Object: M103

Diameter in '
Number of stars
Brightest star (magnitude)
Distance in parsecs (3.26Ly)
: Cassiopeia
: 01 33.4 / +60.39
: 6.0
: 7.4
: 172
: 7.2
: 2493
Details sketch
Date / Time
Observing Location
Seeing / Transparency
Magnification / Field of View '
: 05/09/10 / 23:31
: Landgraaf
: 3 / 4
: Orion Optics UK 350mm
: 12mm Nagler Type 4
: 133 / 37
Observing report
M103 is an easy object. Even at the lowest magnification, 46x and a field of view of 1.5 degrees, the triangle of bright stars that outline M103 are visible. There is also a bright orange star just of the centre.

This attractive open cluster makes a very pleasing object to observe right from the start. It remembers me of a small christmas tree (like the christmas tree cluster in Monoceros). M103 is very well detached from its surroundings. The optimum magnification is 133x which is achieved with the 12mm Nagler T4. The field of view is 37’. It is a small open cluster, and I count about 30+ stars in and around the triangular shape. The variation in brightness is large. There are a few bright stars and a lot of fainter stars, merely on the edge of detection. There is no real central star, but there are a few bright ones that stand out very well. At the northern top of the triangle I see a bright star, that looks a bit bluish-white, and at the north-eastern edge I can see an orange star. The bright star at the north has three very dim companions, that look like an asterism together with the bright star. It resembles a small 4 star hockey stick.

I do not detect any nebulosity or glow of unresolved stars in M103. I see no real empty, starless, areas. In the sketch, I have drawn quite a few of the dimmer field stars, just to get the orientation right. This sketch is far from complete when it comes to field stars. There are so many visible in this patch of the sky, that it is impossible to plot them all. What I did notice, when I touched up and edited my sketch later that night indoors, is that I missed two weaker stars right at the northern tip of M103, which I did see (and sketched) a few years ago through my 8-inch telescope. A strange omission but well, I got the cluster right, and thats what I wanted to sketch in the end.


Image from Voyager by CapellaSoft

M103 is an open cluster that can be observed through the Cassiopeia window, a stretch of the winter milky way that is relatively dust-free. We look from our position right through the Orion spur and the inter-arm gap out to the Perseus arm of the milky way, where we see M103. With a distance of approximately 8,000 lightyears, M103 is the Messier open cluster that lies the most farthest from us.


Image from "where is M13" by Think Astronomy

The stars that looked the most interesting through the eyepiece were the bright bluish-white at the top (north) of the christmas tree, and the orange star on the north-eastern edge. In fact this blue-white star is a triple star Struve 131, which is formed by the bright Magnitude 7.2 blue-white star HD9311 itself, and the two fainter magn. 10.9 companions, that lie directly to the south-southeast, at position angles of 142 resp. 145 degrees. The AB pair are separated by 13.8” and the AC pair by 28.2”. In my 12-inch telescope all three components were visible with direct vision.

The orange star is a M0 cool red giant of Magnitude 8.48, with a temperature of only 3,620 K. As you can see from the table below, the observation of the colours of the two brightest stars is confirmed.

Stacks Image 1712
Atlas der Messier Objekte by Ronald Stoyan
Where is M13 by Think Astronomy
Sky Safari by Southern Stars
Aladin by CDS
The Messier Objects by Stephen James O’Meara
The Washington Double Star Catalog
Sky Vistas by Crossen and Rhemann