Step-By-Step Guide to Observing the Eclipsing Binary Star Algol (The "Daemon Star")

In this article we will cover how to observe an eclipsing binary star. An eclipsing binary is where a dimmer star in a multiple star system passes in front of its brighter counterpart one of the easiest ways to observe this is to find the star Algol which is a triple star system but consists of one main bright star and another dimmer star that passes in front of it from our perspective.

This dimming occurs every two days forty hours and forty-eight point eight minutes the change in magnitude can be from +2 to +3.3 and with magnitude the larger the number the dimmer it is.

Click to enlarge the Algol star chart
To find Algol you must first find the Perseus constellation. To do this find the bright constellation Cassiopeia, near the M103 cluster. Then draw an imagine line down from Gamma Cassiopeia down to the star Ruchbah once you are in the Perseus constellation follow the stars down through the Perseus constellation one by one until you reach the star Algol.

The best time of year to observe an eclipse of Algol is during the Autumn, Winter or Spring skies. It is possible to observe even when the full moon is out in force.

To see when an eclipse of Algol is due to occur just use the Algol Minima Calculator.

Algol is one of those astronomical targets that needs to be observed over an extended period the whole process from maxima, which is the brightest point to minima which is the dimmest point and back again will take about ten hours. However, there is a noticeable difference two hours before or after the minima. So, if you have a short amount of observing time you can compare the change in brightness up to that point, I would also recommend recording the brightness of Algol the night before and then comparing it to the actual minima.

As Algol’s brightness changes over an extended period, it would be good to make sure to observe it when it is at its peak as well as at its minima. I did something similar for this article, I started recording two and a half hours before the minima and then I carried on recording until four and a half hours after the minima taking a photo every fifteen minutes. I would have recorded for longer but unfortunately bad weather started coming in and that stopped my recording.

I then went out and recorded Algol the following night using the same settings on the camera I then combined all those images together to show a sequence of images shown in the episode below. What is really striking is how the brightness of Algol changes over that period.

There is actually quite a lot of history to Algol in ancient times ancient astronomers used to find Algol quite unnerving. So much so that the name Algol actually means the Daemons Head and it’s supposed to represent the Gorgon from ancient Greek myths in the constellation Perseus. In many cultures it has actually got a bit of a bad reputation with links to bloody violence so not entirely sure why I guess they are a bit superstitious of the change in brightness it stands out a bit compared to all of the other stars.

If you would like to see some example footage, or animations of how a eclipsing binary star occurs, please watch the episode below.

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