Caldwell 105

2020-06-16 C105 NGC 4833 Glob Stack_22frames_3753s

Globular Cluster NGC 4833 in Musca

Magnitude: +6.9
Apparent size:14 arc-min.
Diameter:88 light years.
Distance:22,000 light years.
Date:2020-06-16.
Exposure:22 x 170 sec = 62 minutes.
Field of View:80.6 x 56.4 arc-min; up is 261 degrees E of N.

A southern globular cluster located in the constellation of Musca, just 30° from the South Celestial Pole. The distance to C105 is about 22,000 light years. Looking through the Milky Way disc plane, it is partially obscured by galactic dust.

The very bright star (upper left) is orange giant Delta Muscae, a third magnitude K-type star only 91 light years distant, which has left the main sequence.

The second brightest bright star, near Delta, is blue-white giant LS Muscae, a fifth magnitude B-type variable star about 1,000 light years away. It is part of a multiple star system.

You can see an annotated version of this image provided to me by Astrometry.net – a free automated astrometric calibration service which creates accurate astrometric meta-data for any useful astronomical image.

EDIT: There’s a bright star near the globular cluster (in the 11 o’clock direction) which was not identified by Astrometry.net. I looked on Google Sky and found it was just a pinpoint like all the stars around it.

A variable star!

Checked it on Simbad and it is identified as RZ Muscae. It varies between mag +12.01 and +15.19 over a 333 day period. I just happened to catch it somewhere near its peak.

My gif making skills are a bit ham-fisted but here you go……

2020-06-16 C105 (lower right) with variable star RZ Muscae (upper left) My image x Google Sky image
2020-06-16 C105 (lower right) with variable star RZ Muscae (upper left) My image x Google Sky image

The things you learn by chance…..

Stay safe.

That’s all.



Telescope Details
SkyWatcher Esprit 120 mm apochromatic 3-element refractor.
840 mm focal length @ f/7 with field flattener.
Baader L-Booster UHC-S light pollution filter 2458276.
SkyWatcher EQ6-R Pro mount.
Camera: ZWO ASI 071 MC Pro (CMOS 28.4mm 16 Mpx)
ZWO ASI120 guide camera, using PHD2 software.

2 Comments

    1. Yes, I suppose it could be easy to conclude that. However, the size of a star on astronomical images is a function of how bright it is, with excess light spilling into surrounding pixels. 🙂

      Liked by 1 person

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