Challenge #2 – C54, Open Cluster in Monoceros
| Target | Type | R. A. | Dec. | Constellation | Magnitude | Size |
| C54 (NGC 2506) | Open Cluster | 08h 00m 01s | -10° 46′ 00″ | Monoceros | +7.6 | 12 arcmin |
Overview
This cluster of about 100 stars lies nearly 13,000 light-years away from us. William Herschel discovered it in December 1791.
It presents as an elongated, triangular shape and most of its members shine between magnitudes 12 and 14, although there are a few individual stars shining at magnitude 11.
Astronomers estimate the cluster to be about two billion years old, making it one of the oldest that we know of. It’s only small in our telescopes because of its great distance. In reality, the cluster is 37 light-years across, making it larger than M46.
Finder Charts
The first sky chart, from SkySafari 6, shows the view at 10 pm in the middle of March, looking south. Stars are shown to magnitude 5.0 but the moon and planets are not shown.
The brightest, most obvious constellations visible in this direction are Orion and the Sickle asterism in Leo. Monoceros is a lacklustre constellation so use these much more obvious signposts to locate it.
The star in Monoceros that we’re most interested in is Samoht (Alpha Monocerotis) which shines at magnitude four.
To further refine our area of search, use Sirius – the night sky’s brightest star – linked with Muliphein to point us in the right direction. The stars 19 and 20 Puppis are also in the neighborhood and bright enough to be seen with the naked eye under a dark sky.
With stars now shown to magnitude seven, use the 5 Puppis (pointed at by drawing a line from Sirius to Muliphein) as the right angled corner of a triangle that has Samoht and the cluster at its other corners.
We’re now ready to attempt observation of this cluster.
Individual Telescope Views
The following views will help you find C54 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. Each image can be clicked on for a full-screen version. Stars are shown to magnitude 10.0 and the stars 5 Puppis and Samoht are shown in each image.
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
At lower magnifications, this cluster appears very tight and it’s tricky to make out any particular detail.
As your magnification increases, so will the number of stars you can see as discreet dots of light. The brighter pairs of 11th magnitude stars become obvious and strings of fainter stars, perhaps 20 or 30, come into focus.
It’s very easy to imagine strings of stars fleeing from the core of the cluster which remains shrouded in the fog of stars to faint to be conjured to individuality.
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 [C54-1], for example, click the ‘download’ button next to it below and you’ll be able to open it as a printable pdf.