Object Of The Week September 13, 2020 – NGC 7293, The Helix Nebula

[wvreeven]

NGC 7293, Caldwell 63, The Helix Nebula

Planetary Nebula

Constellation: Aquarius

RA: 22 29 38.5
DEC: -20 50 13.7

Mag: 7.6

Size: 13.4'

3998
Image credit: http://www.capella-observatory.com/ImageHTMLs/PNs/HelixDeep.htm

This nebula has been discussed a few times on this forum but it never was Object Of The Week. It is such a well known object with a wealth of details visible that it deserves to be, though!

This nebula was discovered by Karl Ludwig Harding and it is unclear when he actually discovered it. Wikipedia says "probably before 1824", Wolfgang Steinicke says "probably 1824". I think that Wolfgang's text is missing the word "before" because in the table on the same page he mentions "1823, 9?". See

http://www.klima-luft.de/steinicke/ngcic/persons/harding.htm

Harding was working at the Göttingen observatory at the time of this discovery and before that at Johann Schröter's observatory in Lilienthal where he discovered the asteroid (3) Juno. Harding also discovered the variable stars R Virginis, R Aquarii, R Serpentis and S Serpentis and he published a star catalog containing 120,000 stars. Harding's successor at the observatory in Lilienthal was Friedrich Wilhelm Bessel whom we all of course remember for measuring the first annual parallax of a star.

Of all the bright planetary nebulae, the Helix Nebula is one of the closest to Earth. According to Gaia data, the distance is a mere 655 +/- 13 light years. It spans about 0.8 parsecs or 2.5 light years and is estimated to be 10,600 (give 2300 or take 1200) years old. The age was determined based on the expansion rate of some 31 km/s. For more fascinating facts see

https://en.wikipedia.org/wiki/Helix_Nebula

Visually this nebula is quite challenging and requires truly dark skies. The reason of course is the low surface brightness despite its pretty large total brightness. Many starting observers choke on the nebula because they don't expect such a low surface brightness because this nebula is so famous. I remember having troubles seeing it when I first started observing with my 15 cm telescope in the early nineties and was quite disappointed when I finally managed to see it. This was from the Netherlands though, where the nebula doesn't rise very high above the horizon.

Later I observed it again with my 15 cm telescope from southern France and my notes read

"This nebula already is visible in the 50 mm finder. At 64 x I see a large, faint sphere that has a darker inner region. An OIII filter helps a LOT."

I did observe it as well with my 20" telescope but sadly I didn't log any observation with it. So my own article is a good excuse to revisit this nebula :D Hopefully I will be able to do this soon from Chile, where the nebula passes very high in the sky.

I'd like to point at a discussion about the very faint outer details of this nebula that took place on this very forum way back in 2012. See

http://www.deepskyforum.com/showthread.php?262-The-Geometry-of-the-Helix-Faint-Halo-Struktures-in-NGC-7293

and don't forget to check out Johannes' link to this very interesting and challenging observing project.


As always,

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

[Raul Leon]

Hi here's my observation from Chiefland Florida 11/3/2018: Ngc 7293 planetary nebula in Aquarius ; magnitude:7.3 ; size: 13.4' ; large but low surface brightness, needs OIII filter for best view ; dark inner region with brighter sections opposite of each other. Many stars superimposed over nebula. Did not notice outer regions, must revisit this one soon. I used a 21mm Ethos at 75x with my 14.5 Starstructure Dob f/4.3 3999

[Bertrand Laville]

Hi All,

Here is how I saw the NP with a 20" from Namibia, and a 25" from Southern Alps.
More details here: http://www.deepsky-drawings.com/ngc-7293/dsdlang/fr

Clear skies
Bertrand

4000

[Steve Gottlieb]

I'm curious if any on this list has seen the Helix Nebula naked-eye? Although this may seem to be a highly implausible feat, here's part of an online article by Sue French in 2006 (https://skyandtelescope.org/observing/celestial-objects-to-watch/hunting-down-the-helix/) on the Sky & Tel website.


"Amazingly enough, Michael Bakich writes, "From our dark-sky site 50 miles east of El Paso, three of us saw the Helix with the naked eye on August 26, 2000." And in Western Australia, where the Helix passes almost overhead, Maurice Clark and his friends have used its naked-eye visibility as a guide to sky conditions."

Here's Maurice Clark's post (I copied it years ago from AMASTRO, I believe):


"Actually it is not at all difficult to observe the Helix naked eye, especially from Australia where it rises higher in the sky. I have always been able to see it naked eye when the skies were better than LM 6.8. No OIII filter required. If fact, my friends and I used to use its naked eye visibility as a guide to the sky conditions. If the Helix was not visible to the naked eye, then the skies were not good."

The smallest aperture I've seen it in is 50mm binoculars. But what about others?

[wvreeven]

Interesting Steve. Thanks for sharing that. I’ll need to try from a dark location in Chile when I get the chance. For me, too, 50 mm (my finder) was the smallest aperture with which I have seen the Helix Nebula.

[Uwe Glahn]

I could catch up the object up to a 8x30 binocular under dark skies. Impressive to see it in just a small instrument.

In the 16-inch the Helix-structure became visible, although it is not prominent in the [OIII] line.
sketch: 5" binocular, no filter, NELM 6m5+
4009
home (http://www.deepsky-visuell.de/Zeichnungen/NGC7293.htm)

sketch: 16", 100x, [OIII], 7m0+
4008
home (http://www.deepsky-visuell.de/Zeichnungen/NGC7293_16.htm)

As a visual challenge you can try the faint galaxy 2MASX J22290968-2047179 at the NW border. With the 27-inch is was popping in and out of view.

As you mentioned the halo-structure, together with an very experienced observing friend I finally tried the brightest arc with a 36-inch under very good transparency at a High Alpine place (47°N) and we both could not see the arc.

[oliva]

Hi,

I also could spot the nebula with an 10x50 binocular and doing that I would say the object is visible even with smaller apertures.

I observed the Helix-Nebula several times from Namibia were it passes the zenith. Last attempt was two weeks ago from now but I was disappointed because of the bad conditions with slight Calima and low hight that the object reaches compared to southern skies.

What I noticed during my observation is that the helix structure is not so obvious because even in a 25 inch telescope a minimum magnification of 178 is needed using an OIII-Filter. With a 24 inch binocular Newtonian and OIII the nebula becomes really bright and the inner part becomes slightly “fibrous“. The outer halo structure I also spot but I was surprised that is wasn’t visible with the 25 (Mono) Newton several days later under same conditions. Below you find a sketch using the mentioned 24 inch Bino Dobson (with an OIII).

Oliver

4020

[wvreeven]

For those who haven't seen it yet, APOD of 24 November 2020 features a very deep image of this nebula taken with CFHT:

https://apod.nasa.gov/apod/ap201124.html