The Ptolemy Cluster (M7) is a bright open cluster near the sting of Scorpius, easy to see with the naked eye and described by Ptolemy in antiquity.
At a glance
| type | open star cluster |
|---|---|
| distance from the Sun | 900 light-years (276 pc) |
| its light left it | 900 years ago |
| size in the piece | about 21 light-years acrossmodel |
Read off the piece’s own catalogue, the same values it draws with. Model marks a property of the piece’s galaxy model rather than a measurement.
About Ptolemy Cluster (M7)
Just above the curled tail of Scorpius, against the dense Milky Way towards the Galactic centre, lies a large, bright knot of stars. Ptolemy recorded it in the Almagest as a nebula following the sting of Scorpius, and it later entered Messier’s catalogue, which is why it carries both names.
Seen through binoculars it resolves into a rich, loose group of blue-white and yellow stars. It is a genuine cluster, its members born together and moving as one, older than very young clusters such as the Pleiades. The Butterfly Cluster (M6) sits close to it on the sky but lies farther away.
What you see in the piece
With HUMAN EYE look at the tail of Scorpius; fly to it and its members stay together while the crowded Milky Way background falls away.
Questions
How far away is the Ptolemy Cluster?
The Ptolemy Cluster (M7) is about 900 light-years (276 pc) from the Sun.
Why is it called the Ptolemy Cluster?
Because Ptolemy recorded it in the Almagest, as a nebula following the sting of Scorpius.