VISTA views a vast ball of stars

VISTA infrared image of the globular star cluster Messier 55

Packed into a relatively small space, the tens of thousands of stars make up an ancient globular cluster known as Messier55. Astronomers study clusters like this to learn how galaxies evolve and stars age.

The image was obtained using data from the VISTA telescope in Chile, which was conceived and developed by a consortium of universities in the UK, led by Queen Mary.

Professor Jim Emerson from Queen Mary’s Astronomy Unit, commented: “This image helped us confirm how well VISTA works. When we analysed the brightness and colours of all its stars we found the expected correlation between the two quantities, which allowed us to determine the age of this star cluster.”

Globular clusters are held together in a tight spherical shape by gravity; in Messier 55, the stars keep close company: approximately one hundred thousand 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 our galaxy, the Milky Way, mostly toward its bulging centre. 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 own bodies.

Sky watchers can find Messier 55 in the constellation of 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 17 300 light-years from Earth.