NGC 6826 - Caldwell 15 - The Blinking Nebula

General Notes

Planetary nebulae are the final stage of middle to low mass stars. They are essentially gigantic shells of gas surrounding the nucleus of a dying, progenitor star and one of the best examples of this kind is perhaps NGC 6826.

On a cosmic scale, planetary nebulae are considered relatively ephemeral phenomena, lasting anything between 30 to 100 thousand years from formation to complete dissipation. It is possible that our own sun will become a planetary nebula after exhausting its own supply of fuel in about 5,000 million years time.

It is believed that about half the star's mass of NGC 6826 has already been ejected to form the nebula. This disintegration of the dying star causes cosmic wind of newer particles to collide with older material before it forming that hot shell we see from Earth. Planetary nebula have quite a high surface brightness, so are generally easy to find in urban domains and with a small telescope can often be mistaken for an unfocused star.

NGC 6826 is often referred to as the Blinking Nebula yet the name does not refer to the nebula itself but more to the nature of human vision. Our eyes have their least sensitive light spotting cells in the centre of the fovea. Thus, staring at this object makes it appear to dim or even disappear. If you look away using averted vision, it comes on again and seems brighter. So, playing around with looking-at, looking-away, the nebula appears to blink.

NGC 6826 is about 2,200 light years away in the Cygnus constellation. It is located south of the spectacular double star 16 Cygni (not included in this sketch). At about 35x it appears like a blurry, unfocused star and at about 50x is easily resolved into a curious cloud-like object.

Observations from the City & Sketch

It took a couple of nights to discover the nebula, due in part to a misreading of the star atlas, finding myself lost in a maze of double star systems and relatively bad seeing conditions. I have read that NGC 6828 can sometimes appear with a dim, neon green of grayish-blue hue, but I think one needs larger aperture and darker skies to draw out these features and will be absolutely necessary to spot the nebula's dying central star.

Once sighted at about 35x, it appears as an unfocused star and with a little more magnification it takes on a cloud like appearance with quite a significant amount of internal brightness. I found it useful to use a low band nebula filter. In my own case an UHC-S but an OIII will also suffice. When trying to identify an object like NGC 6826 in an urban setting, you ought to repeatt observations with and without the filter, increasing magnification as you go. With the filter in place, stars become less apparent but the nebula gains a higher level of contrast enhancement. It was useful to follow through this process right up to about 140x.

I found that with direct vision the nebula's brightness was clearly reduced and oftentimes would disappear into the darkness. With averted vision it would come alive again, almost as if some playful being was transmitting a slow motion light-code for us to decipher. The Cheeky Winker I noted in my sketch pad, understanding that its message will probably remain a mystery for all time.

The sketch was conducted without the filter in place. I think the rather pretty double star at the top of the sketch is 61 Cygni.