A Mira type variable in the ‘neck’ of the Swan, that has a brightness range of over 10 magnitudes.
In May 2013, Chi Cygni reached an impressive magnitude of 3.8 when at maximum.
2014 brought a marked contrast with the early July maximum only reaching about magnitude 6.4 – possibly the faintest maximum since the discovery of the brightness variations of chi Cygni in the late 17th century.
The 2015 maximum was somewhat brighter, peaking at around magnitude 4.4, whereas that of 2016 was fairly “normal” at around mag 5.0.
As is the case for all Mira type variables, chi Cygni is a red giant star and the brightness variations are primarily due to pulsations in the tenuous outer layers of the star.
The brightness changes that we see are “enhanced” by the fact that during the course of these changes a sizeable part of the output of Chi Cygni moves to and fro between the infrared (which our eyes can’t see) and visible light (which we can see). If we could see both visible light and infrared, we would only see an overall change of about 1.5 magnitudes, but the transfer of output from the infrared to the visible increases this range to around 10 magnitudes.
Additional smaller brightness changes are due to simple molecules that form and absorb light when the pulsations cause the surface temperature to cool and split apart again when the surface temperature rises again.
|Extreme brightness range||3.4 – 14.2|
|More typical range||5.1 – 13.4|
|Period of variation||407 days (approx 13.5 months)|
|Frequency of observation||Worth checking a few times per month|
|Observe using||naked eye for the brighter maxima ; binoculars when fainter ; a telescope to follow it all the way down to minimum|
|Visibility||Can be observed all year round, but is only visible in the morning sky from mid January to April|
|Dates of maxima||late October 2017, early December 2018|
The finder charts which follow show the location of chi Cygni.
The first shows the wider view and some of the brighter comparison stars that are suitable for naked eye observations.
This second chart shows the area around chi Cygni in more detail and labels several comparison stars suitable for use when observing using binoculars.