Observers’ Forum

NGC 7008 – a ‘celestial hook’

One of the delights of the summer sky for 21h 00m 33s and Dec planetary enthusiasts is the number +54o 32' 35" (2000.0), of objects on view. And for visual observers almost midway between who don’t have the luxury of remote warm- a line joining mag 1.3 room observing, all available under a pleas- (α Cygni) and ant summer sky. Even the lack of truly dark mag 2.4 Alderamin (α skies is not a serious problem as many Cephei). As with many planetaries have a high surface brightness and planetaries its stated the use of filters can often improve contrast. magnitude varies widely To many people summer planetaries mean in the literature and the Ring Nebula or the Dumbbell, but there ranges from around 9 to are numerous other objects which deserve 12, but with a size of investigation, and one of them is NGC 7008 only 1.4×1.1 arcmin it in . appears quite bright and Discovered by William Herschel in 1787, he will certainly be visible classified it as a bright nebula rather than a in a 20cm telescope. It is Andrea Tasselli planetary which, considering its shape, was thought to lie at a dis- reasonable. To Herschel planetary nebulae tance of around 3,000 light years, with a to the nebula enhancing the hook like ef- were small round objects which often ap- physical size of around 1 light year. fect, whereas visually at high power it is peared slightly greenish and which reminded At first appearance NGC 7008 seems a clearly separate. High power will also show him of Uranus, which he had discovered 6 strange object. It has a hooked shape, with a variation in intensity across the nebula years earlier. The true nature of what we the hook lying north−south, and a double and what were once thought to be super- now call planetary nebulae remained a mys- (h1606, separation 18 arcsec) at its imposed faint are now thought to be tery until William Huggins turned his spec- southern end. Imagers intent on capturing knots of material associated with the plan- troscope on NGC 6543 in 1864 and realised detail in the nebula often cause the brighter etary itself. The mag 13 central star is off- it was a gaseous object. NGC 7008 lies in component of this double star to become set and lies slightly to the west of centre. Cygnus, close to the Cepheus border, at RA bloated, so that it appears to be attached Spectrographic investigations suggest that

J. Br. Astron. Assoc. 120, 3, 2010 179 Observers’ Forum

Martin Morgan–Taylor Nick Hewitt the nebula consists of two elongated shells field image (field size 38×28 arcmin) was marvellous detail, appearing around 90 arcmin of expanding gas, one inside the other, with obtained with a TMB 115mm f/7 refractor in size with a brighter area to the north-east the inner shell expanding faster than the and SXV-HD CCD camera. The exposure end of the hook and a smaller bright area at the outer and causing a ‘snow plough’ effect was 10min through an OIII filter. Even at southern tip. Careful study will show an in- familiar in planetary nebulae as the inner this scale the nebula displays a clear shape. teresting textured surface which is much more shell piles in to the outer. Martin Taylor’s image was taken through obvious with an OIII filter, and a darker band A selection of images of NGC 7008 received his 20cm Cape Newise using a Canon 20D which appears to cut the nebula in two. A by the Section are included with this article SLR camera. The stacked exposure of UHC filter is of minimal use on this object, (depending upon the optical train in the in- 8×4min gives a close approximation to how the view being only marginally better than dividual telescopes some images are flipped the nebula appears visually. Fred with no filter at all. compared to others). Nick Hewitt’s wide Stevenson’s image was from a much longer exposure, including an H-alpha component. Stewart L. Moore, Director, Deep Sky Taken through his 35cm Meade SCT with a Section DSI Pro III CCD camera the details were 10×4min Hα, 10×4min Red, 7×4min Green and 13×4min Blue. Andrea Tasselli is one of the leading exponents in the country of imaging planetary nebulae and his image on the previous page through an Intes Micro M809 (20cm f/10 Mak−Cass) with SXV- H9 CCD shows tremendous detail. The LRGB image consisted of 75min Luminance and 21 min each RGB through Astronomik IIc RGB filters. Visually in a large telescope (say 30−35cm) × Fred Stevenson and at high power ( 200) this planetary shows

180 J. Br. Astron. Assoc. 120, 3, 2010