Keen Northern Hemisphere stargazers will rejoice that the winter solstice, which occurs on 22 December 2019, brings the longest nights of the year. This is the time when observers in the UK have twelve hours of darkness to enjoy – moonlight notwithstanding – starting at 6pm GMT. If you relish an early evening stargaze before it gets too cold, one readily identifiable constellation that lies virtually overhead at 7pm GMT as seen from the British Isles during the festive season is Cassiopeia (pronounced kas-ee-uh-pee-uh).
Most people learn to identify Cassiopeia as a five-star M-shaped (if you are facing north at this time of year) pattern of second- and third-magnitude stars that can easily be encompassed by the span of an outstretched hand at arm’s length. If you are lucky enough to receive a new telescope for Christmas, why not seek out some of the constellation’s beautiful double and multiple stars set against the rich backdrop of the Milky Way.
Sigma (σ) Cassiopeiae
α = 23h 59.0m, δ = +55° 45’ (J2000.0)
Magnitudes: +5.0 & +7.1
Separation: 3.1 arcseconds
Sigma (σ) Cassiopeiae is a close double star for 10-cm (4-inch) aperture telescopes and larger. A night of steady seeing and a magnification of around 150× is required to see this pairing of a magnitude +5.0 primary and magnitude +7.1 companion separated by 3.1 arcseconds. Look for bluish-white and yellow tints. Sigma Cassiopeiae is about 1,500 light-years distant. And here’s another fact: owing to precession, Sigma Cassiopeiae has a mean right ascension of exactly (give or take a second) of 00h 00m 00s at the current J2019.9 epoch!
Eta (η) Cassiopeiae, aka ‘Achird‘
α = 00h 49.1m, δ = +57° 49’ (J2000.0)
Magnitudes: +3.4 & +7.5
Separation: 13.4 arcseconds
Commonly known as Achird, Eta (η) Cassiopeiae is a fine double star with noticeable colours first recognised as a binary system by William Herschel in the summer of 1779. An easy pair for small telescopes at 50× owing to the wide 13.4 arcsecond separation, its magnitude +3.4 primary is yellow, while the magnitude +7.5 companion is orange. The orbital period of Eta Cassiopeiae is about 500 years and its distance from Earth is 19.4 light-years.
Σ 163 (Cassiopeia)
α = 01h 51.3m, δ = +64° 51’ (J2000.0)
Magnitudes: +6.8 & +8.8
Separation: 34.8 arcseconds
Although the accompanying illustration shows a high-power view, a colourful double in Cassiopeia that doesn’t require large magnifications to resolve is Struve 163 – frequently appreviated to Σ 163 in catalogues – on account of its wide, 34.8 arcsecond separation. A small telescope at 30 to 50× magnification is all that’s required to view the magnitude +6.8 orange primary and its magnitude +8.8 bluish companion. The stars lie about 2600 light-years from the Sun.
Iota (ι) Cassiopeiae
α = 02h 29.1m, δ = +67° 24’ (J2000.0)
Magnitudes: +4.6, +6.9 & +8.4
Separation: 2.6 & 7.6 arcseconds
We shall conclude this very brief tour of the best double and multiple stars Cassiopeia has to offer with what I consider to be the finest triple star in the entire sky. Iota (ι) Cassiopeiae is also a multiple star whose components typify the subtle and subjective nature of star colours. Whereas many perceive just three white stars, a large proportion of observers see a pure white primary, the closer companion as yellow-orange and the wider companion as bluish-white. Your perception of star colours can also vary with the type of telescope and eyepiece used. What do you see? However you interpret its component hues, you will need a night of good seeing and typically a 10-cm (4-inch) aperture ‘scope at around 150×, or more. Iota Cassiopeiae lies about 133 light-years distant.
Cassiopeia in fact and myth
Cassiopeia is an ancient constellation, one of the original 48 listed by Greek astronomer and mathematician Ptolemy (circa 100-170 AD). At the temperate northern latitudes of the British Isles, it’s circumpolar – permanently above the horizon, never rising or setting. By size, Cassiopeia is the 25th largest out of the 88 constellations recognised today.
In Greek mythology, Cassiopeia is queen to King Cepheus and mother of Andromeda. Cassiopeia angered the sea god Poseidon by boasting that Andromeda was more beautiful than sea nymphs known as the Nereids. As punishment, Poseidon decreed that Andromeda should be sacrificed to the sea monster Cetus (but she was rescued by Perseus), while Cassiopeia was condemned to eternally revolve around the north celestial pole seated on her throne.