Object of the Week August 9th, 2015 - NGC7008 The Fetus Nebula

[Dragan]

NGC 7008
PK 93+5.2

Cygnus

RA 21 00 33.1
DEC 54 32 32

Type: Planetary Nebula
Size: 86.0"

Magnitude: 12
Central Star Mag: 13.2

A favorite target of mine during the summer months, I'm actually a little surprised that not only NGC7008 hasn't been selected in the past as an OOTW but it hasn't even been discussed here at DeepSkyForum!

2800 light years distant towards the Swan, NGC 7008 was discovered by William Herschel in 1787. The Fetus Nebula, an alias given it by an amateur astronomer Eric Honeycutt in the 2001 summer edition of Amateur Astronomy Magazine, is not like your typical planetary. In fact, it's so atypical Herschel didn't recognize it as a PN cataloging it in the same category as the Lagoon! Its structure proved to be a catalyst for extensive study, some of which was derived from data collected by the Hubble Space Telescope. NGC7008 is unique in not only its overall shape, but its two separate shells. Where did these shells come from? Could its central star go through to two PN phases? Inquiring minds needed to know.

As best as I can find, there are 3 competing theories into how 7008 became the shape that it is.

1. It's possible that the inner shell is actually expanding faster than the outer shell and it's the overtaking of the outer shell by the inner that is creating its unique shape. To further exacerbate the complex structure, the shells may be interacting with the interstellar medium. Varying densities in the IM may be taking its toll on the shells.

2. The dual shells could have formed by different materials which may have originated by different sources. In other words, NGC7008 may have formed when a close binary system, both similar in age, "died" very close in time to one another. One star may have gone through the asymptotic giant branch (AGB) phase with the other experiencing the same fate not too long afterwards. http://adsabs.harvard.edu/abs/2006A%26A...452..523K

3. The third theory may be that the structure of the PN may have formed due to the dying stars expanding gas shell interacting with planetary material in orbit around the star. This to me was the most interesting. I started to think about this during my research of 7008 and thought to myself this may be the most plausible, especially since PN are typically older stars that may in fact have planetary systems. When I started digging more, I found this fantastic article from 2008 echoing my own thoughts. How cool! This theory would also explain why not all PN are symmetrical and some have the wild shapes that they do.

Some sources online will indicate that a knot, located approximately 20" to eh NW of the central star, is in fact a distinct planetary in and of itself. Unfortunately, it is not as it is nothing but a condensation within NGC7008 itself. http://adsabs.harvard.edu/abs/1983A%26AS...52..399S

NGC7008 is a fantastic object in the eyepiece. Accented with a beautiful gold/blue 18" double star on its southern edge, NGC7008 may be seen in nearly any telescope with sufficient skies. The Fetus Nebula may look as a small uniform glow in scopes around 6" while scopes in the 10"-12" range, observers have noted an oval or kidney been shape with some mottling. North of 20" aperture, 7008 really offers up some tantalizing views! In my 25", you can see exactly why it was give the common name it has. It really does look like its namesake. While observing this object, my mind wanders between two thoughts. One is the ending in 2001: A Space Odyssey. (Don't ask. I just do) And in the other, I tend to see a bright "ear" shape with a brighter region extending from the brighter northern region down to the SW and a darkening in the center on the eastern edge. It's this easterly darkening that likens itself to an "ear canal" to me helping me see a human ear. I also find UHC and OIII filters work best, though I have zero notes mentioning the use of my NPB filter, something I'll try next time I'm out.

So there is this week's OOTW. A great visual PN with some real mystery behind it. Next time you're out, give the 3rd theory above some thought. Imagine what it would be like to be standing on a planet when your parent star decides to give up the goods, provided your civilization survived that far along. Now think about the fact that our own solar system will experience the same fate, colliding and interacting with the expanding shell that was our sun. And who knows. Maybe our sun will be the progenitor of such a unique PN for other civilizations to observe and study.

And as always,

"Give it a go and let us know!
Good luck and great viewing!"

http://apod.nasa.gov/apod/ap080825.html

ngc7008_hagercollab_c800.jpg

[theritz]

Interesting read, thanks! I find 7008 a little hard to starhop to every time I watch it. But it is a really cool PN even in my modest 10" scope.

[Love Cowboy]

Great selection! I loved 7008 when I observed it last year in my 16". The structure was very evident, especially with the OIII filter on. Definitely a uniquely shaped planetary.

But you're right theritz... starhopping to it is tough... right in the middle of a big void

[Paul Alsing]

Dragan, you should see this guy through the 60" at Mt. Wilson, it is spectacular! This is one of the most peculiar planetaries ever, with lots of unusual and interesting details.

That knot you spoke of, on the WNW edge, Kohoutek 4-44 (PK 93+5.1), is not too hard to detect in my own 25", and can probably be detected in smaller telescope, too. It is just another knot, but has its own designation and is therefore a more desirable target, in my view.

This is (or rather, WAS) an object on my own OOTW future to-do list... fortunately, there are lots and lots of other objects that qualify for this branding, I forgive you

[Uwe Glahn]

Nice OOTW Dragan. NGC 7008 is always worth a visit, no matter what aperture.

It is an interesting fact, that science is absolutely uncertain about the formation complex PN structures. I know the discussion with Double Stars and the irregular IM around the PN. But I never heard about planets to create a abnormal PN form. And I don't find any word about that in Kwok's "The Origin and Evolution pf Planetary Nebulae". The story behind your point three sound very interesting but I cannot imagine that a so small Planet system like ours can form such huge structures. But who knows and of course there are some much bigger Planetary Systems than ours out there.

The double shell structure and the knot "Kohoutek 4-4" always were my challenging details within the PN. While I could pick up a dark lane through a brighter knot in the N (the first sign of the double shell) it was always hard to detect the knot in the NW. With bigger aperture >20" it was no problem any more.

In the past I heard a common name for K 4-44 but cannot remember and find any word about this. Something with "... star"??? I'm sure when a common name exist one of you guys knows it.

I only have an older sketch with my former 16" telescope. I think about to sketch the PN with the 27" another time.

16", 250x, UHC, NELM 7m+
NGC7008.jpg

[Ivan Maly]

Very fine write-up, and I find the Space Odyssey and ear references very appropriate. I've read something about the planetary (protoplanetary/"postplanetary"?) material impacting the development of the shells before. In any case, PNs are indeed a very "anthropic" sort of objects. As far as actual observations, here is what I have.

16", 140x, no filter, variable muggy summer conditions at remote dark site (2012). PN in N Cyg near border w Cep. Looks like a small Crescent: there is a bright patch to the N of the central star and a fainter one to the S. They are connected diffusely to the W of the star but not to the E, where there is a dark gap. There is a bright pair of stars to the SE of the central star, on the ESE border of the S patch. A star equal in magnitude to the central star is just SE of the N patch. I previously observed this PN with 7 and 4" (in 2007 and 2009).

[KidOrion]

Agreed about the 2001 reference--absolutely the first thing I think of when I see a photo of this one. Haven't really gotten to observe it under excellent skies, but under mediocre skies it's still impressive. Also agreed about the star-hop to it; it's in a rather blank spot in SA 2000.0, and takes a little more work! I'll have to try it with Interstellarum and the TriAtlas app.

[Love Cowboy]

Very fine write-up, and I find the Space Odyssey and ear references very appropriate. I've read something about the planetary (protoplanetary/"postplanetary"?) material impacting the development of the shells before. In any case, PNs are indeed a very "anthropic" sort of objects. As far as actual observations, here is what I have.

16", 140x, no filter, variable muggy summer conditions at remote dark site (2012). PN in N Cyg near border w Cep. Looks like a small Crescent: there is a bright patch to the N of the central star and a fainter one to the S. They are connected diffusely to the W of the star but not to the E, where there is a dark gap. There is a bright pair of stars to the SE of the central star, on the ESE border of the S patch. A star equal in magnitude to the central star is just SE of the N patch. I previously observed this PN with 7 and 4" (in 2007 and 2009).

This is almost identical to my observation of it, with the same aperture and nearly the same magnification and conditions.

Curious about one thing though... I didn't feel at all certain that one of the embedded stars was the central star. Did you research that perhaps?

[Ivan Maly]

No, this was probably just a descriptive term, not used in the astrophysical sense. Incidentally, the relatively low magnification was an approximation of the one Herschel likely used when this object was discovered.

[Uwe Glahn]

Dragan,

I took your reminder to revisit NGC 7008 last new moon with my 27". Under good but not perfect skies (transparency + seeing) in the Austrian Alps I gave the PN enough time to show all detail what was possible for the sky, telescope and mine. Best view was unfiltered. I tried all filters but the combination of the bright clear PN with all its stars around gave the best picture.

27", 488x, NELM 7m+, Seeing III
NGC7008_27.jpg