Messier 4 (NGC 6121) is a massive globular cluster in Scorpius that is found in the night sky close to M80, another globular cluster in the same constellation. M4 was discovered by Philippe Loys de Chéseaux in 1745, who described it as a “white and round” glow. In 1764, Charles Messier then became the first person to resolve a globular cluster into individual stars when he did so with M4, and subsequently included it in his catalogue of 110 “Messier objects”.
• Constellation: Scorpius
• Coordinates: RA 16h 23m 35.22s | Dec. –26° 31′ 32.7″
• Distance: 7,200 light years
• Cluster Type: Globular
• Mass: 67,000 sol
• Radius: 37.5 light years
• Apparent Magnitude: +5.9
• Age: 12.2billion years
• Best seen: Summer
• Other Designations: NGC 6121
As with M80, the best time to view M4 is during summer, when the constellation Scorpius is relatively high in the sky. While even the smallest telescope will reveal the cluster as a fuzzy ball of light about as big as the full Moon, a medium to large wide-field telescopes will enable you to resolve individual stars. Look for M4 only 1.3 degrees to the westward of the star Antares, but bear in mind that in wide-field instruments, both Antares and M4 will be visible in the field of view.
In terms of its structure and composition, M4 is notable in several ways. Firstly, it is not as densely packed as other, similar globular clusters; secondly it contains a distinctive bar structure across its nucleus; and thirdly, along with NGC 6397 (Caldwell 86), a globular cluster in the constellation Ara, it is one of the two closest globular clusters to the solar system.
The bar structure runs almost north/south across the core of the cluster, and consists mostly of magnitude 11 stars. It was first discovered by William Herschel in 1783, and telescopes of moderate aperture clearly show a thick band of stars that is about 2.5′ long. At its distance of about 7,200 light years, the entire cluster spans 26 minutes of arc, which translates into an effective diameter of 75 light years.
Recent studies into the composition of M4 as a whole have revealed a metalicity of about 8.5% that of the Sun. Metalicity usually refers to an abundance of elements in an object that are heavier than hydrogen and helium, but it is more commonly expressed as the abundance ratio of iron relative to hydrogen in the Sun. In the case of M4, these measurements and values suggest that the cluster contains two distinctly different populations of stars, and although there is some debate around the reason for this, it is commonly thought that it is the result of two separate epochs of star formation within the cluster.
Velocity and Orbit
This is perhaps not surprising, considering the orbit of M4 through the Milky Way. The space velocities (U, V, W) of M4 are –57 ± 3, –193 ± 22, and –8 ± 5 km/s respectively, where “U” represents a positive value with respect to the galactic center, “V” represents a positive value with respect to the galaxy’s direction of rotation, and “W” represents a positive value relative to the direction of the North Galactic Pole. Finally, the clusters’ orbit (that has a period of about 116 ± 3 million years) has an eccentricity of 0.80 ± 0.03.
Taken together, the clusters’ orbit and velocity carry it to within around 1,950 light years of the galactic core, and as far away as 19,240 light years from the core. Given that the orbital inclination of the cluster is 23° ± 6° relative to the galactic plane, it means that M4 is at times carried to a distance of 4,890 light years above and below the galactic plane. In practice, this means that every time the cluster passes through the galaxy, it does so within less than 16,000 light years from the galactic core, which is close enough to subject the cluster to enormous tidal shocks.
Therefore, M4 can suffer the loss of stars due to tidal stripping every time it passes through the galaxy, although these tidal shocks can also trigger episodes of star formation. As of yet, there is no evidence of the latter having happened, although this mechanism can possibly explain the different stellar populations that exist within the cluster. What is certain, however, is that M4 has lost significant numbers of stars during its repeated passages through the Milky Way during its 12-billion year history.
Photographs of M4 taken by Hubble in 1995 have shown that the cluster contains some of the oldest known white dwarf stars in the Milky Way, with some being as old as 13 billion years. Interestingly, one such ancient white dwarf was found to have a pulsar (PSR B1620-26) as a companion in a binary system, with the system being orbited by 2.5 Jupiter-mass planet.