Image Credit: ESO/J. Emerson/VISTA. Acknowledgment: Cambridge Astronomical Survey Unit
Messier 55 (also known as NGC 6809) is a large globular cluster with a diameter of 96 light-years, held together in a tight spherical shape by gravity. It is located only 17,600 light-years away in the constellation Sagittarius.
The cluster contains around 100,000 stars, mostly old red stars, but also an unusual number of blue stragglers (stars that appear younger than the population from which they formed), and has a mass of about 269,000 Sun masses. Only about 55 variable stars have been discovered in the central part of Messier 55.
Messier 55 is one of the 160 globular clusters encircling our Milky Way, mostly toward its bulging centre. Observations of globular clusters’ stars reveal that they originated around the same time (over 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, Messier 55 has a low metallicity, what means that nearly all of the gas in the original cloud was hydrogen, along with some helium and even much smaller amounts of heavier chemical elements such as oxygen and nitrogen. (Our Sun, and the Solar System are just 4.6 billion years old and are infused with heavier elements created in earlier generations of stars, which exploded in supernovae.)
The stars in M55 are among the oldest in the Universe. Astronomers study Messier 55 and other globular clusters, to learn how galaxies evolve and stars age.
As well as the stars of Messier 55, this VISTA image also records many galaxies lying far beyond the cluster.