Object of the Week January 11, 2015 - NGC 1360 A Dissolving Planetary

Object of the Week January 11, 2015 - NGC 1360 A Dissolving Planetary

NGC 1360, Minkowski 1-3, PK 220-53 01, PN G220.3-53.9, Bennett 15
Type III Planetary Nebula
RA: 03h 33m 14.6s Dec: -25° 52’ 17.9”
Size: 11’ x 7.5’
Mag: 9.4v
Surface Brightness: 21.9 mag/arc-second²
Mag (central star): 11.3v

Huge and bright through the winter night
Yet not even plotted on all maps of the sky
In Fornax this object lies
Where mostly galaxies reside
Irregular, peculiar, uncommon they write
Looks more like a football with a distinctly green light
A planetary nebula this happens to be
Has NGC 1360 been seen by thee?

Among the multitude of galaxies in the constellation Fornax, there lurks one of the brightest planetary nebula of the night skies, NGC 1360. How the prolific observers of the period, like John Herschel and James Dunlop, missed this large, bright object remains a mystery to this day. Instead it was two well-known comet hunters, Lewis Swift and Friedrich August Theodor Winnecke, who independently discovered NGC 1360.

The story of Lewis Swift’s discovery is rather interesting. In 1855 Swift purchased a damaged 3-inch objective from a peddler for five dollars. After making a brass mounting and an eyepiece for the telescope he began a survey of the night sky. Two years later he discovered the extremely comet-like glow of NGC 1360. Meanwhile Friedrich Winnecke is credited as a co-discoverer of NGC 1360 when he observed it with his 3.5” comet seeker telescope in January 1868.

These early discoverers of NGC 1360 both described the object as a peculiar object, not at all sure what it was. It wasn’t until 1946 when Rudolph Minkowski first identified it as a planetary nebula. It was Lubos Kohoutek who labeled it an “uncommon planetary” in 1968. A few years later Steven Hynes listed NGC 1360 in his Planetary Nebulae as a Type III planetary (a planetary with an irregular disk).

Today we know that NGC 1360 is typical of a large evolved planetary nebula, where evolved means “aging”. A 2004 paper Physical Structure of Planetary Nebulae. III. The Large and Evolved NGC 1360 describes the planetary nebula as not having any obvious shell morphology, unlike most planetary nebulae that show distinctive, well-marked symmetry, and well-defined boundaries. The authors go on to show that the large kinematic age (study of stellar movement used as a proxy for age) of the shell, which is ~10,000 years, and the low density of the nebula, imply that NGC 1360 has begun to merge (dissolve) with the interstellar medium.

NGC 1360 is one of the few known examples of a large, high-excitation planetary. The central star’s spectrum places it in the Wolf-Rayet category of planetary nebulae, meaning the central star appears to be in a state of flux and is violently ejecting matter into the surrounding shell. The bright (magnitude 11.3) central star has a temperature that is over 14 times hotter than the Sun and 540 times more luminous. It is a hot sub-dwarf with only one-tenth the Sun’s diameter and a bit over half of its mass.

Despite all these unusual characteristics, the object’s green hue, caused by oxygen excited by the hot central star, marks NGC 1360 as a planetary nebula. In photographs a faint red smudge to the northeast of the nebula can be seen, probably traces of material ejected before it became a true planetary nebula.

NGC 1360 is not only bright it is huge. While many planetaries measure arc-seconds across, this one spans nearly 11 arc-minutes (major axis), making its apparent size five times larger than M57, the Ring Nebula. Its size and brightness make it a fairly easy object to find. Just don’t let its location in Fornax put you off as being too far south. The planetary is no farther south than the globular cluster M4 near Antares.

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Observations of the NGC 1360 have been recorded with apertures as small as 2.2”. Of course your mileage will vary based on your sky conditions. Increasing aperture begins to show the full glory of this planetary. It responds quite well to an O-III filter, which brings out an incredible amount of extremely subtle detail. By the way, you are excused if you begin to think the planetary nebula looks like a spiral galaxy.

How many of you have viewed this huge, diffuse, green football-shaped planetary nebula? Better hurry before it totally dissolves into the interstellar medium! :)

“GIVE IT A GO AND LET US KNOW”
GOOD LUCK AND GREAT VIEWING!


P.S. Curiously the second edition of Star Atlas 2000.0 doesn't plot NGC 1360

My latest observation of NGC 1360 is from a few years ago but on what was an excellent night for transparency and darkness at Steens Mountain. My notes:

"This is a huge oval planetary that first looks like a complete oval, and then with a longer gaze comes across more like a big fuzzy "C" - but it's still a complete oval too, which is really shown by using the UHC filter. The perimeter of the nebula is somewhat indefinite and the center is rather dark. The central star is really bright so the dark center could be a contrast illusion - pretty cool either way. 253x, 21.85 SQM."

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My original sketch follows my written description better, and although I don't mention any color, I do remember a distinct greenish-blue hue at low power with my 28 inch. I seldom think to observe this gem, but then it's up during the worst of the winter weather here in Oregon so I don't get many chances.

I found three entries in my list:

4", 44x, NELM 6m5+
bright and flashy object, CS easy direct vision, 1:2 elongated, one side something fainter

16", 129x, [OIII], NELM 7m+
bright wonderful egg around the bright CS, SE side a little bit fainter
Attachment 1512

27", 172x, NELM 7m+
bright and large, 1:2 elongated around bright CS, darker region around CS which is visible to the SW, 1.7' SW of the CS another very faint star is visible within the nebula

This object surprised me first time I took a look. It is as has been said here 'a wonderful egg'. The second time I saw this one I was very pleased that the darker drop-out to the NNE can be detected. This PN responds best with my Lumicon OIII offering a better view than my 'goto' NPB filter.

Here is an 18" 243x observation from a 21.3 SQM type of site (so the OIII helps greatly over unfiltered).

1/4fov[8.2'] 1.8EL N-S slight EL NPB filter The even round glow is about 1/5fov[6.6'] but with some glow extends as a 'nob' off the south side. Central star is apparent with NPB but no other stellar points seen in the glow with NPB. Possible mottled look on the NNE end. Using OIII filter central star still easy. Extension to the south stands out better with the OIII filter and still 1/4fov[8.2'] but maybe as must as 2.0EL in the OIII. NNE darker dropout before the edge gets brighter again is easier to see in OIII. West is at 9pm'

I had a look at this nebula this morning around 04:45 CEST = 03:45 UT with my 12" from a SQM=20.7 observing site near my home. WOW this nebula is big! Even at 48x already it fills a considerable part of the FOV. I also tried 88x with UHC and OIII filters, but I didn't see more than a large ellipse with a central star. The nebula already was moving towards the horizon, so I'll give it another try from a darker location with my 20" when it is higher in the sky.