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Blue Planetary Nebula  (NGC 3918), 


            Bruce Rohrlach

Blue Planetary Nebula (NGC 3918)

Technical card

Resolution: 2036x1595

Dates:April 10, 2019

Frames: 240x15"

Integration: 1.0 hours

Avg. Moon age: 5.00 days

Avg. Moon phase: 25.77% job: 2644572

Locations: Lysterfield, Melbourne, Victoria, Australia

Data source: Backyard


My fourth Planetary Nebula (PN) – this time the Blue Planetary Nebula (NGC 3918) at around 4,900 LY away in Centaurus (also known as “The Blue Planetary” or the “Southerner”). This PN was first discovered by Sir John Herschel in March 1834.

Imaged from the eastern fringe of Melbourne last night. One hour exposure comprising 240 sub-frames, each at 15 seconds exposure (ASI224mc). It is difficult to extract many visual details from this planetary nebula (shells of gas ejected from a star that is transitioning from a red giant to a white dwarf star) due to its tiny angular diameter. The very slightly oval centre is telescopically 8-10 arc-seconds in diameter - whilst the seeing (i.e. the limit of resolution looking through a turbulent atmosphere) is typically around 2 arc-seconds. So not much room to see 'features' in this planetary nebula. The outer gas shells are around 20 arc-seconds in visual diameter (around half the diameter of the visual disc of Jupiter), so this feature is extremely small on the sensor at prime focus.

Nevertheless, the Blue Planetary Nebula is stunning with its extraordinary turquoise blue to green colour against a starry background. The beginning of a bipolar lobate structure can just be made out at the 2 O’clock and 8 O’clock positions. Apparently bipolar planetary nebula (i.e. with bipolar gas jets) are thought to be the result of a binary star system, with one star at the end of its life convulsively ejecting shells of gas whilst the other binary interacts gravitationally with the ejected gas shells. These expulsed shells of gas ejected from the dying star are approaching us at around 17 km/second while the shells of gas are expanding at around 24km/second.



Bruce Rohrlach
License: None (All rights reserved)


Blue Planetary Nebula  (NGC 3918), 


            Bruce Rohrlach