winning the Copley Medal.

Algol also posed a riddle. Heavier stars age faster, yet here the lighter star is the more evolved. The answer is that it was once the heavier one, and poured much of its gas onto its partner.

Algol (Beta Persei) is 90 light-years away: the light we see from it set off in 1936, 6 years after Pluto was discovered.

## Did you know?

- **Satan’s Head.** Hebrew folklore called it Rōsh ha Sāṭān, “Satan’s Head”, and a 16th-century Latin name was Caput Larvae, “the Spectre’s Head”.
- **A brilliant past.** About 7.3 million years ago Algol passed close to the Solar System and shone at around magnitude −2.5 — far brighter than Sirius looks today.
- **First photoelectric light curve.** Joel Stebbins turned an early selenium-cell photometer on Algol, making the first photoelectric study of a variable star. It revealed the faint second dip, when the bright star hides its partner.

A blue-white 2nd-magnitude star in Perseus that fades to 3rd magnitude for about ten hours every 2 days 21 hours. Normally it matches Mirfak and Almach; at its faintest it drops to the level of nearby Rho Persei — check it against them.

## When and where to see it

It is best placed in the evening sky around December, when it crosses the meridian at about 10 pm local time (around midnight in November). It can be seen from everywhere north of 49°S and never sets north of 49°N.

At its highest it stands 81° up from 50°N, 84° up from 35°N, 49° up from the equator, 14° up from 35°S.

## Facts

- Type: Star
- Constellation: Perseus
- Designations: Beta Persei, Demon Star, β Per, 26 Per, HR 936, HD 19356, HIP 14576
- Magnitude: 2.1
- Position: RA 3h 8.2m, Dec +40° 57′ (J2000)
- Spectral type: B8V
- Colour: white (B−V -0.05)
- Brightness: varies from magnitude 2.1 to 3.4
- Distance: 90 light-years
- Discovered: Geminiano Montanari, 1667, Known since antiquity; Montanari first recorded its variability, and John Goodricke found the period in 1783

Open AstroPilot to see when Algol (Beta Persei) is highest tonight from your location, how easy it is with your telescope or binoculars, and an eyepiece view. https://astropilot.space/object/algol