Messier 4 or M4 (also designated NGC 6121) is a

globular cluster A globular cluster is a spheroidal conglomeration of stars. Globular clusters are bound together by gravity, with a higher concentration of stars towards their centers. They can contain anywhere from tens of thousands to many millions of membe ...

in the

constellation A constellation is an area on the celestial sphere in which a group of visible stars forms Asterism (astronomy), a perceived pattern or outline, typically representing an animal, mythological subject, or inanimate object. The origins of the e ...

Scorpius Scorpius is a zodiac constellation located in the Southern celestial hemisphere, where it sits near the center of the Milky Way, between Libra to the west and Sagittarius to the east. Scorpius is an ancient constellation that pre-dates the Gre ...

. It was discovered by

Philippe Loys de Chéseaux Jean-Philippe Loys de Cheseaux (; 4 May 1718 – 30 November 1751) was a Swiss astronomer. Biography Loys de Cheseaux was born on 4 May 1718 in Lausanne, Vaud, to Paul-Etienne Loys de Cheseaux, a banneret, and Estienne-Judith de Crousaz. His ...

in 1745 and catalogued by

Charles Messier Charles Messier (; 26 June 1730 – 12 April 1817) was a French astronomer. He published an astronomical catalogue consisting of 110 nebulae and star clusters, which came to be known as the ''Messier objects''. Messier's purpose f ...

in 1764. It was the first globular cluster in which individual

star A star is an astronomical object comprising a luminous spheroid of plasma (physics), plasma held together by its gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked ...

s were resolved.

Visibility

M4 is conspicuous in even the smallest of

telescope A telescope is a device used to observe distant objects by their emission, absorption, or reflection of electromagnetic radiation. Originally meaning only an optical instrument using lenses, curved mirrors, or a combination of both to observe ...

s as a fuzzy ball of light. It appears about the same size as the

Moon The Moon is Earth's only natural satellite. It is the fifth largest satellite in the Solar System and the largest and most massive relative to its parent planet, with a diameter about one-quarter that of Earth (comparable to the width of ...

in the sky. It is one of the easiest globular clusters to find, being located only 1.3 degrees west of the bright star

Antares Antares is the brightest star in the constellation of Scorpius. It has the Bayer designation α Scorpii, which is Latinised to Alpha Scorpii. Often referred to as "the heart of the scorpion", Antares is flanked by σ Scorpii and τ S ...

, 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 of

apparent magnitude Apparent magnitude () is a measure of the brightness of a star or other astronomical object observed from Earth. An object's apparent magnitude depends on its intrinsic luminosity, its distance from Earth, and any extinction of the object's li ...

10.8.

Characteristics

M4 is a rather loosely concentrated cluster of class IX and measures 75

light-year A light-year, alternatively spelled light year, is a large unit of length used to express astronomical distances and is equivalent to about 9.46 trillion kilometers (), or 5.88 trillion miles ().One trillion here is taken to be 1012 ...

s 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 Frederick William Herschel (; german: Friedrich Wilhelm Herschel; 15 November 1738 – 25 August 1822) was a German-born British astronomer and composer. He frequently collaborated with his younger sister and fellow astronomer Caroline H ...

in 1783. At least 43

variable star A variable star is a star whose brightness as seen from Earth (its apparent magnitude) changes with time. This variation may be caused by a change in emitted light or by something partly blocking the light, so variable stars are classified as ...

s have been observed within M4. M4 is approximately 7,200 light-years away, making it the closest globular cluster to the

Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Solar S ...

. It has an estimated age of 12.2 billion years. In astronomy, the abundance of elements other than

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic, an ...

and

helium Helium (from el, ἥλιος, helios, lit=sun) is a chemical element with the symbol He and atomic number 2. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas and the first in the noble gas group in the periodic table. ...

is called the

metallicity In astronomy, metallicity is the abundance of elements present in an object that are heavier than hydrogen and helium. Most of the normal physical matter in the Universe is either hydrogen or helium, and astronomers use the word ''"metals"'' as a ...

, and it is usually denoted by the abundance ratio of

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...

to hydrogen as compared to the

Sun The Sun is the star at the center of the Solar System. It is a nearly perfect ball of hot plasma, heated to incandescence by nuclear fusion reactions in its core. The Sun radiates this energy mainly as light, ultraviolet, and infrared radi ...

. For this cluster, the measured abundance of iron is equal to: :

=\ -1.07 \pm 0.01

This value is the

logarithm In mathematics, the logarithm is the inverse function to exponentiation. That means the logarithm of a number to the base is the exponent to which must be raised, to produce . For example, since , the ''logarithm base'' 10 o ...

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. The space velocity components are (U, V, W) = (, , ) km/s. This confirms an orbit around the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye ...

of a period of with

eccentricity Eccentricity or eccentric may refer to: * Eccentricity (behavior), odd behavior on the part of a person, as opposed to being "normal" Mathematics, science and technology Mathematics * Off-center, in geometry * Eccentricity (graph theory) of a v ...

0.80 ± 0.03: during peri

apsis An apsis (; ) is the farthest or nearest point in the orbit of a planetary body about its primary body. For example, the apsides of the Earth are called the aphelion and perihelion. General description There are two apsides in any ellip ...

it comes within from the galactic core, while at apo

it travels out to . The

inclination Orbital inclination measures the tilt of an object's orbit around a celestial body. It is expressed as the angle between a Plane of reference, reference plane and the orbital plane or Axis of rotation, axis of direction of the orbiting object ...

is at (an angle of) from the galactic plane, thus it reaches as much as above the disk. When passing through the disk, this cluster does so at less than 5 kpc from the

galactic nucleus An active galactic nucleus (AGN) is a compact region at the center of a galaxy that has a much-higher-than-normal luminosity over at least some portion of the electromagnetic spectrum with characteristics indicating that the luminosity is not pr ...

. The cluster undergoes

tidal shock A tidal shock occurs when a star cluster or other distributed astronomical object passes by a large mass such as an interstellar cloud, resulting in gravitational perturbation on a time scale that is much less than the mean time for a star to comp ...

during each passage, which can cause the repeated shedding of stars. Thus the cluster may have been much more massive.

Notable stars

Photographs by the

Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope that was launched into low Earth orbit in 1990 and remains in operation. It was not the first space telescope, but it is one of the largest and most versa ...

in 1995 found

white dwarf A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: its mass is comparable to the Sun's, while its volume is comparable to the Earth's. A white dwarf's faint luminosity comes fro ...

stars in M4 that are among the oldest known stars in our galaxy; aged 13 billion years. One has been found to be a

binary star A binary star is a system of two stars that are gravitationally bound to and in orbit around each other. Binary stars in the night sky that are seen as a single object to the naked eye are often resolved using a telescope as separate stars, in wh ...

with a pulsar companion,

PSR B1620−26 PSR B1620−26 is a binary star system located at a distance of 3,800 parsecs (12,400 light-years) in the globular cluster of Messier 4 (M4, NGC 6121) in the constellation of Scorpius. The system is composed of a pulsar (PSR B ...

and a

planet A planet is a large, rounded astronomical body that is neither a star nor its remnant. The best available theory of planet formation is the nebular hypothesis, which posits that an interstellar cloud collapses out of a nebula to create a you ...

orbiting it with a mass of 2.5 times that of

Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

(). One star in Messier 4 was also found to have much more of the rare light element lithium than expected. CX-1 Is located in M4. It is known as a possible millisecond pulsar/neutron star binary. It orbits in 6.31 hours.

Spinthariscope analogy

The view of Messier 4 through a good telescope was likened by Robert Burnham Jr. to that of hyperkinetic luminous alpha particles seen in a

spinthariscope A spinthariscope () is a device for observing individual nuclear disintegrations caused by the interaction of ionizing radiation with a phosphor (see radioluminescence) or scintillator. Invention The spinthariscope was invented by William Crook ...

.''Celestial Handbook, Volume three: Pavo through Vulpecula'' Robert Burnham Jr. page 1703.

References

External links