How to see it
If you’ve never found a deep-sky object on your own before, Messier 4 (M4) is a grand place to start. The M4 globular star cluster is rather easy to find, because it’s right next to the first-magnitude star Antares, the brightest in the constellation Scorpius the Scorpion. Your first step to locating M4 is to find Antares, the Scorpion’s heart star.
Antares and M4 are best seen when they are due south and highest in the sky. In early June, Antares is highest in the sky around midnight (1 a.m. daylight savings time). Remember, the stars return to the same place in the sky some 2 hours earlier every month. Therefore, Antares is highest up around 10 p.m. (11 p.m. daylight savings time) in early July, and 8 p.m. (9 p.m. daylight savings time) in early August. In short, summer evenings are probably your best bet for catching M4.
Use binoculars to find the M4 globular cluster on a dark, moonless night. Antares and M4 readily fit inside the same binocular field of view, with M4 appearing a bit more than 1 degree to the west (or right) of Antares. For reference, a typical binocular field has a diameter of 5 to 6 degrees. M4 looks like a rather dim, hazy star in binoculars, and you really need a telescope to begin to resolve this cluster into stars.
History and Science
The comet hunter Charles Messier (1730-1817) listed M4 as object #4 in his famous Messier catalog. The Messier catalog listed over 100 deep-sky objects that look like comets, but really aren’t. Charles Messier wanted to steer comet hunters away from these faint fuzzies that masquerade as comets.
Modern astronomy tells us that M4 is a globular star cluster – a globe-shape stellar city packed with perhaps a hundred thousand stars. Unlike open star clusters – such as the Pleiades and the Hyades – the Milky Way galaxy’s 200 or so globular star clusters are NOT part of the galactic disk. Instead, globular clusters populate the galactic halo – the sphere-shaped region of the Milky Way circling above and below the pancake-shape galactic disk.
At about 7,000 light-years from Earth, M4 is one of the closest of the Milky Way’s 200 or so globular clusters. Most globulars reside tens of thousands of light-years away. In fact, the farthest of globular clusters, M54, is thought to be 70,000 light-years distant.
Globular clusters are tightly packed with tens to hundreds of thousands of stars, whereas open clusters are loosely-bound stellar confederations with only a few hundred to a thousand stars. Globular clusters contain primitive stars that are billions of years old and almost as old as the universe itself. On the other hand, open clusters consist of young, hot stars that tend to disperse after hundreds of milllions of years.
M4′s position is at Right Ascension: 16h 23.6m; Declination: 26 degrees 32′ south