Watch for Algol at its minimum

On clear February evenings, the variable star Algol (Beta Persei), in the constellation Perseus, is high overhead from central UK locations. An eclipsing binary, this star “winks” in brightness every 2.87 days, dipping from its normal magnitude 2.1 to a minimum of around 3.4, as its dimmer orange-yellow secondary passes in front of, or eclipses, the brighter bluer primary.

Algolfinder Feb24
How to find Algol on the evening of 21 February 2024

The night of Wednesday 21 February provides an excellent opportunity to watch the gradual fade and recovery of Algol. The orbit of the two stars is known so accurately that we can predict within fractions of a second, centuries ahead, when the fade will begin.  That’s why we know that at exactly 22:11 GMT on Wednesday night, Algol will be at its minimum.

Perseus is an easy constellation to find as it’s close to the well-known W-shape of Cassiopeia. This year, the bright planet Jupiter also helps to signpost the way, as shown in the map above. Also look for the striking Pleiades star cluster, or Seven Sisters, above Jupiter and to the left of Perseus. The photo below shows how the stars appear in the sky.

Perseus Rising
Algol rises near minimum on a misty night, making the constellation Perseus look somehow misshapen. Credit: Mark Hardaker

The entire eclipse will last around nine hours, four to five either side of the minimum.  This means that you will notice Algol is already fading as darkness falls at around 18:30 and will continue to do so as the evening progresses. Even as Algol sets below the horizon at around 04:00, it will have not yet fully recovered its usual lustre.

Plotting Algol’s falling brightness over time makes a fascinating project. All you need is our comparison chart, a watch and your naked eye – oh, and clear skies of course! Note the brightness of Algol every 20 minutes or so. comparing it to nearby stars. You can follow the magnitude changes by comparing Algol’s brightness with that of the lettered comparison stars marked on the  finder chart, below.

Comparison Chart for Beta Persei Credit: SPA

The brightness changes of Algol had been known for over a thousand years but were first formally noted in 1667 by Italian astronomer Geminiano Montanari. However, the periodic nature of this variability was not understood until more than a century later when, in 1783, John Goodricke correctly explained that the Algol system actually contains two stars in close orbit around each other. The orbital plane is edge on from our vantage point and so the combined brightness drops when the fainter of the two stars passes in front of the brighter star, blocking out its light. For his work, Goodricke was awarded the Copley Medal of the Royal Society.

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Part of the light curve of Algol, based on observations made by members of the Society for Popular Astronomy in 1999 showing the dip in brightness to minimum. The star’s brightness falls by a factor of more than three in a matter of hours.

If it’s a clear night on Wednesday, go outside and take a look at Algol as it fades to its regular minimum and send your observations in to our variable star section.