Guide to AF Cygni

A star that steadily fluctuates up and down in brightness, going through several cycles of variation each year.

af cyg

A pretty reliable variable star that is easy to locate given that it is located not too far from Delta Cygni (mag 2.9). The brightness changes don’t repeat exactly from one cycle to the next and so every new cycle is a new observing experience. The amplitude in each cycle is mostly close to a magnitude, but from time to time it does go through cycles in which the amplitude may somewhat larger or somewhat smaller.

During 2014, the 92 day period seemed to be replaced with a period close to double that value but in the second half of 2015, as can be seen in the above light curve, the 92 day period seemed to be becoming more apparent again. A similar apparent doubling occurred in early 2017. It is likely that the 92 day period is present throughout but with some maxima being much less prominent than others.

AF Cygni is a red giant star. The brightness changes are related to pulsations in the star’s outer layers. The star probably has more than one pulsation period and the combination of these different periods results in a light curve whose shape and amplitude change from one cycle to the next.

Extreme brightness range 6.4 – 8.4
More typical range 6.7 – 7.7
Period of variation About 3 months (but seemed to be longer in 2014)
Frequency of observation Worth checking a few times per month
Observe with 40mm or 50mm binoculars will suffice for most of the time, but 50-80mm binoculars may be required during the deeper minima
Visibility Can be observed all year round, but is rather low in the evening sky from January to April

Here is a finder chart (approx 5 degrees x 4 degrees) that labels comparison stars that you can use when making brightness estimates of AF Cygni:

Figure 2: Finder chart showing comparison stars for AF Cygni