Ordinary Meeting, 2006 December 16
The Winter Deep Sky
Dr Moore began by defining the 'winter sky'. Though constellations normally associated with summer were still visible in the west at dusk, and those associated with spring were already visible in the east at dawn, he would concentrate upon those which presently transited at around midnight, such as Orion, Gemini and Taurus. Reading the popular literature, one might be forgiven for concluding that the Orion Nebula (M42) was the only deep sky object among these constellations. Dr Moore argued that there were, in fact, many other beautiful but widely-neglected objects on offer; given that he would say nothing further about M42, he conceded that his title should perhaps have been 'The Alternative Winter Deep Sky'.
The sword of ORION contained an often-ignored string of nebulae apart from M42. Immediately north of M42, and physically associated with it, was M43 – a complex of emission/reflection nebulosity which repaid detailed study. A little further north still, one came to another cluster of nebulae: NGC 1973, 1975 and 1977. These were likewise associated with M42 and actually formed a single continuous whole; Dr Moore explained that the small-field telescopes used in the compilation of the NGC had often hindered the identification of large-scale nebulosity. At the northern end of the sword lay NGC 1981 – a beautiful open cluster.
Turning to more challenging targets in Orion, Dr Moore mentioned planetary nebula Abell 12 – easy to find on account of being a mere 1.2' from mag 4 star μ-Ori, but very tricky to distinguish from its glare. This nebula was more accessible to CCDs than to visual observers, but in both cases an OIII filter was essential; the speaker showed a recent CCD image by Andrea Tasselli. Despite its fame, the Horsehead Nebula (B33) often disappointed. It too was a highly challenging target, requiring a large aperture – minimum 14" for visual work – good transparency, and Hβ filter. As with Abell 12, it had become much easier to image since the advent of CCDs.
Further north lay M78 – a comet-shaped region of both emission and reflection nebulosity, appearing to have two nuclei. It repaid both filtered and unfiltered observation to reveal its emission and reflection components respectively. It had achieved brief fame in 2004 as the site of McNeil's Nebula, though this had now faded from sight. Dr Moore urged members to keep watching this sky region; McNeil's Nebula had previously been seen in 1966-7, and so might recur. He concluded his tour of Orion by briefly mentioning two further bright objects: planetary nebula NGC 2022 and open cluster NGC 2169.
He then moved south to LEPUS, a constellation seemingly ignored by many UK amateurs. Though it lay south of Orion, its centre still culminated at 20° altitude, making it accessible from all but the most light-polluted areas. Dr Moore started his tour with M79, a bright globular cluster; at 9' across it was an ideal binocular target. It was midway in concentration and had a bright central condensation.
The Spirograph Nebula (IC 418) was a visually easy planetary nebula. Through a 6" aperture, a disk of 12" diameter was visible; through a 24" aperture, signs of mottling began to appear within this. At high power, it was possible to resolve the mag 10 star which lay at its centre. Using an OIII filter, an outer ring was also discernable.
Finally, Dr Moore gave mention to open cluster NGC 2017 and carbon star R Leporis, which Hind had described as "the most intense crimson, resembling a blood-drop on the black background of the sky". It was a long-period Mira-type variable star, and its colour varied from deep crimson at minimum to a more coppery hue at maximum.
He then turned to GEMINI. The most famous globular cluster here was M35, in which more than 150 stars could be counted within a 1° diameter. But the speaker urged members to look also at NGC 2158, less than 30' away, and much more compact on account of being six times more distant. Two open clusters also lay nearby: IC 2157 and NGC 2129; a little further afield was NGC 2266.
Gemini offered two easy planetary nebulae. The Eskimo Nebula (NGC 2392) was well known, but the double object NGC 2371-2 less so. Measuring 55" across, this latter object was larger than Jupiter; it had a bipolar appearance, especially apparent through an OIII filter. The Medusa Nebula (Abell 21) was much more of a challenge; through the speaker's 14" it was discernable but not bright. A final challenge for Gemini was supernova remnant IC 443.
Dr Moore then turned to MONOCEROUS – not a widely known constellation, probably on account of its complete lack of bright stars. It could be found immediately to the east of Orion. Its most famous deep sky offering was the Rosette Nebula (NGC 2237, 2238, 2239, 2246), a vast swath of emission nebulosity, in the centre of which lay open star cluster NGC 2244; this was a site of very recent star formation. Its most appealing open cluster was M50, a grouping of 50+ stars within a 15" diameter. Visually, it appeared arrow shaped. Monocerous was also home to Hubble's Variable Nebula (NGC 2261), a bright and easy patch of emission/reflection nebulosity, varying in both brightness and structure on timescales of months.
The speaker concluded his talk with TAURUS – home to M1 and M45, but also to some lesser-known objects. The Hyades (Mel 25) were an appealing sight through binoculars – more so than through a telescope – it was nice to see this large cluster with its surrounding field.
Dr Moore closed by remarking that December 14 had been the 200th birthday of the Revd Thomas William Webb (1806–1885); Webb had been one of the first popularisers of astronomy, and it was in his honour that the modern-day Webb Society was named.
Following the applause, the President adjourned the meeting until 2007 January 31 at the present venue.
© 2007 Dominic Ford / The British Astronomical Association.