M21 – Open Cluster NGC6531
The open cluster M21 is a young group of stars formed only about 4.6 million years ago. It contains dozens of very hot, bright blue and white spectral-type B stars, as well as several more evolved giants. It lies very close to the M20 nebula but is not physically connected to it. It has no associated nebulosity because most of its gas and dust has already been dispersed by stellar winds. It stands out for its compactness and strong contrast against the Milky Way's stellar background. It is approximately 4,250 light-years from Earth, has an apparent size of 13.0 arcminutes and a visual magnitude of +6.5.
General information
- Constellation
- Sagittarius
- Category
- Clusters and stars
- Date
- 25 June 2025
Capture
- Total time
- 6h 50min
- Subframes
- 300s (Ha: 42x + OIII: 40x) bin1
- Gain / Offset
- 125 / 30
- Calibration
- N/A
Equipment
- Telescope
- TS102 f/5.6
- Camera
- ASI294MM at -10ºC
- Filters
- Ha and OIII, 6 nm Astronomik
- Mount
- Sky Watcher Wave 150i
- Guiding
- Guide scope 60mm f/4 + ASI220MM with PHD2
- Focuser
- PegasusAstro PPBMicro
- Filter wheel
- ZWO EFW 7pos
Processing and environment
- Processing
- PixInsight 1.9-3
- Acquisition
- NINA
- Location
- l'Ampolla
- Sky
- Bortle 4