Messier 4

Messier 4 / Spider Globular Cluster is a globular cluster located in the constellation of Scorpius, at 6.000 light-years away. It was discovered by Phillipe Loys de Cheseaux in 1745 and catalogued by Charles Messier in 1764. Messier 4 is the closest globular cluster to Earth and therefore it has a small apparent magnitude: +5,6.
M4 is the first globular cluster where individual stars were resolved. It is easy to find in the night’s sky, being only 1,3 degrees West from Antares, the Alpha star of Scorpius. The star density is not very high, making their separation easier. The cluster is 75 light-years across and has a central bar made of 11-th magnitude stars. There are 2 types of stars in the cluster, with different metallicities, suggesting 2 separate star-forming stages.
In M4 there are white dwarves almost as old as the Universe – 13 billion years old. There have been found 43 variable stars, pulsars and one star very rich in lithium, a rare element. There might also be a central black hole 800 times heavier than the Sun, based on the chaotic movements of the central stars.