Globular clusters are held together in a tight spherical shape by gravity. In M55, the stars certainly do keep close company: Approximately 100,000 stars are packed within a sphere with a diameter of only about 25 times the distance between the Sun and the nearest star system, Alpha Centauri.
About 160 globular clusters have been spotted encircling the Milky Way, mostly toward its bulging center. The largest galaxies can have thousands of these rich collections of stars in orbit around them.
Observations of globular clusters’ stars reveal that they originated around the same time — more than 10 billion years ago — and from the same cloud of gas. As this formative period was just a few billion years after the Big Bang, nearly all of the gas on hand was the simplest, lightest, and most common in the cosmos — hydrogen, along with some helium and much smaller amounts of heavier chemical elements such as oxygen and nitrogen.
Being made mostly from hydrogen distinguishes globular cluster residents from stars born in later eras, like our Sun, that are infused with heavier elements created in earlier generations of stars. The Sun lit up some 4.6 billion years ago, making it only about half as old as the elderly stars in most globular clusters. The chemical makeup of the cloud from which the Sun formed is reflected in the abundances of elements found throughout the solar system — in asteroids, in the planets, and in our bodies.
Skywatchers can find M55 in the constellation Sagittarius the Archer. The notably large cluster appears nearly two-thirds the width of the Full Moon and is not at all difficult to see in a small telescope, even though it is located at a distance of about 17,000 light-years from Earth.
The French astronomer Nicolas Louis de Lacaille first documented the stellar grouping around 1752, and some 26 years later another French astronomer, Charles Messier, included the cluster as the 55th entry in his famous astronomical catalog. The object is also cross-listed as NGC 6809 in the New General Catalogue, an often-cited and more extensive astronomical catalog created in the late 19th century.