Jump to content


- - - - -

Cosmic Challenge: NGC 6886 and NGC 6905

Discuss this article in our forums

Cosmic Challenge: NGC 6886 and NGC 6905

September 2016


Phil Harrington

This month's suggested aperture range

6- to 9.25-inch (15-24cm) telescopes

Last month, I offered up two planetary nebulae for smaller apertures. This month, we again hunt for a pair of planetaries. This time, however, we may need a little more oomph to get the job done.

Above: Summer star map from Star Watch by Phil Harrington.


Above: Finder chart for this month's Cosmic Challenge.

Chart adapted from Cosmic Challenge by Phil Harrington.
Click on the chart to open a printable PDF version in a new window.

Let's begin with NGC 6886, a relatively bright, but very small planetary nebula in eastern Sagitta, the Arrow. To a casual eye glancing through a telescope, it will look just like another field star. And since it is poised along the eastern edge of the summer Milky Way, there are plenty of imposters to weed out before the true challenger is uncovered. 

"Now, wait a minute," you're thinking. Look at those numbers in NGC 6886's data table. The nebula's apparent diameter is listed as 6 arc-seconds. That's small, but certainly resolvable as a disk with less than 100x. Unfortunately, numbers can be misleading, as we know from every automobile advertisement that we read. "Your mileage may vary." And it certainly will here.

NGC 6886 is classified as a 2+3 planetary nebula, a shorthand way for describing NGC 6886 as having a smooth disk surrounded by an irregular shell. This planetary-nebula rating system, called the Vorontsov-Velyaminov scale, was devised by the Russian astrophysicist Boris Vorontsov-Velyaminov (1904-1994). His 6-point system describing the morphology of planetary nebulae is summarized in the table below.

Vorontsov-Velyaminov Scale of Planetary Nebulae Morphologies


Stellar image


Smooth disk (a, brighter toward center; b, uniform brightness; c, traces of a ring structure)


Irregular disk (a, very irregular brightness distribution; b, traces of ring structure)


Ring structure


Irregular form, similar to a diffuse nebula


Anomalous form

Planetary nebulae with more complex structures are characterized by combinations of classes. For instance, NGC 6886 is rated "2+3" for its complex disk morphology.  NGC 6905, below, is rated "3+3."


Photographs taken with the Hubble Space Telescope explain NGC 6886's multipart listing by revealing two "wings'' extending an additional 2 arc-seconds on either side of the nebula's circular inner shell. This brings its total diameter to 6 arc-seconds. Through our backyard telescopes, however, only the inner disk will be apparent, which only measures about 2" across.

Begin your journey toward NGC 6886 from 5th-magnitude Eta Sagittae, marking the pointy tip of the Sagitta Arrow. NGC 6886 lies 1.8° due east of Eta. If you're like me and prefer to let someone else do the work for you, aim at Eta, turn off your telescope's clock drive if it has one, and then sit back and relax for exactly 7 minutes 12 seconds. In that time, Earth's rotation will steer your telescope away from Eta and exactly toward the planetary. With a 50x eyepiece in place, look for a small isosceles triangle made up of three faint stars pointing toward the northeast. The "star" at the triangle's southwestern corner is actually NGC 6886. Can't tell for sure? Increasing magnification four-fold may help, but to confirm which is the planetary, blink the field with a narrowband or, better still, an oxygen-III filter. The nebula's central star, once four times more massive than our Sun, now shines at barely 18th magnitude.

Can you catch the Blue Flash? That's the nickname of our second challenger, NGC 6905, a 12th-magnitude planetary nebula in the constellation Delphinus. John Mallas, a prolific deep-sky observer in the 1950s-1970s, bestowed the epithet in an article he penned entitled "Visual Atlas of Planetary Nebulae" in the July/August 1963 issue of Review of Popular Astronomy magazine. That was 181 years after NGC 6905 was discovered by William Herschel.

Unlike NGC 6886, the test posed by NGC 6905 does not stem from its small size. Quite the contrary. NGC 6905's inner shell measures 42"x84", large as planetaries go; and its outermost edge is nearly double that diameter.

No, the challenge presented by the Blue Flash is in the hunt. NGC 6905 is in northwestern Delphinus, where nary a naked-eye star is found. Admittedly, that means little if you are using a GoTo telescope. Just punch "N-G-C-6-9-0-5" into the hand controller and off it goes in a whiz. If that's your preferred method, then that's fine. NGC 6905 should be easy to snag once aimed at the right field. But if you'd prefer the challenge of the quest, then follow me.

NGC 6886 lies halfway between Eta and NGC 6905. Keep NGC 6886 centered as you switch to a low-power eyepiece, and then carefully head due east. As a reference, you will pass an 8th-magnitude star in about 40'. Keep going another 1½° until you spot a crooked pentagon of five 7th- and 8th-magnitude stars. Can you see a tight triangle of 11th- and 12th-magnitude stars just north of the pentagon's center? NGC 6905 will look like a bluish disk along the western side of that tiny triangle.

If you try and try again without success, use the same approach to find NGC 6905 as we did NGC 6886. That is, let Earth be your GoTo telescope mount. Center on Eta, turn off the tracking motor, and set a spell. As before, in 7 minutes 12 seconds, NGC 6886 will be passing through the field, but sit tight and let more time pass. Since NGC 6905 is also due east of Eta, the Earth will turn you toward it in due time -- 16 minutes to be precise.

Once it is in view, the nebula's pale blue-green color should help to set it apart from the crowd of field stars. At 150x, my 8-inch exposes a slightly oval disk elongated north-south. The cloud's ellipticity is confirmed in photographs, which show two "wings" extending away from the bright central core. Spotting the 14th-magnitude central star is just possible through 8- and 9.25-inch scopes under ideal conditions.  The eyepiece rendering below was made through my 18-inch reflector.

Have a favorite challenge object of your own?  I'd love to hear about it, as well as how you did with this month's test.  Contact me through my web site or post to this e-column's discussion forum.

Remember, half of the fun is the thrill of the chase.  Game on!

About the Author:

Phil Harrington writes the monthly Binocular Universe column in Astronomy magazine and is the author of 9 books on astronomy.  Visit his web site at www.philharrington.net to learn more.

Phil Harrington's Cosmic Challenge is copyright 2016 by Philip S. Harrington.  All rights reserved.  No reproduction, in whole or in part, beyond single copies for use by an individual, is permitted without written permission of the copyright holder.


  • John O'Hara likes this


Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics