C15, Blinking Planetary Nebula in Cygnus

Non-Messier Deep Sky Objects C15, Blinking Planetary Nebula in Cygnus

Target	Type	R.A.	Dec.	Constellation	Magnitude	Size
C15 (NGC 6826)	Planetary Nebula	19h 44m 48s	+50° 31′ 30″	Cygnus	8.9	2.1 arcmin

Overview

It’s a great name, isn’t it? The Blinking Nebula!

It comes from the sense that you can’t see the nebula when you look at it directly, but you can when you look away from it, i.e. when you use averted vision. This effect is a consequence of our eyes’ biology. When we look directly at the object, its central star is too bright for our eyes to see the nebulosity around ti but, when we look away, the periphery of our vision can now see the nebula. So it appears to ‘blink’ on and off.

Although this nebula is bestowed with the ‘blinking’; name, it is not a unique phenomena to this object; other nebuyka can also seem to blink when the central star is bright enough.

William Herschel recorded his observations of this relatively bright object, which is the remains of the central star casting off its outer layers, back in September 1793. This nebula is only 4,000 light-years from us.

NGC 6826, otherwise known as Caldwell 15, has a central star that seems to blink on and off as you observe it directly and with averted vision (source)

In the next section, you’ll learn how to see the blinking nebula in your own eyepiece.

Finder Charts

The first finder chart, which uses SkySafari 6, shows the Summer Triangle asterism, composed of Vega, Altair and Deneb, at 10pm in the middle of June. Stars are shown to magnitude five but the moon and planets are not shown.

We need the star Deneb, because it’s the distinctive cross-shape of the swan constellation, Cygnus, that’s going to help us find C15 (NGC 6826).

[C15-1] Use the Summer Triangle to find Deneb in the constellation of Cygnus. Click for full-screen.

In the chart below, still only showing stars to magnitude 5.0, see how we need to trace one of Cygnus’s ‘wings’ to the magnitude 3.8 star at its end, Iota2 Cygni.

Pulling back from there just a little, locate the slightly fainter, but still easily seen, Theta Cygni, shining at magnitude 4.5.

[C15-2] Use one ‘wing’ of Cygnus to locate the star Theta Cygni. Click for full-screen.

We finally need to locate 16 Cygni, which is a very obvious double star shining at magnitude six, so we do need a magnifying finderscope or binoculars to pinpoint it. Stars on the chart below are shown to magnitude 7.

To help locate it, imagine that Theta and Iota2 Cygni are two points on a right-angled triangle, with the right-angle itself being at Theta Cygni. 16 Cygni, the double star that we need, is the third point in this triangle.

C15/NGC 6826 is inside the same 1° field of view as 16 Cygni.

[C15-3] Use Iota2 and Theta Cygni to track down 16 Cygni and then the nebula. Click for full-screen, or download the pdf below.

With the blinking nebula now in your eyepiece, let’s explore what you should be seeing.

Individual Telescope Views

The following views will help you find C15 in different telescope types by presenting the images as your telescope will show them. Stars are shown to magnitude 11 and each chart includes the double star 16 Cygni.

The first image is with a black sky and white stars, the second picture is the same image but presented in inverse monochrome. Black stars on a white background is often easier to use at the telescope. Each image can be clicked on for a full-screen version.

Upright View – This is what your eyes see unaided and through a reflex or red-dot finderscope

Upside-down view – This is what reflectors and magnifying finders show, and refractors / Cassegrains without a star diagonal

Mirrored View – Refractors and Cassegrain models with a star diagonal show this view

With this nebula now in your eyepiece, what should you expect to see?

Observation

For many DSOs we begin with a wide field of view and low magnification to show as large a star field as possible. With the Blinking Nebula, the downside to this approach is that, at this scale, it’s hard to distinguish the nebula from the stars around it.

To tease it out, first make sure you have the double star 16 Cygni on the correct side of your view so that you know C15 must be also visible (use the relevant eyepiece view chart above to achieve this). Then, use averted vision to get the nebula to reveal itself with a growing/shrinking illusion.

When you’re sure you have it, it’s time to increase magnification.

You’ll see the bubble-like nature of this star and, with careful study, the sense of blue coloration. What’s more obvious is the star at the center of this gigantic gas cloud, shining brighter than magnitude 11.

Use higher magnification still, say in excess of 100x, and you’ll discover the disc of nebulosity feels more like a brighter ring around airy darkness with the star at its center. Now it’s time for the blinking… Look directly at the star and see the outer ring of nebulosity disappear!

In larger telescopes, you might see two slightly brighter patches of light on opposite edges of the nebula disc on its longer axis. These are FLIERs (Fast Low-Ionization Emission Region) and, as yet, these jets of ionised gas are unexplained by scientists!

PDFs for Printing

Each of the star maps above is reproduced as a pdf below. Each star map has a number in [square brackets] beneath it which corresponds to the file number below. If you want image [C15-1], for example, click the ‘download’ button next to it below and you’ll be able to open it as a printable pdf.