Regular solar observer & correspondent Harry Roberts reports on his observations of the Saturn nebula, NGC 7009, a planetary nebula in the constellation of Aquarius.
Puzzling things planetaries! They emit mainly H-alpha, OIII and H-beta light: that is, they are almost pure LRGB narrow-band sources. Here we contrast a superb HST(c) image (Fig1, lhs) with the same image remade with no RED channel, 50% GREEN and 100% BLUE (assuming HST team used a normal filter set). This ‘recipe’ approximates human visual sensitivity. The result is, in fact, close to what we see in a ten inch ’scope (Fig1,rhs).
Why can’t we see H-alpha in deep sky objects? This is because our Sun’s photosphere emits little in that band and H-alpha is close to the limit of human sensitivity, while we see blue and green quite well. This is why many planetaries look nothing like their big-scope images that are dominated by the red H-alpha channel (and often other faint bands.)
Central Star. My first view of NGC7009 was at Sydney in 1985 with the new C8 and an LPR filter: “a startling blue planetary; slightly elongated E-W…impression of being annular”. Recent views confirm this but add a large ‘nimbus’ of blue surrounding the entire thing. Where is the central star? (Fig2)
While Lord Rosse could not see the central star with his giant ‘scope, some find it easily: “Unlike the central stars of many planetaries, NGC7009’s is fairly easy to see in small telescopes.” (S. J. O’Meara “The Caldwell Objects” Cambridge Uni. Press. 2002. P219). I’m afraid I can’t agree with him.
I eventually got the central star by ‘blinking’ with an orange #21 filter in a 10 inch Dob and a tracking 10 inch LX90 (Thanks, Bob). The central star is a hot one, 55,000K, with strong UV emissions but does have a continuous spectrum – and while the #21 filter would block much of the star’s light too, it blocks ALL the nebula’s light: finally revealing the 11.5mag star! It’s not easy to see! (Fig2: inset).
What is a planetary nebula? The erroneous name was first used for these objects as they resembled planets in appearance. A current definition is: “A planetary nebula, often abbreviated as PN or plural PNe, is a kind of emission nebula consisting of an expanding, glowing shell of ionized gas ejected from old red giantstars late in their lives.”( Frankowski, Adam; Soker, Noam (November 2009) Wikipedia).
While most well-known planetaries are roundish in shape, the majority have very complex ‘morphologies’ – perhaps the strangest within amateur reach is NGC5189 in southern constellation Musca, which we will consider in a future article.
Planetaries are common enough in the night sky – if you have charts (or go-to’s) to find them. The biggest is the faint “Helix” nebula in Aquarius. The brightest may be the “Ring” in Lyra or the magnificent “Dumbell” in Vulpecula: both are showy bright objects for any ‘scope bigger than 3 inches (75mm) in aperture. Our target, the “Saturn” nebula is bright but small: tracking ‘scopes and high magnification are needed for its elusive details.
Harry Roberts is a Sun and Moon observer, a regular contributor to the Sydney Observatory blog and a member of the Sydney City Skywatchers.