Challenge #1 – C46, Hubble’s Variable Nebula in Monoceros
| Target | Type | R. A. | Dec. | Constellation | Magnitude | Size |
| C46 (NGC 2261) | Bright Nebula | 06h 39m 12s | +08° 45′ 00″ | Monoceros | 9.2 | 4.0×2.0 arcmins |
Overview
This ghostly nebula is lit by the star R Monocerotis, which can’t be seen directly. William Herschel discovered the apparition in December 1783, describing it as “Considerably bright and fan shaped.”
You probably won’t be surprised to learn that it has its name for the famous astronomer who discovered that the nebula’s brightness changes over time. However, the period of change is not predictable and varies from weeks to months. It’s believed that the variable density of the dust clouds in the region are the most likely cause for the nebula’s changing levels of illumination, although this is yet to be proved.
The reason we can’t see R Mon. directly is because it’s shrouded in dust. Astronomers have peered through the dust with infrared telescopes and observed a binary star in there that is about 2,500 light-years away from us.
Learn how to find this delicate nebula with your telescope.
Finder Charts
This first sky chart, from SkySafari 6, which shows the sky looking south at 10:00 p.m. in mid-February. The green box marked NGC 2261 is our nebula and you should note its proximity to the great constellation of Orion, which is marked with an orange oval.
Stars on this chart are shown to magnitude 5.0 and the moon and planets are not shown.
We now need the stars 13 and Epsilon Monocerotis, which both shine at magnitude four. Note the location of Betelgeuse in Orion to help find these two.
Draw an imaginary line from Epsilon to 13 Mon. and extend it on, and you’ll arrive at the nebula.
In this third chart, stars are shown to magnitude seven, comfortably visible in binoculars and magnifying finderscopes. Note that our nebula is inside the same field of view as 15 Monocerotis, another magnitude four star as far from 13 Mon. as Epsilon Mon. but in the opposite direction. 15 Mon. is easy to identify thanks to its incredible shade of blue.
Now you have this variable nebula in your eyepiece, what should you expect to see?
Individual Telescope Views
The following views will help you find C46 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 15 Mon. 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
If you’re lucky enough to observe this nebula at its brightest, it presents as a clear stream of nebulosity at low magnification. Remember though, this structure demands a dark sky to be seen well.
When you do spy it, you’ll be forgiven for thinking that you’re looking at the fantail of a comet. At one end is the brightly shining shrouded star, the other, northern end, is the splayed, delicate gossamer fan.
If you have a wider aperture telescope, don’t be afraid to gaze upon this sight with more magnification. Do you see that the sides of the fan appear to be brighter than the rest of it?
Unfortunately, observing during one of its dimmer periods means it’ll be hard – if not impossible – to discover with a small aperture telescope.
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 [C46-1], for example, click the ‘download’ button next to it below and you’ll be able to open it as a printable pdf.