Skywatch Winter: Cassiopeia
6 minute read
Astronomer Brian Jones focuses his attention on Cassiopeia, a constellation visible from the UK at all times of the year but easiest through the longer, darker nights of winter. Whether you’re camping, bivvying or staying in a bothy you should be able to spot the Queen of the Northern Skies.
This chart shows the stars and constellations that are described as being circumpolar as seen from mid-northern latitudes, including Great Britain, and depicts the groups that occupy the region of sky surrounding Ursa Minor (the Little Bear) and its prominent leading star Polaris (as described in this year’s Skywatch Autumn).
If a star or constellation is described as being circumpolar it means that it never sets as seen from a given location, remaining above the observer’s horizon and visible all year round. Whether or not a constellation is circumpolar depends on where you are on the Earth. For example, as far as observers at mid-northern latitudes are concerned, the stars and constellations depicted on this chart are circumpolar and are always on view. However, the further south you travel, the fewer circumpolar stars there will be (for more information on Circumpolar Stars see www.starlight-nights.co.uk/circumpolar-stars).
The area of sky we see here is home to the better known constellations Ursa Major (the Great Bear), Ursa Minor (the Little Bear) and Cassiopeia (Cassiopeia), as well as a number of fainter and less well known groups such as Draco (the Dragon), Cepheus (Cepheus) and Camelopardalis (the Giraffe). Our focus for now, however, is directed towards the prominent ‘W’ or ‘M’ formation of stars known as Cassiopeia which, along with the Plough (part of the larger constellation Ursa Major, the Great Bear) and Ursa Minor (the Little Bear), completes the trio of conspicuous star groups that grace the far northern skies.
The constellation represents the mythological Queen Cassiopeia, wife of King Cepheus and mother to Andromeda, the beautiful maiden rescued by Perseus from the terrible sea monster (for more about the exploits of Perseus see www.starlight-nights.co.uk/perseus). Cassiopeia stands out quite well and should be spotted fairly easily by observers at mid-northern latitudes, being located a little to the west of the overhead point during winter evenings.
When favourably placed, the entire constellation can be seen from locations to the north of latitude 12°S and portions of Cassiopeia can be glimpsed from Australia, South Africa and all but the southern reaches of South America.
A general guide to locating Cassiopeia is to use the two end stars in the ‘bowl’ of the Plough as pointers, these being the two stars we use to locate Polaris, the Pole Star. The Plough can be found low down in the north eastern sky when viewing from mid-northern latitudes at this time of year. Following the imaginary line from Merak through Dubhe and past Polaris as shown in this chart will eventually bring you to Caph in Cassiopeia (for more on star names see www.starlight-nights.co.uk/star-names) from where the rest of the group can be picked out.
The constellation lies within the Milky Way and the whole area is seen to abound with stars. On a really dark, clear and moonless night you should spot around fifty naked eye stars within the group, although binoculars will reveal many more scattered across this region of sky. Although most of these stars are below naked-eye visibility, their combined light produces the effect we call the Milky Way.
While not usually very clear to city-dwellers, the Milky Way can be a superb sight when viewed under a really dark and moonless sky.
Cassiopeia contains six prominent stars, the brightest of which is Cih whose light has taken around 550 years to reach us. Cih generally shines at magnitude 2.15, although this star is slightly variable and prone to sudden and unpredictable increases in brightness. Shedar, its name derived from the Arabic ‘al-sadr’, meaning ‘the Breast’, is a magnitude 2.24 orange giant star (for more on magnitudes see www.starlight-nights.co.uk/magnitude). Shining from a distance of around 220 light years, the orange tint of Shedar is easily seen in binoculars. Slightly fainter than Shedar is Caph, a white giant shining at magnitude 2.28 and lying at a distance of just 55 light years. Caph derives its name from the Arabic ‘al-kaff al-khadib’ meaning ‘the Stained Hand’, an allusion to the Eastern tradition of staining the finger-tips with henna leaves. Achird, located between Shedar and Cih, is a comparatively nearby star, its magnitude 3.46 glow reaching us from a distance of just 19.4 light years.
Marking the knee of Cassiopeia is magnitude 2.66 Ksora, the light from which set off on its journey towards us around a century ago. An alternative name for this star is Ruchbah, both names being derived from the Arabic ‘rukbat dhat al-kursiy’ meaning ‘the Knee of the Lady in the Chair’. The star Segin completes the distinctive shape of Cassiopeia. Segin shines at magnitude 3.35 from a distance of around 425 light years, the light we are seeing from this star today actually having set off towards us before the invention of the telescope in the early 1600s!
This prominent constellation was alluded to by the English astronomer Joseph Henry Elgie when he was recalling his original efforts to find his way around the night sky. As he informs us, a number of the constellations that he struggled to identify were rather faint and difficult to detect, although he had no problems with Cassiopeia, writing that: ‘Facing due north, and looking high upward, the gaze meets with five stars of nearly equal brightness, in form resembling a sprawling capital ‘W’. They make the principal outline of the constellation Cassiopeia, the Lady in the Chair. I cherish a kindly remembrance for Cassiopeia; it was the first star group I ever recognised, when, under almost heart-breaking difficulties, I was trying to learn the geography of the sky.’
In the next post we will be checking out the small but distinctive Canis Minor (the Little Dog), one of the mighty Orion’s hunting dogs, together with its leading star Procyon, which will be well placed in the night sky throughout the winter months. In the meantime, don’t forget to look for the distinctive shape of Queen Cassiopeia, and perhaps bring to mind Elgie’s wonderful comments on this beautiful constellation. Happy stargazing!
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