M4 is conspicuous in even the smallest oftelescopes as a fuzzy ball of light. It appears about the same size as theMoon in the sky. It is one of the easiest globular clusters to find, being located only 1.3 degrees west of the bright starAntares, with both objects being visible in a wide-field telescope. Modestly sized telescopes will begin to resolve individual stars, of which the brightest in M4 are ofapparent magnitude 10.8.[9]
M4 is a rather loosely concentrated cluster ofclass IX and measures 75light-years across. It features a characteristic "bar" structure across its core, visible to moderate sized telescopes. The structure consists of 11th-magnitude stars and is approximately 2.5' long and was first noted byWilliam Herschel in 1783. At least 43variable stars have been observed within M4.[9]
M4 is approximately 6,000 light-years away,[10] making it the closest globular cluster to theSolar System. It has an estimated age of 12.2 billion years.[7]
In astronomy, the abundance of elements other thanhydrogen andhelium is called themetallicity, and it is usually denoted by the abundance ratio ofiron to hydrogen as compared to theSun. For this cluster, the measured abundance of iron is equal to:
This value is thelogarithm of the ratio of iron to hydrogen relative to the same ratio in the Sun. Thus the cluster has an abundance of iron equal to 8.5% of the iron abundance in the Sun. This strongly suggests this cluster hosts two distinct stellar populations, differing by age. Thus the cluster probably saw two main cycles or phases of star formation.[6]
Thespace velocity components are (U, V, W) = (–57 ± 3,–193 ± 22,–8 ± 5) km/s. This confirms an orbit around theMilky Way of a period of(116 ± 3) million years witheccentricity 0.80 ± 0.03: during periapsis it comes within(0.6 ± 0.1)kpc from the galactic core, while at apoapsis it travels out to(5.9 ± 0.3) kpc. Theinclination is at (an angle of)23° ± 6° from the galactic plane, thus it reaches as much as1.5 ± 0.4 kpc above the disk.[11] When passing through the disk, this cluster does so at less than 5 kpc from thegalactic nucleus. The cluster undergoestidal shock during each passage, which can cause the repeated shedding of stars. Thus the cluster may have been much more massive.[6]
Photographs by theHubble Space Telescope in 1995 foundwhite dwarf stars in M4 that are among the oldest known stars inour galaxy; aged 13 billion years. One has been found to be abinary star with a pulsar companion,PSR B1620−26 and aplanet orbiting it with a mass of 2.5 times that ofJupiter (MJ).[12] One star in Messier 4 was also found to have much more of the rare light element lithium than expected.[13]
CX-1 Is located in M4. It is known as a possible millisecond pulsar/neutron star binary. It orbits in 6.31 hours.[14]
The view of Messier 4 through a good telescope was likened byRobert Burnham Jr. to that of hyperkinetic luminous alpha particles seen in aspinthariscope.[15]
Simulation of stellar motions in Messier 4, where astronomers suspect that anintermediate-mass black hole could be present.[5][16] If confirmed, the black hole would be in the center of the cluster, and would have asphere of influence (black hole) limited by the red circle.
In 2023, an analysis ofHubble Space Telescope and European Space Agency's Gaia spacecraft data from Messier 4 revealed an excess mass of roughly 800 solar masses in the center of this cluster, which appears to not be extended. This could thus be considered as kinematic evidence for anintermediate-mass black hole[5][16] (even if an unusually compact cluster of compact objects likewhite dwarfs,neutron stars or stellar-massblack holes cannot be completely discounted).
^Shapley, Harlow; Sawyer, Helen B. (August 1927). "A Classification of Globular Clusters".Harvard College Observatory Bulletin.849 (849):11–14.Bibcode:1927BHarO.849...11S.
^abCaputo, F.; Castellani, V.; Quarta, M. L. (February 1985). "Reddening, distance modulus and age of the globular cluster NGC 6121 (M4) from the properties of RR Lyrae variables".Astronomy and Astrophysics.143 (1):8–12.Bibcode:1985A&A...143....8C.
