Astrophotography
Capturing Algol’s fluctuations.
Algol (Beta (`) Persei) is an eclipsing binary star in the constellation of Perseus. In Greek mythology, Perseus cut off the head of Medusa, a Gorgon who, if you looked into her eyes, would turn you to stone. He later used the severed head’s still-functioning power to defeat the sea monster Cetus, so that he could rescue the princess Andromeda. Algol – which is also sometimes referred to as the ‘Winking Demon’ – supposedly represents one of Medusa’s eyes, but instead of turning you to stone, it presents a wonderful opportunity to see a variable star in action.
Algol is actually a triple system made up from Beta Persei Aa1, Aa2 and Ab. The
variations in perceived brightness occur when the hot, bright primary Aa1 is covered by the larger but cooler secondary, Aa2. From Earth, the line of sight is such that both stars appear to pass in front of one another at regular intervals. When Aa2 passes in front of Aa1 a dip of 1.3 magnitudes is observed. When Aa1 passes in front of Aa2, the dip is so small that it can only be detected using specialist photoelectric equipment.
Aa1 and Aa2 are 0.062AU or 9.3 million km apart while the third component Ab orbits 2.69AU away from the main pair. The orbital period of Ab and the eclipsing pair is 681 days but Ab doesn’t contribute any brightness variations to the system as we see it from Earth.
The large dips occur at regular intervals of 2d 20h 48.9m but not being an integral number of days, the visibility of a dip requires it to occur at the right time – ie, when it’s dark! During October and November optimally occurring dips occur on 20 October at 04:00 BST (03:00 UT), 23 October at 00:48 BST (22 October 23:48 UT), 12 November at 01:30 UT and 14 November at 22:18 UT. The eclipse dip sequence lasts for 9.6 hours.
Recording a dip with a camera is relatively easy to do but care needs to be taken to ensure that you don’t bias the results in any way by adjusting the camera settings between shots. Similarly care should be exercised to avoid sky conditions that may create variations which are not linked to the true changes in brightness of the star.
The simplest way to show Algol’s variation in brightness is to create a strip image by taking shots at regular intervals and then arranging them next to one another (as in the main image on this page). If you do this over the entire course of an eclipse dip the images should demonstrate the changes in brightness very effectively.
Another less work intensive method is simply to show the whole constellation of Perseus as a before-and-during shot. Before would be timed to record the whole constellation when Algol was at maximum brightness. During would show the whole constellation when Algol was in the middle of an eclipse. Placing both images together would then show just how dramatic the variation is.
The techniques introduced here can be re-employed for other eclipsing binary systems and it may be an interesting exercise to create a gallery, or album, revealing how each one varies compared to the others.
However you decide to do it, revealing variability in stars is a fundamental part of astronomy that has allowed us to decode many of the mysteries of the heavens. If you’ve never witnessed the variation of Algol before, attempting to photograph it will be something of a revelation. Good luck!
Send your images to: hotshots@skyatnightmagazine.com