Algol, frequently referred to as the "Demon Star", is a bright eclipsing binary system located in the constellation of Perseus. It was the first of its type to be discovered and one of the best known and most frequently observed variable stars in the night sky. For most of the time Algol shines at magnitude 2.1 and therefore is the second brightest star in Perseus, only marginally fainter than Mirfak (α Per - mag. 1.8). But then every two days, 20 hours and 49 minutes it suddenly dips in brightness to magnitude 3.4 and remains dim for about 10 hours, before returning back to its original state.

Algol system

The Algol system consists of at least three-stars (β Per A, β Per B and β Per C). The main star (A) is a B8 main sequence star that is nearly 100 times more luminous than our Sun, while the B star is a K2 type subgiant 3 times more luminous than the Sun. These two stars orbit very close together, with a separation of only 0.062 astronomical units (AU). The third star (C) in the system is of type A5, 4 times more luminous than the Sun and is located at an average distance of 2.69 AU from the AB pair. The total mass of the system is about 6 solar masses, with the mass ratios of A, B and C about 3.6 to 0.8 to 1.7. In addition, spectroscopic investigations indicate that the system might also contain a fourth component.

From our perspective, the orbital plane of Algol A and Algol B is in the line of sight of Earth and hence the pair forms an eclipsing binary. The dips in brightness of the Algol system occur when the dimmer B star moves in front of and therefore eclipses the brighter A star. The eclipse lasts for about 10 hours, which corresponds to the time when Algol's dims to magnitude 3.4. They occur every two days, 20 hours and 49 minutes, which is equal to the orbital period of the A and B stars. There is also a secondary eclipse when the brighter star occults the fainter secondary, but this results in a very small dip in brightness that can only be detected photoelectrically.

Algol System (Dept. of Physics and Astronomy - Univ. of Tennessee at Knoxville)


The variability of Algol was first recorded by Geminiano Montanari in 1667, although it's probable that it had been known for a long time before this. The star was associated with a demon like creature in Greek and Arabic tradition and ancient Egyptians used a calendar of lucky and unlucky days, over 3000 years ago, corresponding to the variability period of Algol. This suggests that the variability had long been known, but no conclusive evidence has yet been found.

Although Montanari was the first to record Algol's variability, its periodic nature was not recognized until more than 100 years later. The British amateur astronomer John Goodricke presented his findings to the Royal Society in May 1783 and even suggested that the dimming was caused by a dark body passing in front of the star. Algol was finally confirmed as an eclipsing binary in 1889 when Potsdam astronomer Hermann Carl Vogel made spectroscopic measurements.


Algol is located in Perseus amongst the stars of the Milky Way. It is positioned west of magnitude 0.1 star Capella (α Aur) and southeast of the "W" of Cassiopeia. At it's brightest the star is the second brightest in Perseus, at it's faintest it drops down the list to only seventh brightest. The finder chart below shows the position of Algol along with magnitudes of some of the surrounding stars for comparison purposes.

Finder Chart for Algol (Beta Persei)

Finder Chart for Algol (Beta Persei) - pdf format

The system is currently located 93 light-years from the Sun, but in the very distant past it was much closer to Earth. About 7.3 million years ago Algol was only 9.8 light-years from the Solar System and would have shone at an apparent magnitude of -2.5, more than twice as bright as Sirius appears today.


Algol is a famous variable star in the night sky. It's a popular target since its bright, has a short period of less than 3 days and as a result the complete cycle of the star can be observed over just a few nights. It also dims quite dramatically and hence the change is brightness is obvious to the naked eye. For those new to variable star observing, Algol is an excellent first choice.

Beta Persei (Algol) Data Table

NameBeta (β) Persei
RA (J2000)03h 08m 10s
DEC (J2000)+40d 57m 20s
Apparent Mag. (v)2.1 -> 3.4
Absolute Mag.-0.15
Period (days)2.86736
Variable TypeEclipsing binary
Distance (light-years)93
Mass (Solar)3.59 (A) / 0.79 (B) / 1.67 (C)
Radius (Solar)4.13 (A) / 3.0 (B) / 0.9 (C)
Luminosity (Solar)98 (A) / 3.4 (B) / 4.1 (C)
Spectral TypeB8V (A) / K2IV (B) / A5V (C)
Age (years)< 300 million
Other DesignationsAlgol, Gorgona, Gorgonea Prima, Demon Star, El Ghoul

Sky Highlights - July 2017

The Planets
This Month's Guide

Algol Minima
Algol eclipse dates and times for July

Meteor Shower
Southern Delta Aquariids (Aquarids) meteor shower peaks on July 29

Northern Hemisphere
West:- Mercury (mag. -0.5 to +0.3) (second half of month)
Southwest:- Jupiter (mag. -2.0)
South:- Saturn (mag. +0.2)
West:- Jupiter
South:- Saturn
East:- Neptune (mag. +7.8)
Southwest:- Saturn
South:- Neptune
Southeast:- Uranus (mag. +5.8)
East:- Venus (mag. -4.1)

Southern Hemisphere
West:- Mercury (second half of month)
Northwest:- Jupiter
East:- Saturn
West:- Jupiter
North:- Saturn
East:- Neptune
West:- Saturn
North:- Neptune
Northeast:- Venus, Uranus

Deep Sky

Small telescopes:-
Messier 13 - M13 - Great Hercules Globular Cluster
Messier 92 - M92 - Globular Cluster
Messier 11 - M11 - The Wild Duck Cluster (Open Cluster)
Messier 7 - M7 - The Ptolemy Cluster (Open Cluster)
Messier 6 - M6 - The Butterfly Cluster (Open Cluster)
Messier 4 - M4 - Globular Cluster
Messier 8 - M8 - Lagoon Nebula (Emission Nebula)
Messier 16 - M16 - Eagle Nebula (Emission Nebula with Open Cluster)
Messier 20 - M20 - Trifid Nebula (Emission and Reflection Nebula)

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