Messier 4, also designated NGC 6121, is a globular cluster in the constellation of Scorpius. It was discovered by Philippe Loys de Chéseaux in 1746, and catalogued by Charles Messier in 1764. It appears about the same size as the Moon in the sky. It is one of the easiest globular clusters to find, being located only 1.3 degrees west of the bright star Antares, as bright as mag 5.6 visually. M4 is a rather loosely concentrated cluster of class IX and measures 75 light years across. It features a characteristic “bar” structure across its core, and is approximately 7,200 light years away, making it the second closest globular cluster to our Solar System.
M4 would be one of the most splendid globulars in the sky, if it were not obscured by heavy clouds of dark interstellar matter. Interstellar absorption also reddens the color of the light from the cluster, and gives it a slightly orange or brownish appearance on colour images. Its compressed central core was measured linearly at 3.6 light-years, with half the cluster’s mass concentrated in an inner spherical volume of 16 light-years diameter. It is receding from us at 70.4 km/sec and contains at least 43 known variable stars.
In 1987, the first millisecond pulsar was discovered in this globular cluster. This pulsar, 1821-24, is a neutron star rotating and pulsating once every 3.0 milliseconds, or over 300 times per second, which is even 10 times faster than the Crab pulsar in M1. In August 1995, the Hubble Space Telescope photographed white dwarf stars in M4, which are among the oldest stars in our Milky Way Galaxy. In July 2003, investigations with the Hubble Space Telescope led to the identification of a planet orbiting one of these white dwarfs; they form a triple system with a pulsar called PSR B1620-26. This planet, of a mass 2.5 times that of Jupiter, is presumably about as old as the globular cluster M4, a figure currently estimated to be almost three times the age of our Solar System.
Credits: SEDS, Wikipedia.