Messier 7 – Ptolemy Cluster

Claudius Ptolemaeus Pelusiniensis (Ptolemy) was a Greek philosopher and astronomer who lived in Alexandria, Egypt in the first and second centuries. Among other things, he identified 48 constellations we still use today. He identified the open star cluster that would later become Messier 7 (M7) around 130 A.D.

M7 is roughly 980 light years away from Earth, contains about 80 stars, and likely came into being about 220 million years ago. M7 is the most southerly Messier object and spans approximately 20 light years.

The New Horizons space probe used M7 to calibrate its camera to prepare for its arrival at Pluto and Charon:

Open star clusters (or “open clusters”) are collections of stars—perhaps a few dozen to a few thousand—that likely formed in the same molecular cloud, but are not gravitationally bound to each other. They eventually wander away from each other and find their own paths through the galaxy. Our own sun was probably once part of an open cluster at one time but has since moved out on its own.