Haunting the barren starfields of the large serpentine constellation Hydra, the Ghost of Jupiter Nebula is a glorious glowing sapphire of ionized gas! Also catalogued as NGC 3242, this jewel of a planetary nebula has been enchanting astronomers since its discovery in 1785 by William Herschel. Its popular name stems from both its brightness and size in telescopic observing being comparable to the planet Jupiter in the sky.
Consisting of a multiple shell structure, NGC 3242 is a high excitation planetary nebula that is of great interest to professional astronomers and has been the subject of a number of studies. In addition to an inner AGB halo, this telescopic image reveals the brightest portion of a large ISM-type halo, which is the irregular cloud with gentle shades of pink and purple. The colours of the various structural elements signify the central shell and inner halo as consisting predominantly of OIII (oxygen) emission and the larger 40×36 arcminute halo structure is comprised of both overlapping Ha (hydrogen) and OIII emission.
An AGB halo provides a record of mass loss of the original progenitor star during the late asymptotic giant branch phase, which precedes the main planetary nebula phase. Haloes are generally 10 times fainter (or more) than the central shell and require deep photography to elucidate their faint gossamer forms. Once considered rare, haloes have been found around a large sample of planetary nebulae in deep imaging surveys conducted in the past few decades and more continue to be discovered even by amateur astronomers!
The ISM-type halo might have a size of approximately 39×33 light years and was first discovered in photographic plates by the astronomer Rudolph Minkowski and independently by others in the 1960’s. The AGB halo was produced by the central star prior to the planetary nebula phase. However the giant ISM-type halo isn’t material ejected by the star but is simply part of the interstellar medium that is part of the space surrounding the planetary nebula. It is visible through the process of ultraviolet radiation leaking out of the planetary nebula shell and ionizing the surrounding material. Other examples of ISM-type haloes have been identified around a small number of planetary nebulae including HDW 2 and Abell 36.
The complexity of this beguiling planetary nebula also extends to its main central shell with deep professional images revealing multiple knots, which belong to a category of low ionisation structures associated with planetary nebulae known as FLIERs. Standing for “fast low ionisation emission regions”, they consist of a pair of nebulous knots that usually share the symmetry axis of the central star. Professional images have also revealed the presence of multiple rings around the central shell. Rings have been found around other planetary nebulae and are a puzzle awaiting to be solved although their formation and presence is likely to be related to episodic mass loss in the early evolutionary stages of planetary nebulae.EAPOD Archive
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