Messier 4

Lore & Background

M4 is a rather loosely concentrated cluster of class IX and measures 75 light-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 by William Herschel in 1783. At least 43 variable stars have been observed within M4. M4 is approximately 6,000 light-years away, making it the closest globular cluster to the Solar System. It has an estimated age of 12.2 billion years. The measured abundance of iron is equal to [Fe/H] = −1.07±0.1, meaning 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, implying two main cycles or phases of star formation. The space velocity components are (U, V, W) = (−57±3, −193±22, −8±5) km/s, confirming an orbit around the Milky Way of a period of (116±3) million years with eccentricity 0.80±0.03. The cluster undergoes tidal shock during each passage through the galactic disk, which can cause the repeated shedding of stars, so it may have been much more massive.

Reader's Guide

M4 is notable as the first globular cluster in which individual stars were resolved, and it remains one of the easiest globular clusters to find, located only 1.3 degrees west of the bright star Antares. It is conspicuous in even the smallest telescopes as a fuzzy ball of light, appearing about the same size as the Moon in the sky. Modestly sized telescopes will begin to resolve individual stars, the brightest of which are of apparent magnitude 10.8. The view through a good telescope was likened by Robert Burnham Jr. to that of hyperkinetic luminous alpha particles seen in a spinthariscope. Photographs by the Hubble Space Telescope in 1995 found white dwarf stars in M4 that are among the oldest known stars in our galaxy, aged 13 billion years. One star has been found to be a binary with a pulsar companion, PSR B1620−26, and a planet orbiting it with a mass of 2.5 times that of Jupiter. In 2023, an analysis of Hubble and Gaia data revealed an excess mass of roughly 800 solar masses in the center, which could be kinematic evidence for an intermediate-mass black hole, though an unusually compact cluster of compact objects cannot be completely discounted.

Did You Know?

• It is the closest globular cluster to the Solar System, at about 6,000 light-years away.
• M4 contains a binary star with a pulsar (PSR B1620−26) and a planet with 2.5 times Jupiter's mass.
• In 2023, data suggested an intermediate-mass black hole of roughly 800 solar masses may exist at its center.

A Comet Hunter's Unintended Legacy

Charles Messier never set out to build an astronomy catalogue. His true obsession was tracking comets across the night sky, and the bright, fixed objects that kept appearing in his telescope—star clusters, nebulae, and distant galaxies—were nothing more than annoying obstacles that mimicked the faint smudges of incoming comets. Working alongside his assistant Pierre Méchain, Messier compiled a running list of these frustrating look-alikes so he could skip them in future surveys. What began as a practical workaround for a single astronomer's pet project grew into one of the most celebrated inventories of the night sky ever assembled. The catalogue spans an extraordinary variety of deep-sky targets, from the Crab Nebula's supernova remnant to the Andromeda Galaxy's sweeping spiral arms, and includes globular clusters like Messier 4. Today, objects across the list are still routinely identified by their M-numbers, a testament to how a comet hunter's irritation produced a reference work that outlived his original purpose by centuries.

From Forty-Five to One Hundred and Ten

The Messier catalogue did not arrive fully formed. A preliminary edition surfaced in 1774 within the Memoirs of the French Academy of Sciences, listing forty-five objects that carried no numerical designations. Eighteen of those had been spotted by Messier himself; the remainder were borrowed from earlier observations by other astronomers. By 1780 the count had swelled to seventy entries, and the definitive version—published in 1781 in the Connaissance des Temps for the year 1784—reached one hundred and three objects. Yet the story did not end there. Marginal notes in Messier's and Méchain's manuscripts revealed that additional targets had been known but never formally published. Nicolas Camille Flammarion recovered Messier 104 in 1921 from a note in a copy of the 1781 edition. Helen Sawyer Hogg added M105 through M107 in 1947, Owen Gingerich contributed M108 and M109 in 1960, and Kenneth Glyn Jones appended M110 in 1967. The long-standing confusion over Messier 102—likely a duplicate sighting of M101, though some scholars identify it with the galaxy NGC 5866—kept the official count at one hundred and two for many years before the full tally of one hundred and ten was accepted.

Bright Enough for a Paris Rooftop

Because Messier studied the sky from the streets of Paris using a modest refracting telescope with an aperture of roughly one hundred millimetres, every object in his catalogue is, by definition, bright enough to be resolved by a small instrument. This single constraint means the list captures nearly all of the most spectacular examples of the five principal deep-sky categories—diffuse nebulae, planetary nebulae, open clusters, globular clusters, and galaxies—visible from European latitudes. Most of these targets also rank among the closest members of their respective classes, which is why professional observatories continue to study them with large-aperture instruments capable of revealing fine structural detail. For the modern amateur astronomer, the catalogue remains an irresistible checklist: the objects are bright, numerous, and distributed across the sky in a way that makes them accessible with larger telescopes than Messier ever possessed. In early spring, dedicated observers even attempt a "Messier marathon," racing to sweep through every entry in a single night, a tradition that underscores how a two-century-old list still serves as a practical observing roadmap.

The Limits of a Parisian Sky

Charles Messier conducted his observations from the Hôtel de Cluny in Paris, a building that today houses the Musée national du Moyen Âge. His position in the French capital imposed a hard geometric limit on what he could see: his catalogue encompasses only objects whose celestial latitudes fall between the north celestial pole and approximately minus thirty-five point seven degrees. Anything lying farther south—most notably the Large and Small Magellanic Clouds—simply never entered his field of view and therefore never made the list. This geographic constraint means the catalogue is, in effect, a northern-hemisphere survey, capturing the deep-sky treasures most readily available to observers in Europe and the northern United States. The result is a collection that skews toward the brightest and most visually rewarding targets in each class, since fainter southern objects were invisible from his Parisian vantage point. For amateur astronomers in the Northern Hemisphere, this makes the Messier list not just a historical curiosity but a genuinely practical guide to what their telescopes can realistically reveal on any given clear night.