Carbon Star - WZ Cassiopeiae
The position of WZ Cassiopeiae (arrowed) in the night sky [Charted in Stellarium]
This is a typical carbon star, whose surface temperature is a relatively cool ~2,500K and its atmospheric composition has a greater abundance of carbon than oxygen. As with all stars of this class, WZ Cas undergoes pulsations in brightness with a semi-regular periodicity. AAVSO data (below) shows the light variations going back to the 1930s, when observations of this star were first recorded:
As can be seen, the oscillations in light would be quite exciting to observe and occur with some erraticness, even though there is a sinusoidal regularity. Based upon the above analysis, I have deduced the primary period to be of the order of ~186 days (consistent with Cholopov et al, 2000). The star hardly ever stays close to maximum or minimum brightness for much longer than about a month at a time. Furthermore, there is an underlying dynamical attractor which suggests a secondary period of ~300 days maybe at work. The mean topline range is from mag. +6.5 to +8.5, but the reddish light of WZ Cas could occasionally wander by about half a magnitude on either side of this, taking it well outside of the nominal range.
An 8.3-magnitude bluish companion
Over the years, WZ Cassiopeiae has proved more popular with night sky watchers compared to other carbon stars in its league for a whole variety of reasons. The star is circumpolar in most of Europe and North America, which means it can be studied on any chosen night of the year and its variability followed through the seasons without discontinuity by any one observer. But what really sets it apart is that the star is accompanied by a bluish, 8th magnitude companion that is of fixed brightness. According to WDSC records, this companion star to WZ Cas was first observed by Otto Struve and measured in 1874. The companion star (WZ Cas B) is a hot bluish-white star of spectrum A0, and it sits 58 arcseconds away from the primary. This means the pair are of a deep red and blue-green colour contrasted components, making them a spectacle for sore eyes in good binoculars and even in the smallest of telescopes.There is something deeply magical about viewing colour-contrasted doubles, where the intrinsic colour of a white star can turn a turquoise green simply because it is viewed next to a reddish companion. Hence, I have long sought to identify the most colour contrasted double star of all... and having analysed over 50 carbon stars dotted across the sky, I believe I have found it!
Since the primary of WZ Cas is of an extreme red colouration (B-V= +3.08) and its companion is virtually white (B-V= 0.0), with over three whole magnitudes of difference between their colour indices, this is arguably *THE* most colour-contrasted double star system in the whole night sky! (RS Cygni comes close - but that system is complicated by the presence of a third star nearby in the same field of view.)
WZ Cassiopeiae and its companion, imaged in the Palomar Observatory Sky Survey (POSS 2). [Credit: SIMBAD/Aladin Previewer]
As beautiful as they are to watch, the pairing of this star with its companion is most likely to be a "line of sight" optical alignment rather than a dynamically associated true binary system; the available stellar catalogue data on each star shows differing proper motions and radial velocities.
Another point worth noting is that if magnitude estimates are to be made visually on the primary star in this system by comparing it to the brightness of the secondary (which is of fixed light, at V mag +8.3), care must be taken since red stars affect the human retina in stranger ways than might be realised because of something known as the Purkinje effect. A detailed chart of the starfield surrounding WZ Cas, giving suitable comparison stars and their relative magnitudes is to be found at the BAA's Variable Stars Section.
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