Messier 71

For many years, scientists believed that Messier 71 (M71) was an open cluster of stars not gravitationally bound together. But in the 1970s they determined that it is merely a relatively small (24 light years in diameter), loosely populated globular cluster whose stars are glued together with gravity.

Part of what fueled that debate is that M71 is richer in heavier elements than most globular clusters.

Globular clusters are ancient. They are many billions of years old—usually 10 to 13 billion years old—almost as old as the universe itself. The stars they contain tend to be very old as well, because star formation inside the cluster has largely ceased. The formation of globular clusters is not well understood. Current research leans toward the idea that they formed from very dense molecular clouds in the early universe. Some larger globular clusters may once have been dwarf galaxies whose larger star populations were stripped away from the core by larger galaxies.

M71

Distance: 13,000 light years

Number of stars: 20,000

Diameter: 27 light years

Age: 10 billion years