Challenge #1 – C22, The Blue Snowball Nebula in Andromeda
| Target | Type | R. A. | Dec. | Constellation | Magnitude | Size |
| C22 (NGC 7662) | Planetary Nebula | 23h 25m 54s | +42° 32′ 06″ | Andromeda | 8.3 | 0.5×0.5 arcmins |
Overview
As object names go, they don’t get much more appropriate that this.
What we’re looking at is a planetary nebula that’s a great object for backyard astronomers to track down. First discovered in October 1784 by William Herschel, it’s described in the New General Catalog as “A magnificent planetary or annular nebula, very bright, pretty small, round, blue, variable nucleus.”
Planetary nebulae are born from red giants – stars nearing the end of their life – throwing off their outer shells of gas in a violent explosion. We see the expanding ring(s) of gas now circling the inner remnants of the star.
We can see two shells in the image below, an inner and outer, but there is a third, circular halo surrounding both of these. From their rate of expansion, scientists estimate this object to be just over 3,000 years old. Its distance is not well established but believed to be around 5,700 light-years away.
The central star is more than 5,000 times more luminous than the sun but less than thwo-thirds of its mass.
Finder Charts
This first sky chart, from SkySafari 6, which shows the sky looking northwest at 10:00 p.m. in mid-December. Dauntingly, it also shows the nebula C22 (NGC 7662) in the middle of a part of the sky barren of bright stars.
Fear not, this nebula is condensed enough to pick up and is near a readily-identifiable star.
Stars on this chart are shown to magnitude 5.0 and the moon and planets are not shown. On it, I’ve circled the obvious constellations of Cassiopeia and the Great Square of Pegasus.
We now need to find this little asterism of four stars that I’ve ringed with an orange oval. The three brightest are comfortably visible to the naked eye (all are brighter than magnitude five) and we need to locate Iota Andromedae, shining at magnitude 4.3.
Iota And. is in the east, diagonally opposite is the star Omicron And, which is brighter at magnitude 3.6. An imaginary line connecting both of these passes close by C22.
In this third chart, stars are shown to magnitude seven, comfortably visible in binoculars and magnifying finderscopes. Find 13 Andromedae, a magnitude 5.8 star. This is om the same 1.5° field of view as C22.
Now you have this snowball in your eyepiece, what should you expect to see?
Individual Telescope Views
The following views will help you find C22 in different telescope types by presenting the images as your telescope will show them. 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. Stars are shown to magnitude 10.0 and 13 And. is marked in each chart, along with a circle showing a 1.5° field of view.
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
Observation
Despite its intrinsic brightness, the magnitude 13 central star at the heart of this nebula is not visible in a regular backyard telescope. However, what we can readily see is the blueish disc of the nebula itself.
When first sweeping the area on a low magnification, the Blue Snowball appears star-like and doesn’t especially jump out. If you use averted vision, then it will reveal itself as being a distinctly different object from the surrounding stars. Switch to a higher magnification and its small, delicate disc is revealed. careful examination shows the center is brighter and there’s a distinct boundary between inner and outer zones.
Additional power will show you that the core brightness actually splits into two denser regions on the northwest and southeast. With a larger telescope and stronger magnification, you will see an light-blue hue to the light from it.
In all cases (except the lowest magnification) notice that the nebula is oval, rather than circular. Use a narrowband filter to extract better contrast.
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 [C22-1], for example, click the ‘download’ button next to it below and you’ll be able to open it as a printable pdf.