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Astronomical Observations & Research
* * * Analysis, sky location and images for selected entries in my Red Stars Catalog * * *

Carbon Star - WZ Cassiopeiae

Article posted: 10th June 2010

WZ Cassiopeiae (also commonly known as "HIP 99") is a late type giant red star located near the 'W' shape of Cassiopeia, in the far northern part of the sky. Based upon its small annual proper motion (< 12 milliarcsecs/year) and a Hipparcos parallax of ~1.27(±0.70) mas/year , the star is likely to be no less than 1,500 light years away from us.

The position of WZ Cassiopeiae (arrowed) in the night sky.

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 may be 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.

The double star with the greatest colour contrast!

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 the Washington Double Stars Catalog records, this companion star to WZ Cas was first observed by astronomer, 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 large binoculars and even in the smallest of telescopes.

There is something deeply magical about viewing colour-contrasted double stars, where the intrinsic colour of a white star can turn into a shade of 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 Cassiopeiae 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 numerically *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.)
Visual descriptions of double star colours are notoriously subjective and have been known to vary markedly between individual observers. With WZ Cassiopeiae and the method of "colour differencing" I have described here (based on B-V of each component in a double star system), the high colour contrast of this pair is based on "objective" and "numerical" grounds and, of course, astronomy is after all a numerical science and we desire objectivity in all aspects of our work. So... there should not be any argument here about WZ Cas being the most colour-contrasted out of all double stars we observe in the night sky! :)

A past discussion on WZ Cas is archived here.

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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