Challenge #1 – C28, Open Cluster in Andromeda
| Target | Type | R. A. | Dec. | Constellation | Magnitude | Size |
| C28 (NGC 752) | Open Cluster | 01h 57m 41s | +37° 47′ 00″ | Andromeda | +5.7 | 75 arcmins |
Overview
It is Caroline Herschel who’s credited with discovering this object in 1783 although there is evidence that Batista Hodierna recorded it before 1654.
Spanning over one degree of the night sky, this loose cluster of around 60 stars shines bright enough to be seen with the naked eye in very dark skies, even though no individual member is brighter than magnitude nine. As you might expect, it is easy to see with binoculars and looks lovely in a telescope eyepiece.
Beyond the naked eye, astronomers have identified over 300 stars that are gravitationally bound to this cluster.
Thought to be around 1.3 billion years old, this cluster lies 1,300 light-years away from us.
Finder Charts
The first sky chart, from SkySafari 6, shows the view looking south at 10 pm in the middle of January. The moon and planets are not shown and stars are only shown to magnitude 5.0.
Cassiopeia and the Great Square of Pegasus are the two most obvious constellations visible when we look northwest at this time and they’re circled orange in the first chart. We need to identify the two ‘legs’ of stars extending from one corner of the Great Square, which, together, form the constellation of Andromeda.
The star Alpheratz, on the corner of the Great Square, is the starting point for Andromeda. Of the two ‘legs’ extending out from it, we want to trace the southeastern one to its end, where we’ll find the bright star Almach, as shown in the star chart below.
We should also pick out the small constellation of Triangulum and its brightest star Beta Trianguli, shining at an easy-to-see magnitude of 3.0.
This third and final finder chart shows stars to magnitude seven. The cluster, C28, is located inside the green circle which you can see is roughly midway between Beta Trianguli and Almach. The star 58 Andromedae will also help you locate the cluster.
Remember, the cluster is bright enough to be seen with the naked eye and will be easily picked up by binoculars or a magnifying finderscope when you’re pointed in the right location.
Now you’ve found this delicate cluster, what should you expect to see?
Individual Telescope Views
The following views will help you find C28 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 11.0 and star Almach is visible in each view.
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
A decent pair of binoculars reveal a few dozen of the stars in this spread cluster, covering about one degree. Once you switch to even a small telescope, you’ll see dozens more, perhaps 80 or 90 altogether. It’s hard to be sure how many belong to the cluster because the lack of a concentrated core makes identifying its edges difficult.
What is hard to see is the beauty of these myriad diamonds of light filling the eyepiece. See how your eye is drawn to identify crossing chains of stars, as if they were designed in drawn lines. You’ll also discovere many apparent pairs of stars in the mix, as well as the golden color of the brightest, shining at magnitude nine.
Talking of color, take your time to spy the various hues and shades of the stars in C28. It’s because of C28’s vast age that we see so much variety: many of the stars have lived long enough to evolve into red giants.
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 [C28-1], for example, click the ‘download’ button next to it below and you’ll be able to open it as a printable pdf.