NGC 6397 (also known as Caldwell 86) is a

globular cluster A globular cluster is a spheroidal conglomeration of stars that is bound together by gravity, with a higher concentration of stars towards its center. It can contain anywhere from tens of thousands to many millions of member stars, all orbiting ...

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 first constellati ...

Ara that was discovered by French astronomer

Nicolas-Louis de Lacaille Abbé Nicolas-Louis de Lacaille (; 15 March 171321 March 1762), formerly sometimes spelled de la Caille, was a kingdom of France, French astronomer and geodesist who named 14 out of the IAU designated constellations, 88 constellations. From 1750 ...

in 1752. It is located about 7,800

light-year A light-year, alternatively spelled light year (ly or lyr), is a unit of length used to express astronomical distances and is equal to exactly , which is approximately 9.46 trillion km or 5.88 trillion mi. As defined by the International Astr ...

s from

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

, making it one of the two nearest globular clusters to Earth (the other one being Messier 4). The cluster contains around 400,000 stars, and can be seen with the

naked eye Naked eye, also called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnification, magnifying, Optical telescope#Light-gathering power, light-collecting optical instrument, such as a telescope or microsc ...

under good observing conditions. NGC 6397 is one of at least 20 globular clusters of the

Milky Way Galaxy The Milky Way or Milky Way Galaxy is the galaxy that includes the 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 in other arms of the galaxy, which are ...

that have undergone a core collapse, meaning that the core has contracted to a very dense stellar

agglomeration Agglomeration may refer to: * Urban agglomeration, in standard English * Megalopolis, in Chinese English, as defined in China's ''Standard for basic terminology of urban planning'' (GB/T 50280—98). Also known as "city cluster". * Economies of agg ...

Astronomical research

Estimating the age of the Milky Way

In 2004, a team of

astronomer An astronomer is a scientist in the field of astronomy who focuses on a specific question or field outside the scope of Earth. Astronomers observe astronomical objects, such as stars, planets, natural satellite, moons, comets and galaxy, galax ...

s focused on the cluster to estimate the age of the

. They used the UV-Visual

Echelle Spectrograph An optical spectrometer (spectrophotometer, spectrograph or spectroscope) is an instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, typically used in spectroscopic analysis to identify mater ...

of the

Very Large Telescope The Very Large Telescope (VLT) is an astronomical facility operated since 1998 by the European Southern Observatory, located on Cerro Paranal in the Atacama Desert of northern Chile. It consists of four individual telescopes, each equipped with ...

Cerro Paranal Cerro Paranal is a mountain in the Atacama Desert of northern Chile and is the home of the Paranal Observatory. Prior to the construction of the observatory, the summit was a horizontal control point with an elevation of ; now it is above sea ...

to measure the

beryllium Beryllium is a chemical element; it has Symbol (chemistry), symbol Be and atomic number 4. It is a steel-gray, hard, strong, lightweight and brittle alkaline earth metal. It is a divalent element that occurs naturally only in combination with ...

content of two stars in the cluster. This allowed them to deduce the time elapsed between the rise of the first generation of stars in the entire Galaxy and the first generation of stars in the cluster. This, added to the estimated age of the stars in the cluster, gives an estimate for the age for the Galaxy: about 13.6

billion Billion is a word for a large number, and it has two distinct definitions: * 1,000,000,000, i.e. one thousand million, or (ten to the ninth power), as defined on the short scale. This is now the most common sense of the word in all varieties of ...

years, which is nearly as old as the universe itself. This estimate assumes that the NGC 6397 is not older than the Milky Way.

Lower mass limit for stars

In 2006, a study of NGC 6397 using the

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

was published that showed a clear lower limit in the

brightness Brightness is an attribute of visual perception in which a source appears to be radiating/reflecting light. In other words, brightness is the perception dictated by the luminance of a visual target. The perception is not linear to luminance, and ...

of the cluster's population of faint stars. The authors deduce that this indicates a lower limit for the

mass Mass is an Intrinsic and extrinsic properties, intrinsic property of a physical body, body. It was traditionally believed to be related to the physical quantity, quantity of matter in a body, until the discovery of the atom and particle physi ...

necessary for stars to develop a core capable of fusion: roughly 0.083 times the mass of the

Sun The Sun is the star at the centre of the Solar System. It is a massive, nearly perfect sphere of hot plasma, heated to incandescence by nuclear fusion reactions in its core, radiating the energy from its surface mainly as visible light a ...

Black holes and white dwarfs

In February 2021, the core of NGC 6397 was reported to contain a relatively dense concentration of compact objects (

white dwarfs A white dwarf is a stellar core remnant composed mostly of electron-degenerate matter. A white dwarf is very dense: in an Earth sized volume, it packs a mass that is comparable to the Sun. No nuclear fusion takes place in a white dwarf; what ...

neutron stars A neutron star is the gravitationally collapsed core of a massive supergiant star. It results from the supernova explosion of a massive star—combined with gravitational collapse—that compresses the core past white dwarf star density to th ...

and

black holes A black hole is a massive, compact astronomical object so dense that its gravity prevents anything from escaping, even light. Albert Einstein's theory of general relativity predicts that a sufficiently compact mass will form a black hole. Th ...

), based on the movement of stars near the core derived from data gathered by the Hubble Space Telescope and the European Space Agency's Gaia spacecraft. The respective study argued that black holes could dominate the mass budget of this concentration, if the black holes were not ejected by means of dynamical interactions. Indeed, another group of scientists shortly responded, claiming that because NGC 6397 has undergone core collapse, it should have started dense enough to speed its rates of dynamical interactions, and its original black hole population should be almost entirely gone. This group provided dynamical simulations that showed that a concentration of white dwarfs could explain the first measurement. Finally, in 2022, a work was published by the former scientists, along with one of the leaders of the latter group, and other experts of Hubble and Gaia data. This new work showed that the fits of the central mass excess in NGC 6397 from observed data agreed remarkably well with numerical simulations accounting for a population of hundreds of massive white dwarfs, and essentially no black holes.

Distant globular clusters

Hubble pictures revealed many distant globular clusters in galaxies far behind NGC 6397.

Gallery

File:Hubble's view of dazzling globular cluster NGC 6397.jpg, Hubble's view of globular cluster NGC 6397 File:L R.BedinNGC6397.gif, alt=NGC 6397 region double observation HST& JWST, NGC 6397 region double observation HST and JWST File:Euclid’s view of globular cluster NGC 6397 ESA25170813.jpg, NGC 6397 imaged by

Euclid Euclid (; ; BC) was an ancient Greek mathematician active as a geometer and logician. Considered the "father of geometry", he is chiefly known for the '' Elements'' treatise, which established the foundations of geometry that largely domina ...

References

External links

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SEDS
– NGC 6397 {{DEFAULTSORT:NGC 6397 Ara (constellation) Globular clusters 6397 086b