Discovered by Philippe Loys de Chéseaux in 1746.
M4 is one of the nearest globular clusters in the sky; according to newer results, its distance is perhaps only about 7,200 light years, which may be the smallest for a globular. M4 can be detected by the naked eye under very dark skies (1.3 degrees west of Antares), and is prominent with the slightest optical aid.
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 brown-ish appearance on color images. Its angular diameter, seen on deep photographs, is more than that of the Full Moon; this corresponds to a linear diameter of about 75 light years. On typical photos it appears somewhat smaller.
M4 is one of the most open, or loose, globulars. It is receding from us at 70.4 km/sec.
Globular cluster M4 was discovered by De Chéseaux in 1745-46 and listed by him as No. 19, and included in Lacaille's catalog as Lacaille I.9. Charles Messier cataloged it on May 8, 1764 and was the first to resolve it into a "cluster of very small [faint] stars;" this is the only globular cluster he could resolve with his moderate instruments, and thus the first globular cluster ever to be resolved. Only about 20 years later, William Herschel was able to resolve all Messier globulars with his large telescopes.
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 has 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 at about 13 billion years, or almost three times the age of our solar system.
M4 can be easily found as it is only 1.3 deg west of bright Antares (Alpha Scorpii). A round diffuse patch in binoculars, it is a circular glow in small telescope, and even a 4-inch resolves the brightest stars; the reseolved stars appear irregularly distributed. Larger telescopes show a halo of stars around the bright central portion of the cluster to a diameter of over 16 arc minutes.======================
Flesherton, Ontario, Canada
May 13, 2007