Messier 4

M4, also designated NGC 6121, is a bright globular cluster located in the southern constellation Scorpius. It lies at an approximate distance of 7,200 light-years from Earth at right ascension 16h 23m 35.22s and declination: -26°31’32.7” and has an apparent magnitude of 5.9. Messier 4 has a density classification IX, which means that it is rather loosely concentrated. (The densest clusters are those classed I.) The cluster has an estimated age of 12.2 billion years. M4 has a characteristic bar structure across its central region roughly 2.5′ long and consists of 11th magnitude stars. Messier 4 is one of the easiest clusters to find in the night sky. It lies only 1.3 degrees west of Antares, the brightest star in Scorpius constellation and the 17th brightest star in the sky. Observers can see both the red supergiant star and the cluster in a wide field telescope. The best time of year to observe M4 from northern latitudes is in the summer months when Scorpius is visible above the southern horizon in the evening. From southern latitudes, Scorpius is best seen during the months of May, June and July, when it rises high in the sky.

M4 was discovered by the Swiss astronomer and mathematician Jean-Philippe Loys de Chéseaux in 1746, who added it to his catalogue as number 19. Charles Messier catalogued the cluster on May 8, 1764. Other globular clusters in the Messier catalogue were not resolved until about 20 years later when William Herschel observed them in his larger telescopes. French astronomer Nicholas Louis de Lacaille discovered the globular cluster independently on April 13, 1752, and included it in his catalogue as Lacaille I.9. In 1783, Sir William Herschel resolved this object into stars; and gauging it by a modification of the method which he applied to fathom the Galaxy, he concluded that his 10-foot reflector, having the power to show stars exceeding that of the eye 28.67 times, gave the profundity of this cluster of the 344th order.

Messier 4 has roughly the same apparent size as the full Moon. It is visible to the naked eye, but only in exceptionally good conditions. If it weren’t obscured by clouds of dark interstellar dust, the cluster would appear far brighter. The cluster’s overall spectral type is F8. In deep images, M4 has an angular diameter of 36 arc minutes, corresponding to 75 light-years, while in regular images the cluster appears a bit smaller, at about 26 arc minutes. The tidal radius of M4 is estimated at 32.49′, corresponding to about 70 light-years. In other words, the stars within the region of 140 light-years in diameter that M4 occupies can’t escape the cluster’s gravitational influence.

Messier 4 is relatively small, home to more than 100,000 stars, fewer than most other globular clusters. Half the mass of M4 is concentrated in the cluster’s inner region, spanning 16 light-years in diameter. The brightest stars have an apparent magnitude of 10.8. The cluster contains at least 65 variable stars. It is believed to host at least two distinct stellar populations, which is to say populations of stars formed at two or more different points in time. If this is the case, the cluster underwent two or more cycles of star formation. The cluster has an orbit that takes it through the Milky Way’s disk, at distances less than 5,000 parsecs from the galactic nucleus. As a result, M4 undergoes tidal shock every time which, in turn, results in repeated loss of stars. Messier 4 is receding from us at 70.4 km/s. It has an estimated mass of 100,000 solar masses. A millisecond pulsar, PSR 1821-24, was discovered in Messier 4 in 1987. It has a period of 3 milliseconds, which is roughly 10 times faster than the Crab Pulsar in Messier 1.

Messier 4 is home to some of the oldest known stars in our galaxy. Between 1995 and 2001, the National Optical Astronomy Observatory (NOAO) and NASA also uncovered the oldest burned-out stars in our Milky Way Galaxy within this cluster. These small, burned-out stars – called white dwarfs. The cluster contains up to 40,000 white dwarfs — the cores of ancient, dying stars whose outer layers have drifted away into space. As white dwarfs age, they grow cooler, fainter, and more difficult to detect. Therefore, a globular cluster’s age can be inferred from the age of its faintest white dwarf. Because the stars in these clusters are some of the oldest in the universe, up to 13 billion years old, astronomers are able to use them to estimate the age of the universe. Observations of the full cluster were performed by the Kitt Peak National Observatory in March 1995. About 15 per cent of the stars in M4 are binaries. In a recent survey of the cluster, one of the stars was discovered to have an unusually large amount of lithium, an element that is usually destroyed over a star’s lifetime. The unusual star has either retained its original lithium or produced a new supply. The source of the element is still unclear.

The M4 cluster (top) with sections showing white dwarf stars shown in the bottom left and right. The blue circles indicate the dwarfs. Credit: NASA/JPL/NOAO/HST.

In July 2003, astronomers discovered a planet in the cluster. The planet has 2.5 times the mass of Jupiter and orbits a binary star system consisting of a white dwarf and a pulsar (rapidly rotating neutron star). The binary system designated PSR B1620-26 lies at a distance of 12,400 light-years from Earth, just outside M4’s core region. The planet, PSR B1620-26 b, has the unofficial nicknames “the Genesis planet” and “Methuselah” because it is believed to be 12.7 billion years old. It is one of the oldest extrasolar planets known, roughly three times the age of our solar system.

Credits: NASA, Messier Objects, Universe Today, Wikipedia.