Algol, also known as Beta Persei, is an eclipsing binary star in the constellation Perseus. The name derives from an Arabic word meaning "ghoul" and into English as Demon Star!
Every 2.87 days Algol drops in brightness from magnitude +2.1 to +3.4. This variability was discovered in 1782 by the English astronomer John Goodricke. The large, bright primary star is partially eclipsed by a smaller, fainter companion and the eclipses last around 10 hours. Eclipse times are widely available online.
I've wanted to capture a pair of images Algol eclipsed and uneclipsed for a long time. I happened to wander outside a couple of days ago and noticed that Algol was at, or near, minimum brightness. There's a star next to Algol in the sky (rho Per) which makes comparison easy: Algol is usually much the brighter of the two, but during eclipse they shine at roughly equal brightness.
I took a 30 second exposure (ISO3200, 18mm) on the camera at around 10.30pm on September 29th; this was a couple of hours after the middle of the eclipse. The following night I was able to take another picture with the same camera settings. Here are the two images.
The pictures were taken late evening but the camera position was slightly different and the position in the sky was slightly different. I cropped out similar sized areas around Algol and used the software Iris to align the images:
Zooming in on Algol (the bright star on the left in each picture) you can see how it varies in brightness compared to the bright star on the right (rho Persei, which is itself and variable star but over much longer periods than Algol). The stars are slightly sausage shaped because they trailed during the 30s exposure. The images haven't been enhanced or retouched in any way!
Finally....a GIF to compare the variation in brightness:
Possibly in the future I'll do some guided exposures with the CCD and attempt some photometry on images captured during the 10 hour eclipses.
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Dr Adrian Jannetta. Amateur astronomer, maths teacher and science enthusiast.