Great Star Cluster in Hercules
About this image
This is M13, the Great Globular Cluster in Hercules, one of the finest globular clusters visible from the northern hemisphere. Unlike a galaxy, M13 is a huge ancient ball of stars bound together by gravity, sitting in the halo of our own Milky Way.
M13 is about 25,000 light-years away and contains over 100,000 stars, packed into a region roughly 145 light-years across. In the centre, the stars are incredibly crowded, with the stellar density around a hundred times greater than the neighbourhood around our own Sun. This is why the core appears so bright and almost granular in the image.
The cluster was first recorded by Edmond Halley in 1714, and later added to Charles Messier’s catalogue in 1764. Under very dark skies it can even be seen with the naked eye as a faint fuzzy patch, but through a telescope or long-exposure image it begins to resolve into a rich swarm of individual stars.
What I like about this annotated version is that it also shows how deep the image goes. In the same field there are tiny background galaxies, far beyond the stars of M13. The small object marked IC 4617 is a faint spiral galaxy in Hercules. It is only really visible in long-exposure photography, with a blue magnitude around 16, and appears less than one arcminute across in the sky.
There is also PGC 2074909 near the lower-left of the frame. This is another very faint background galaxy, appearing as a tiny smudge compared with the bright stars of M13. These galaxies are not part of the cluster; they just happen to lie in the same line of sight. That is one of the amazing things about astrophotography: in one image you can capture stars in our own galaxy, an ancient globular cluster, and tiny galaxies sitting far deeper in the universe.
Annotated image
Image details
| Object type | globular star cluster |
|---|---|
| Catalogue | M13, NGC 6205, M 13 – NGC 6205 – MEL 150 |
| Constellation | Hercules (Her) |
| Observed | 2026 Jul 03 – 23:40 UTC |
| Location | Portsmouth, UK |
| Moon | Waxing Gibbous – 95% illuminated – age 12.6 days |
| WordPress categories | Not selected |
Equipment
| Telescope | Skywatcher 127 Mak |
|---|---|
| Camera | ZWO ASI2600MC Pro |
| Mount | Juwei 17 |
| Filter | None |
| Focal length | 1500mm |
| F-ratio | f/11 |
| Pixel scale | 0.52 arcsec/pixel |
Acquisition
| Exposure | 240 x 30s |
|---|---|
| Gain | 100 |
| Guiding RMS | 0.6 -1.00 |
Observation notes
Warm evening. Last week has been very hot, breaking all records. This is my second experiment using this setup. RA and DEC agression 35%. FORGOT TO DITHER!
Processing notes
Stacked in AstroPixelProcessor. Processed in PixInsight. Lack of dither has given me a lot of walking noise
Posted from AstroLog.