Globular clusters in the Andromeda Galaxy

 tl;dr - Since our big telescope is away for maintenance, we are currently working with a small refractor. On 3 October 2026, we took advantage of its short focal length to take a deep-sky image of the Andromeda Galaxy and searched it for globular clusters. Using an online catalogue we were able to identify almost 500 of them.

In August, during the school holidays, our big Ritchey-Chrétien (RC) telescope left us for a well-deserved break. Its journey south brought it close to Venice, back to its manufacturer's workshop, where it will spend several months for overdue maintenance work. However, we had prepared for its absence. We replaced it with a small yet beautiful refractor, which now sits on our massive GM2000QCI mount – admittedly, quite an odd couple.

The refractor's aperture of only 106 mm means that over the coming months to come we will have to cope with 16 times less light-gathering power. On the other hand, we will benefit from a shorter focal length and thus a significantly wider field of view. With our full-frame camera, we get a FOV of more than 10 square degrees with a resolution of 1.46 arcseconds per pixel. That spans seven full moons on the long side of the sensor.

With our big RC several celestial objects were unsuitable imaging targets simply because of their sheer size. Now that has changed, and we are eager to take a look at some of the crown jewels of the northern night sky. One of these is undoubtedly the famous Andromeda Galaxy, M31.

Lili and Tommy at the observatory.

When sweeping through the autumn night sky, a pit stop at the Andromeda Galaxy is a must. Starting at Alpheratz and taking a sharp right at Mirach, the distant island universe can easily be discerned as a dim patch of light. Knowing that this light left its point of origin two and a half million years ago … that's something!

The refractor pointing at its target.

On the evening of 3 October 2026, we were not only interested in taking a pretty picture of M31, we also wanted to give the endeavour a slight scientific touch.

The Milky Way contains a wide variety of interesting objects: stars and planets, compact objects such as white dwarfs, neutron stars, and black holes, gas and dust clouds, planetary nebulae … It is also home to about 160 globular clusters. These compact balls full of stars are among the oldest objects in the universe. They do not follow the usual disk's rotation but orbit our Galaxy on inclined planes, very much like a swarm of bees. Other galaxies, of course, have them too.

Globular cluster M13, Dominik Moser, 2019

While collecting „ancient“ Andromeda photons we started a search for globular clusters right away in the very first subframe. And yes, they were there! While panning and zooming we could match several wee and inconspicuous dots of light to a comparison image we found on the web, which had dozens of globular clusters marked.

A few days later, we met at school to finish the job. We stacked our twenty 5-min subs to produce a colour image with a total exposure time of 1 hour and 40 min. We plate-solved it in order to have celestial coordinates, then we used an AI-generated subset of the Revised Bologna Catalogue of M31 Clusters and Candidates to annotate it.

Small version of our final image of M31. Follow the link in the article to get to the original version.

The result is not only impressive to look at. We also noticed an intriguing deviation from the expected spherically symmetric distribution of the clusters. A quick search revealed that, unlike the Milky Way, M31 also features a disk population of globular clusters – a fact that is still under investigation by professional astronomers.

In this crop of the annotated image, the concentration of the clusters along the spiral arms is evident.

Link to the full size images of M31. Click to see the annotated image. 

https://www.wiedemair.com/M31_GC/m31_image_viewer.html

Christof Wiedemair