Beside the "famous three" Protoplanetary Nebula (PPN) in the summer skies (M 2-9, M 1-92 and CRL 2688), Steve wrote about in the June edition of S&T, we found a fourth brighter one at the NW edge of Sagittarius - Henize 3-1475 the "Garden Sprinkler".
Discovered and published [1976ApJS...30..491H] first by Karl Gordon Henize, this object was found on a photographic plate from a 10.5-inch lens, which was lend to the Lamont-Hussey Observatory in South Africa from 1949 - 1951 to cover the southern sky. Equipped with a objective-prism, the plates shows all in all 1929 "new" emission-line stars in H? - one of it, number 1475 was the current OOTW.
From the physical aspect, a PPN represents the evolutionary stage between the end of the asymptotic giant branch (AGB) stage and the planetary nebula (PN) stage of an intermediate-mass star. Within this short period, the strong infrared shining central star (CS) illuminates the ejected shells. The envelope shape changes from spherically symmetric (concentric rings) to axially symmetric and forms the frequently observed bipolar structures, depending on the line of sight. The remaining dust, usually donut-shaped around the CS, blocks the light and forms the constricted morphology between the bipolar outflows. The star itself is still not hot enough to ionize the surrounding remnants, causing the visible nebula to be a reflection nebula.
Because of the young age and the limited illumination power of the CS, the objects are small and they hence appear usually much smaller than the familiar PNs. Good seeing and high magnifications are required for successful observations. Due to the absent ionization, nebula filters don't have any effect.
Hen 3-1475 is around 18000 light-years away from us. Its special feature are its high velocity jets. The formation and collimation of jets in PPNe are still in discussion [2004ASPC..313...73R]. Science suggested the morphology of the jets are created by a central source that ejects streams of gas in opposite directions and precesses once every thousand years. Science also speaks about a slowly rotating garden sprinkler - like its nickname.
A quick look into the DSS shows a bright and slightly elongated object. The elongation indicates, that the axis has to be over around 10" and could be in reach for mid-size telescopes. The given magnitudes are around 12vmag. With my 27" and exceptional good seeing I wrote: bright, direct vision main body; main body itself 3:2 elongated with stellar peak at its SE edge; NE extension visible from 837x up; best view 1137x; extension thin but steadily visible with averted vision; starting from the main body first brighter part, than small but fainter part till a stellar peak at its NW end; no thin extension to the SE, but only a slightly brightening; no reaction with UHC.
But now it is your turn to get this PPN into the eyepiece.
Nice post, Uwe. A few years ago, I looked at Hen 3-1475 in my 10" SCT. All I could see was its central star. But I observed it again in my new-to-me 16" f/4.5 on the morning of the 7th. The central star was easily visible at only 70x, but it took 600x before I could see that the star seemed to extend slightly NW. I could suspect maybe something on the star's SE side, but the tiny but bright glow just poking out from the star on the NW side was much more noticeable. Uwe's drawing does a good job of showing that high surface brightness knot I saw. It was my sixth preplanetary nebula sighting and the first one that I couldn't also see in nothing more than my 10" SCT.
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