Ordinary Meeting, 2005 October 26
The Autumn Deep Sky
Dr Moore remarked that his talk would provide a striking contrast to Mr Boles' earlier Presidential Address: whereas the supernova patrol work described there had been at the cutting edge between amateur and professional work, the present talk would contain a lot of pictures, showing some of the beautiful gems of the nights sky. The autumn was often thought a poor season among amateur astronomers. The winter constellations – Orion, Taurus, Gemini, etc. – rose very late in the night, while the summer constellations – those at right-ascensions closest to the Galactic Centre, including Patrick Moore's famous Summer Triangle – began to sink into the twilight of dusk. As for the autumn's own constellations – Andromeda, Triangulum, Aquarius, etc. – they seemed to be dominated by rather faint and unappealing stars.
But he wished to right this view, for what these constellations lacked in bright stars, they made good with a fine showing of deep sky objects. Concentrating on the vicinity of Pegasus, the speaker began to tour the sights.
Perhaps the most famous among them was the Andromeda Galaxy, M31, an Sb spiral, and its companions, M32, a dwarf galaxy, and M101, an elliptical. Here was a fine illustration of the gregarious nature of galaxies – a triplet separated by a mere degree on the sky. Showing a three-dimensional map of the Local Group of galaxies, the speaker illustrated how M31's relationship to its two companions was very similar to that of the Milky Way to the two Magellanic Clouds; in both cases, smaller satellite galaxies orbited around a more massive parent.
M31 itself was ~3° across, and readily visible as a misty patchy to even mediocre eyes given sufficiently dark skies. Given its brightness and size, it was an easy target to find telescopically; the star α And (aka δ Peg) provided a nearby reference for star-hopping. Through a telescope with 30' field, however, it was often a disappointment: only the central region could be seen, which seemed to merely fade away towards its edges. A good pair of binoculars generally provided a much more appealing view. The speaker's personal favourite of the trio, however, was M101 – though the faintest of the three, it was set in an attractive star-field, and its mottled core and a faint obscuring dust lane made it appear slightly spiral, despite its elliptical classification.
Despite their fame, however, Dr Moore thought that Andromeda had better sights to offer – NGC 891, for example. It was easy to locate, being 18' of right ascension to the east of mag 2 star γ And – one could simply find this star, turn off the drive, and drink tea for 18 minutes! An edge-on Sb spiral galaxy, it appeared spindle-like, with a strikingly dark dust lane running through its centre, and two mag 12-13 stars marking either end. Another nearby edge-on spiral (classified Sbc) was NGC 7640, appearing as a thin needle of light – a long thin streak of faint nebulosity with a slightly grainy core. It was a remarkable sight, but despite being quite bright at mag 11, its low surface brightness required a good sky.
The speaker turned next to NGC 7331 – a discovery of William Herschel's; Messier appeared to have missed it. An Sb spiral, it had a cluster of associated close companions, of which NGC 7335 was the brightest. Despite being a beautiful sight in its own right, it was perhaps best known as a stepping-stone to Stephan's Quintet – a short star-hop of around 30' to the south-west. Discovered by Frenchman Jean Marie Eduard Stephan with his 40-cm refractor from Marseilles in 1877, this latter object – five galaxies contained in a mere 5' field – was something of a historical curiousity. Four had measured redshifts in the range 5750-6750 km/s, while one, NGC 7320, measured a mere 790 km/s. Conventional cosmology would, by associating redshift with distance, place this one galaxy much closer than its "companions", and argue it to be unassociated with them; their closeness on the sky would be thought purely coincidental. However, in the 1960s, Halton Arp, amongst others, had proposed a now largely discredited view, that NGC 7320 could be seen to be interacting with its companions, and therefore must be associated with them; he had argued, therefore, that the redshifts of objects might not measure their distances so reliably as many thought.
Moving now to the constellation Triangulum, the speaker showed the famous Pinwheel Galaxy, M33 – to be found 4.3° north-west of α Tri. Despite sounding rather bright at mag 5.7, it actually had a rather faint surface brightness on account of its size – at 71' × 42', larger than a Full Moon. It was the third largest member of the Local Group, and within its sweeping spirals could be found many deep sky treats, including the bright HII region NGC 604. However, the speaker wished to draw more attention to nearby NGC 672 and its companion IC 1727 (8' distant). Both were beautiful Sb spirals, but under-observed, despite their being visible even with a mere 6" aperture. The Pinwheel, by contrast, could be a challenge with a 10" aperture.
As a final Messier galaxy, Dr Moore mentioned M74, though warning that it was perhaps the most difficult of all objects in the Messier catalogue to observe. A fine face-on spiral, visible as a square bright patch in lesser sky conditions, rewarded those who persevered, however. The speaker recommended fine frosty nights as the times most likely to yield the required sky conditions. He closed his summary of galaxies with NGC 253. At a declination of –25° it never rose above 14° altitude in the UK, and there was some debate as to its observability from these shores. However, this was surely unquestionable: it had been discovered from the UK by Caroline Herschel in 1783. Measuring 29' × 7', and mag 7.3, it was quite bright, visible in even comparatively poor seeing. Given its declination, the windows for observation were limited, however, and the speaker recommended 10pm in late October as a good time.
Turning to star clusters, M2 in Aquarius and M15 in Pegasus were surely the finest globulars of the season, measuring 12' and 15' across respectively. NGC 7006 in Delphinus was also fine, though rather more distant – 185 thousand light-years (kly), as opposed to 30 and 37 kly for M2 and M15 respectively. Consequently, a mere 1.5' across, it was too compact to be resolved. NGC 7492 in Aquarius was also worthy of note, though at δ=-16° it never rose more than 23° above the London horizon, and at 6' across, was tricky to resolve.
Cassiopeia was littered with many tens of open clusters, but surprisingly only one was a Messier – M103 – and even this a rather stubby unexciting specimen. Rather more interesting, to the south-west of the 'W', was NGC 7789 – missed by Messier and later discovered by Caroline Herschel – so often recorded in history as a mere secretary to her brother. Some had associated nebulosity, including NGC 281 / IC 1590 – the former referring to the nebulosity, the latter the stars themselves – this example being best viewed with an ultra-high contrast (UHC) filter; it appeared rather poor through an OIII filter. IC 59 and IC 63 were also good candidates for an UHC filter.
Two planetary nebulae were particularly well-placed in the autumn sky: NGC7662, the Blue Snowball Nebula, in Andromeda, and NGC 7094 in Pegasus. The former was visually easy and required no filter; it could be found close by galaxy NGC 7640. John Herschel had first noted it to be "blue in colour"; its nickname derived from Leyland Copeland's remarking upon its "looking like a light blue snowball". This trademark colouration was quite tricky to make out, as the eye's most sensitive cells, the rods, were colour insensitive, but it could be discerned with a reasonable aperture. NGC 7094, 1.5° away from M15, was rather harder – it was rather fainter, and greatly benefited from the use of an OIII filter. It presented all that a planetary nebula should, though: a bright shell surrounded a central star, with faint nebulosity filling the shell.
At half the size of the Full Moon, the largest, and at 0.45 kly, also the closest, of all planetary nebulae was the Helix Nebula, NGC 7293, which presently culminated at around 20h00 UT. At δ=-20° it never rose more than 18° above the London horizon, and was easiest viewed from the Continent, but was beautiful sight through a large aperture. It could be seen through a 6" aperture, but a 12" brought out much more structure; in both cases an OIII filter was recommended.
Following the applause for Dr Moore's fine, well illustrated tour of the autumn sky, the President adjourned the meeting until December 17 at the English Heritage Lecture Theatre on Savile Row.
© 2005 Dominic Ford / The British Astronomical Association.