What kind of star is Algol?
Algol, catalogued as Beta Persei Aa1 (β Per Aa1), appears in the official constellation Perseus. Its apparent visual magnitude is varies from about 2.1 to 3.4, while its estimated distance is about 93 light-years. Astronomers classify the object as B8 V + K-type semidetached eclipsing binary in a triple system. These labels describe temperature, luminosity class or system components; they should not be read as a claim that every naked-eye point is a single isolated star.
The close Algol A–B pair is a semidetached binary viewed nearly edge-on. Its cooler, less massive subgiant fills its Roche lobe and transfers gas to the hotter main-sequence star. This creates the Algol paradox: the less massive star is more evolved because it originally had more mass and later donated much of it. A third star orbits the inner pair on a wider path. Primary eclipses occur when the cooler star blocks part of the hot component; a shallower secondary eclipse occurs half an orbit later. Timing changes help reveal mass transfer and additional orbital effects.
The coordinate pair RA 03h 08m 10.13s, Dec +40° 57′ 20.3″ fixes the object's direction on the celestial sphere for the J2000 reference epoch. Right ascension works like celestial longitude measured in hours, while declination works like latitude north or south of the celestial equator. Proper motion and improved measurements can refine catalogue positions, but these coordinates are sufficiently precise for a finder chart or planetarium search.
Algol data at a glance
| Proper name | Algol | Catalogue designation | Beta Persei Aa1 (β Per Aa1) |
|---|---|---|---|
| Constellation | Perseus | Apparent magnitude | varies from about 2.1 to 3.4 |
| Spectral or system type | B8 V + K-type semidetached eclipsing binary in a triple system | Estimated distance | about 93 light-years |
| Right ascension (J2000) | 03h 08m 10.13s | Declination (J2000) | +40° 57′ 20.3″ |
Values are rounded for an observer-friendly overview. Consult the linked SIMBAD record for component-level identifiers, bibliography, measurement provenance and newer catalogue values.
The name Algol
Algol comes from Arabic raʾs al-ghūl, “the demon’s head.” It marks the severed head of Medusa carried by Perseus in the classical figure.
Algol’s ominous name long predates a physical explanation of its variability. Geminiano Montanari recorded its changing brightness in 1669, and John Goodricke established the periodicity in 1783, proposing an orbiting dark body as the cause. Spectroscopy eventually confirmed an eclipsing stellar binary. Algol became the defining example of its class and a cornerstone in understanding mass exchange. The IAU name applies specifically to Beta Persei Aa1, the hot primary.
Modern professional catalogues retain identifiers such as the Bayer designation alongside a proper name. The International Astronomical Union standardizes which name belongs to which component—an important distinction for systems whose combined light looks like one star from Earth.
How to find Algol
Algol’s primary eclipse repeats every 2.867 days and lasts roughly ten hours from beginning to recovery. Compare it with nearby stars when predicted near minimum: it fades from second to third magnitude, an unmistakable naked-eye change when conditions and comparison stars are consistent.
A northern target best on autumn and winter evenings; circumpolar at high northern latitudes. Visibility on a particular night still depends on latitude, season, time, horizon obstructions, weather and local light pollution. Use the interactive atlas below to check the surrounding field before observing.
Observation checklist
- Use an AAVSO or observatory ephemeris to choose a predicted primary minimum.
- Begin comparisons several hours before minimum and continue through recovery if possible.
- Use the same nearby comparison stars and avoid switching instruments mid-sequence.
- No telescope is required; binoculars help when haze or city light makes third-magnitude comparisons difficult.
Interesting facts about Algol
- Algol’s primary eclipse repeats every 2.867 days.
- It typically fades by more than one magnitude, easily visible to the unaided eye.
- Mass transfer explains why the lower-mass component is the more evolved star—the Algol paradox.
- The system contains a third star beyond the eclipsing pair.
- John Goodricke accurately determined its periodic behaviour while still a teenager.
Continue from Algol into the night sky
Use the Perseus constellation guide to place this star within its larger pattern. When you are ready to observe, compare dark-sky places, find a nearby observatory, or follow our practical guide to finding a star in the night sky. Symbolically registered names are kept separately in the searchable Star-Register catalogue.
Sources and further reading
Measurements are rounded for readability. Multiple systems, variable stars and distant supergiants can have component-specific or model-dependent values. Survey panels show photographic sky fields, not the physical surface of the star.