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| IC 1396.... |
Constellation: Cepheus
Distance: 2,450 light years
Equipment: Atik 460/EFW 2, Samyang 135mm lens@ F2, Vixen GPDX mount, guiding with Lodestar X2/PHD
Date: July 18th. 2021
Subframes: 12 x 300s for Ha, SII and OIII each, no flats, no darks (hot pixel removal in Astroart).
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| Stellarium map showing image field of view |
A distinct feature of IC 1396 is the radial arrangement of several bright rimmed globules that form a loose and slowly expanding ring around the illuminating stars. The ring of loosely arranged dark globules has a radius of about 40 light years with HD206267 at its centre. Although several of the globules are optically conspicuous, the most prominent is catalogued as IC 1396A, nicknamed the “Elephant’s Trunk” (previously imaged here). IC 1396A contains the well known reflection nebula vdB 142. Low and intermediate mass stars appear to be actively forming within the globules. The star formation within the globules has been induced by a process known as "radiation driven implosion" where the ultraviolet flux from a massive star like HD206267 compresses the cold molecular gas within the globules, thus triggering collapse of the cloud and subsequent formation of lower mass stars.
The history of IC 1396 suggests a complex interplay of sequentially-triggered star formation and cloud-cloud interactions. An initial burst of star formation occurred between 13 and 18 million years ago and gave rise to the first generation of stars, which includes the existing nearby cluster NGC 7160. The more massive members from that first generation of stars went on to destroy themselves in supernovae explosions and no longer exist today. About 7 to 8 million years ago the effects of the first generation of supernova driven shock fronts and powerful stellar winds from existing stars created a huge 400 light year diameter bubble known as the Cepheus bubble. The expanding bubble compressed and flattened surrounding molecular clouds triggering a second burst of star formation which went on to form the Cepheus OB2 association some 7 million years ago.
Also formed in this second generation of star formation were Trumpler 37 and its dominant star, HD206267 some 4 million years ago. Under the influence of the ionizing radiation field from the new star cluster, the HII cloud IC 1396 and its globules formed some 2 to 3 million years ago. Triggered by the expanding Cepheus bubble, many well known HII regions have formed along its perimeter including IC 1396, Sh2-129, 133, 134, and 140.
As IC 1396 expanded from the stellar winds of HD206267, surviving fragments of molecular clouds in the form of globules formed into an expanding ring around the central exciting star. The third and youngest generation of lower mass stars is currently forming within the dark globules of IC 1396A by the process of radiation driven implosion (see above).
The bright orange star just to the north of IC1396 is the supergiant μ (mu) Cephei. It formed during the first generation of stars which created the Cepheus bubble. It has the distinction of being one of the most luminous stars in our galaxy, emitting 350,000 times the power of our sun. Also known as Herschel's Garnet Star, it is the prototype of μ-Cephei type variables. It is a red supergiant of irregular brightness, varying from about magnitude 3.5 to 5 over a period of two to two and a half years. Its mass is estimated at 15 times that of the Sun, and it is one of the largest stars known (the best part of two billion miles across at maximum size). It is a very cool star (only about 3700°C), so most of its radiation is in the infrared. Its visible radiation is about 40,000 times that of the Sun, and taking into account absorption of its light by interstellar dust and the large proportion of infrared radiation it must give off, its total luminosity must be nearly ten times greater, or about 350,000 solar luminosities. The star is near the end of its life, having already begun to fuse helium into carbon in its core, and within a few million years will expire in a supernova explosion, leaving behind nothing but (most likely) a black hole or (less likely) a neutron star.
The image above is a 1:1:1 RGB compilation of the narrowband data (Ha=R, SII=G, OIII=B). I did do a Starnet version with RGB stars added back in but it looked essentially like the image above, but with muted stars, and I think the star-strewn field looks more natural in this case. There was a bad dose of field rotation in the stacked frames and my polar alignment must have been off, something I will check next time out.
Curiously, the IC 1396 complex itself doesn’t have a popular nickname, often being regarded as merely an extension of its “Elephant’s Trunk” component. I always think it looks like a large figure-of-eight. For some reason, it reminds me of one of those sickly ice-cream lollies from the early Eighties, “Funny Faces”, though the thought of IC 1396 being called the “Funny Face nebula” is too awful to contemplate.
References:
http://annesastronomynews.com/photo-gallery-ii/nebulae-clouds/ic-1396/
http://www.starrywonders.com/ic1396NBsmall.html
https://cseligman.com/text/atlas/ic13a.htm
http://www.robgendlerastropics.com/IC1396text.html
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