An unusual cometary globule in IC 1396.
Credit: Bernard Miller.
The Dark Tower in Scorpius. Credit: Martin Pugh/Rocco Sung.
The full size image of today's APOD, the unusual globule in IC 1396, is stunning!
I'm showing you an image of another cometary globule, the Dark Tower in Scorpius, to point out the characteristic similarities between these globules. They are dark dusty elongated shapes with bright rims, and they are particularly bright at their "heads", just like comets. They are bombarded with ultraviolet light and stellar winds from hot stars, which makes their "heads" and sometimes their sides glow in red hydrogen alpha light, as hydrogen in their outermost parts facing the hot star(s) is being ionized.
(And stars may form in the heads of these globules
, as is the case with the globule in IC 1396 in today's APOD.)
Structures very similar to cometary globules are seen in planetary nebulas, but there they can be much more numerous:
Cometary globules and finger-like structures in the dusty shell surrounding central stars in planetary nebulas are formed because the evaporating dusty shell around any newborn hot star (and central stars of a planetary nebulas are "newborn" in their current form) is uneven in thickness. The thinnest parts evaporate away quickly, while the thickest clumps remain. These clumps shield the area immediately below them, creating long tails of dust below the cometary "head".
Finally, I can't resist showing you a stunning RGB+Hα image of more of IC 1396, where you can see the central hot blue star as well as many ghostly dusty shapes inside this fascinating nebula:
IC 1396. Credit: Pradhu Astrophotography.
I picked the Pradhu Astrophotography picture because the ionizing central star, O6V-type HD 206267, looks so blue here. You knew I would.
And the image looks just ghostly in a way that is very attractive, if you ask me.
The POSS-II/T. Masterson picture doesn't show us that the central star is blue, but it shows us nebula IC 1396 in its entirety. Also it's an RGB image, probably with added Hα. I will not post Hubble palette images if I can find good RGB image substitutes.
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