The Mira type variable chi Cygni is due at maximum at around the end of October, but how bright will it be at its peak?
The average peak magnitude is around mag 5.1. However, the brightest ever peak reached mag 3.4, whereas the faintest only managed mag 6.4.
Following its previous maximum in September 2016, chi Cygni was predicted to reach minimum during May. Minima can go as faint as 14th magnitude, but observations by SPA VSS member Don Matthews showed it to be around mag 12.1 in early May, after which it started to slowly brighten. Summer twilight and cloudy skies hindered observations during June.
In early July, however, many observers were surprised to find that chi Cygni was already visible in binoculars. By the end of the month it had brightened to around mag 8.0. The rate of brightening increased during August, with chi Cyg reaching around mag 5.6 by the end of the month. If this rate of increase was to continue, then we would be set for a particularly bright and/or early maximum.
During September, however, the rate of brightening has slowed considerably, only adding a few tenths of a magnitude by mid-month.
Could this be a sign that we are getting close to maximum and we are set to see a fairly average maximum several weeks before the predicted date? Predictions for the dates of maxima of Mira type variables are always only a rough guide and it isn’t unusual for maxima to occur a couple of weeks before or after the predicted date. In addition, many maxima of chi Cygni are fairly “flat-topped”, which means that there may be little change in brightness for a fortnight or so around the actual peak date.
Another possibility though is that chi Cygni will continue to edge upwards in brightness for another month or so and thus produce a brighter than average peak close to the predicted date.
We will have to wait and see.
More information about chi Cygni, including a finder chart with comparison stars, can be found in this guide