Messier 4 — Globular Cluster

Messier 4 is one of the nearest globular clusters of stars in the sky, located at a distance of about 7200 light-years towards the constellation of Scorpius (the Scorpion).

The most prominent feature is the central bright bar structure, reported for the first time by English astronomer Sir William Herschel, made up of various stars of 11th magnitude. Heavy clouds of dark interstellar matter obscure and absorb the starlight from the cluster, causing it to shimmer in various hues of orange and brown.

Messier 4 contains several hundred thousand stars, stretching on the sky over an area of the full Moon and receding from us at the rate of 250 000 kilometres per hour.

Astronomers peering inside this cluster have found 43 variable stars. In 1987, scientists detected a pulsar in the heart of the cluster. This is a neutron star that rotates over 300 times per second. In August 1995, the Hubble Space Telescope captured images of white dwarf stars in Messier 4, and these are thought to be some of the oldest stars known in our Galaxy. In July 2003, an intriguing new object turned up — a planet with a mass 2.5 times that of Jupiter, orbiting one of the white dwarfs. Estimated to be of the same age as the cluster, this system is around 13 billion years old, or almost as old as the Universe itself!

Messier 4 was discovered by the French astronomer Philippe Loys De Chéseaux in 1746, but it is best known as Messier’s Object Number 4, after another French astronomer, Charles Messier, who catalogued it in 1764. About 20 years later, Herschel, aided with his large telescope, reported this object as a cluster, as he was able to observe individual stars in it.

Messier 4 is bright enough to be detected by the unaided eye under very dark skies, just south of the line from the giant star Antares (Alpha Scorpii) to another Scorpius star, called Sigma Scorpii.