Ptolemy's Cluster
M 7
Scientific overview
Ptolemy's Cluster is an open cluster in the constellation Scorpius. It shines at apparent magnitude 3.3 and spans about 22.2 arcminutes of sky.
Observation guide
Guidance follows standard observing practice, derived from the object's real magnitude and position. A dark, moonless sky always helps.
Location in the sky
Precision measurements
Distances and velocities from SIMBAD · NGC 6475
| Distance | 911.607082 ly | SIMBAD · catalogued |
| Physical size (major axis) | 5.886902 ly | SIMBAD · derived |
| Radial velocity | -14.7 ± 0.2 km/s | SIMBAD · measured |
| Right ascension (ICRS) | 268.447083 deg | SIMBAD · measured |
| Declination (ICRS) | -34.841111 deg | SIMBAD · measured |
Full field-level provenance
- Distance: Centre de données astronomiques de Strasbourg (CDS) · SIMBAD · mesDistance · dist · 2022ApJ...931..156P · catalogued value · source value 279.5 pc · retrieved 2026-07-11
- Physical size (major axis): Centre de données astronomiques de Strasbourg (CDS) · derived (OpenNGC angular size × SIMBAD distance) · physical size = angular size × distance (small-angle); angular size from OpenNGC, distance from SIMBAD mesDistance · derived · retrieved 2026-07-11
- Radial velocity: Centre de données astronomiques de Strasbourg (CDS) · SIMBAD · basic · rvz_radvel · 2017A&A...600A.106C · direct catalogue measurement · retrieved 2026-07-11
- Right ascension (ICRS): Centre de données astronomiques de Strasbourg (CDS) · SIMBAD · basic · ra · 2021A&A...647A..19T · direct catalogue measurement · retrieved 2026-07-11
- Declination (ICRS): Centre de données astronomiques de Strasbourg (CDS) · SIMBAD · basic · dec · 2021A&A...647A..19T · direct catalogue measurement · retrieved 2026-07-11
Physical size is derived from the OpenNGC angular size and the catalogued distance (small-angle: size = angle × distance). This is a Galactic object: its Doppler shift is shown as a radial velocity, not a cosmological redshift. Fields a source did not provide are omitted, never invented.
Other objects in Scorpius
| Object | Type | Mag | Size (′) |
|---|---|---|---|
| NGC 6231C 76 · NGC 6231 | Open cluster | 2.6 | 13.8 |
| IC 4592 | Reflection nebula | 3.9 | 60 |
| Butterfly ClusterM 6 · NGC 6405 | Open cluster | 4.2 | 15.6 |
| IC 4605 | Nebula | 4.7 | 30 |
| M 4M 4 · NGC 6121 | Globular cluster | 5.4 | 28.2 |
| NGC 6281NGC 6281 | Open cluster | 5.4 | 10.2 |
| NGC 6374NGC 6374 | Open cluster | 5.5 | 6.9 |
| NGC 6124C 75 · NGC 6124 | Open cluster | 5.8 | 13.5 |
Sources
The primary and reference sources this topic draws on.
- OpenNGCOpenNGC (M. Verga) — NGC/IC, Messier, and Caldwell catalogue data
Openly-licensed deep-sky catalogue: object types, positions, magnitudes, sizes, morphology, and cross-identifiers.