How to observe Algol, the ‘demon star’

A fairly bright star in the constellation of Perseus makes an intriguing observing target because its brightness swiftly fades significantly every few days before it brightens again.

The rapid brightness change, from magnitude 2.1 to 3.4, is very obvious and fascinating to watch for amateur astronomers.

The star is called Algol, or Beta Persei, and its dimming and then brightening is a regular event that happens in less than 10 hours. The stars takes 5h 48m to fade and then the same amount of time to brighten again.

The star is so easy to see in the night sky, and the fade so evident, that ancient skywatchers must have been aware of the phenomenon. Indeed, Algol, the name given to the star by Arabic astronomers, translates to “demon star”, suggesting they knew it was odd.

The first official record of the star’s variability was made by an Italian astronomer Geminiano Montanari in 1667. But it was not until 1783 that the reason for the brightness changes was explained, by a young Dutch-born amateur astronomer called John Goodricke, who lived most of his short life in England.

Goodricke realised that Algol was not one star, but a bright one with a darker companion that were orbiting each other edge-on as viewed from Earth.

When the darker object, a dim star, passed in front of the brighter star, it blotted out some of its light, causing the dimming. Thus the phenomenon is rather like an eclipse of the Sun. This type of variable star is known as an eclipsing binary.

The stars’ orbit around each other takes 9.6 days, so the eclipses occur at highly regular intervals. This means they can be predicted so that we know when to watch for them. (We now know there is a third star in the Algol system, but it is not involved in the eclipses.)

There occurs a much smaller eclipse when the fainter star disappears behind the main bright star in the system, but it is a very slight fade that is hard to detect.

The main eclipse occur a little under three days apart (2.867 days), so they don’t happen always at the same time of the day or night. An astronomical handbook can give you predicted dates and times of minimum, or you can find predictions online.

Bear in mind that from early spring into summer in the northern hemisphere, from where Algol is best placed for viewing. During the rest of the year, your best opportunities come when minimum occurs close to midnight so that you can follow the fade and brightening over many hours.

It makes an interesting project to take separate short exposures of Perseus – up to 20 seconds with a standard lens, if you want to avoid stars trailing – and then to compare the appearance of Algol.

Incidentally, John Goodricke was remarkable in achieving so much, despite becoming deaf when a young child, and dying at the early age of 21 after suffering pneumonia. He was elected a Fellow of the Royal Society just four days before his death.

Comparison stars

Here are some nearby stars of fixed brightness, which you can use as comparisons to help you judge the brightness of Algol during an eclipse. (Not all the comparison stars are included on the chart here. But you can find maps of the other constellations on Wikipedia.)

Related: How to observe variable stars