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{{titolo errato|titolo=Sandbox n°2 - Omnia Vincit Sirivs}}

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==Ammasso aperto==

[[Image:Pleiades large.jpg|thumb|right|250px|The [[Pleiades (star cluster)|Pleiades]] is one of the most famous open clusters.]]

Un '''ammasso aperto''' è un gruppo di massimo un centinaio di [[stelle]] che si sono formate dalla stessa [[nube molecolare gigante]] e sono debolmente legate [[Gravità|gravitazionalmente]] le une con e altre. Si differenziano dagli [[ammassi globulari]], le cui componenti sono invece fortemente legate dalla gravità. Gli ammassi aperti possono essere osservati solo in [[Galassia spirale|galassie spirali]] ed [[Galassia irregolare|irregolari]], in cui avviene frequentemente la [[formazione stellare]]. Di solito la loro età è inferiore a poche centinaia di milioni di anni, poiché vengono dissolti dalle numerevoli forze presenti nelle galassie, dalla rivoluzione attorno al [[centro galattico]] e dall'avvicinamento di nubi di [[gas interstellare]], tutte forze che ne disperdono le componenti.

Gli ammassi aperti più giovani sono ancora avvolti dalla [[nube molecolare]] dalla quale si sono formati, che viene illuminata dalle nuove stelle, ionizzandosi e diventando [[Regione H II|regioni H II]]; col tempo, la [[pressione di radiazione]] generata dall'ammasso tenderà a far disperdere queste nubi residue. Di solito, prima che la nube sia dispersa, circa il 10% della sua massa sarà collassata per generare nuove stelle.

Gli ammassi aperti sono degli [[Oggetto celeste|oggetti celesti]] di notevole importanza nello studio dell'[[evoluzione stellare]]; poiché le loro componenti sono simili fra loro sia come età che come [[Elemento chimico|composizione chimica]], gli effetti delle piccole variazioni delle proprietà delle singole stelle sono studiabili con più facilità rispetto a quelle rilevabili su stelle isolate.

==Historical observations==

The most prominent open clusters such as the [[Pleiades (star cluster)|Pleiades]] have been known and recognised as groups of stars since antiquity. Others were known as fuzzy patches of light, but had to wait until the invention of the [[telescope]] to be resolved into their constituent stars. Telescopic observations revealed two distinct types of clusters, one of which contained thousands of stars in a regular spherical distribution and was found preferentially towards the centre of the [[Milky Way]], and the other of which consisted of a generally sparser population of stars in a more irregular shape and found all over the sky. Astronomers dubbed the former [[globular cluster]]s, and the latter open clusters. Open clusters are also occasionally referred to as ''galactic clusters'', because they are almost exclusively found in the [[galactic plane|plane of the Milky Way]], as discussed below.

It was realised early on that the stars in the open clusters were physically related. The Reverend [[John Michell]] calculated in 1767 that the probability of even just one group of stars like the Pleiades being the result of a chance alignment as seen from Earth was just 1 in 496,000.<ref>Michell J. (1767), ''An Inquiry into the probable Parallax, and Magnitude, of the Fixed Stars, from the Quantity of Light which they afford us, and the particular Circumstances of their Situation'', Philosophical Transactions, v. 57, p. 234–264</ref> As [[astrometry]] became more accurate, cluster stars were found to share a common [[proper motion]] through space, while [[Astronomical spectroscopy|spectroscopic]] measurements revealed common [[radial velocity|radial velocities]], thus showing that the clusters consist of stars born at the same time and bound together as a group.

While open clusters and globular clusters form two fairly distinct groups, there may not be a great deal of difference in appearance between a very sparse globular cluster and a very rich open cluster. Some astronomers believe the two types of [[star cluster]]s form via the same basic mechanism, with the difference being that the conditions which allowed the formation of the very rich globular clusters containing hundreds of thousands of stars no longer prevail in our galaxy.

==Formation==

[[Image:Trapezium cluster optical and infrared comparison.jpg|thumb|left|250px|[[Infrared]] light reveals the dense open cluster forming at the heart of the [[Orion nebula]].]]

A large fraction of stars are originally formed in [[binary star|multiple systems]]<ref>{{cite journal | journal=Annual Reviews of Astronomy and Astrophysics|year=1994|author=Mathieu, R. D.|volume=32|pages=465-530|title= Pre-Main-Sequence Binary Stars ||url=http://adsabs.harvard.edu/abs/1994ARA%26A..32..465M | doi = 10.1146/annurev.aa.32.090194.002341 <!--Retrieved from CrossRef by DOI bot-->}}</ref> because only a cloud of gas containing many times the [[mass]] of the [[Sun]] will be heavy enough to collapse under its own [[gravity]], but such a heavy cloud cannot collapse into a single star.<ref>Boss A.P. (1998), ''The Jeans Mass Constraint and the Fragmentation of Molecular Cloud Cores'', Astrophysical Journal Letters v.501, p.L77</ref>

The formation of an open cluster begins with the collapse of part of a [[giant molecular cloud]], a cold dense cloud of gas containing up to many thousands of times the [[solar mass|mass of the Sun]]. Many factors may trigger the collapse of a giant molecular cloud (or part of it) and a burst of star formation which will result in an open cluster, including shock waves from a nearby [[supernova]] and gravitational interactions. Once a giant molecular cloud begins to collapse, star formation proceeds via successive fragmentations of the cloud into smaller and smaller clumps, resulting eventually in the formation of up to several thousand stars. In our own galaxy, the formation rate of open clusters is estimated to be one every few thousand years.<ref>Battinelli P., Capuzzo-Dolcetta R. (1991), ''Formation and evolutionary properties of the Galactic open cluster system'', Monthly Notices of the Royal Astronomical Society, v. 249, p. 76–83</ref>

Once star formation has begun, the hottest and most massive stars (known as [[OB star]]s) will emit copious amounts of [[ultraviolet radiation]]. This radiation rapidly ionizes the surrounding gas of the giant molecular cloud, forming an [[H II region]]. [[Stellar wind]]s from the massive stars and [[radiation pressure]] begin to drive away the gases; after a few million years the cluster will experience its first [[supernova]]e, which will also expel gas from the system. After a few tens of millions of years, the cluster will be stripped of gas and no further star formation will take place. Typically, less than 10% of the gas originally in the cluster will form into stars before it is dissipated.<ref>also see Battinelli P as previous reference.</ref>

Linee guida da sempre applicate ma "ufficialmente" ancora da ufficializzare

Another view to cluster formation is that they form rapidly out of a contracting molecular cloud core and once the massive stars begin to shine they expel the residual gas with the sound speed of the hot ionised gas. From the time of start of cloud-core contraction to gas expulsion takes typically not more than one to three million years. As only 30 to 40 per cent of the gas in the cloud core forms stars, the process of residual gas expulsion is highly damaging to the cluster which loses many and perhaps all of its stars <ref>Kroupa P., Aarseth S.J., Hurley J. (2001), "The formation of a bound star cluster: from the Orion nebula cluster to the Pleiades", Monthly Notices of the Royal Astronomical Society, v. 321, 699-712 [http://arxiv.org/abs/astro-ph/0009470 preprint]</ref>. All clusters thus suffer significant infant weight loss, while a large fraction undergo infant mortality. The young stars so released from their natal cluster become part of the Galactic field population. Because most if not all stars form clustered, star clusters are to be viewed the fundamental building blocks of galaxies. The violent gas-expulsion events that shape and destroy many star clusters at birth leave their imprint in the morphological and kinematical structures of galaxies <ref>Kroupa P. (2005), "The Fundamental Building Blocks of Galaxies", in Proceedings of the Gaia Symposium "The Three-Dimensional Universe with Gaia" (ESA SP-576). Held at the Observatoire de Paris-Meudon, 4-7 October 2004. Editors: C. Turon, K.S. O'Flaherty, M.A.C. Perryman, p.629 [http://arxiv.org/abs/astro-ph/0412069 preprint]</ref>.

Salve. Vorrei segnalare che sparse su Wikipedia abbiamo un gran numero di pagine di aiuto e di servizio che sono ''taggate'' ancora col template {{tl|WikiBozza}} (ossia appare la scritta "questa pagina o sezione non ha ancora ricevuto il consenso della comunità"). La massima parte di queste pagine viene regolarmente e ovviamente citata un po' ovunque, proprio perché le indicazioni in esse contenute hanno un senso. Ciò significa che un consenso sulla loro validità esiste. Per altro è piuttosto imbarazzante linkare, per esempio, ad un utente nuovo delle regole sensate e più o meno universalmente riconosciute come tali che riportano in cima "questa pagina non ha il consenso della comunità". Io direi di metterci un attimo e "sbozzare" queste pagine, in modo da levare un po' di questi avvisi. L'elenco delle pagine che includono il template è lungo, ma alcune di queste sono ormai di importanza fondamentale; fra queste ricordo le seguenti:

It is common for two or more separate open clusters to form out of the same molecular cloud. In the [[Large Magellanic Cloud]], both [[Hodge 301]] and [[R136]] are forming from the gases of the [[Tarantula Nebula]], while in our own galaxy, tracing back the motion through space of the [[Hyades (star cluster)|Hyades]] and [[Praesepe]], two prominent nearby open clusters, suggests that they formed in the same cloud about 600 million years ago.<ref>Eggen O. J. (1960), ''Stellar groups, VII. The structure of the Hyades group'', Monthly Notices of the Royal Astronomical Society, v. 120, p.540</ref>

*[[Wikipedia:Wheel war]]

*[[Wikipedia:Fonti attendibili#Quali fonti hanno la precedenza?]]

*[[Wikipedia:Regola dei tre ripristini di pagina]] (la "regola dei tre rollback")

*[[Wikipedia:Bot/Etica del manovratore]]

*[[Wikipedia:Wikipedia non è l'asilo]]

*[[Aiuto:Nomi stranieri]]

*[[Aiuto:Timeline]]

Ce ne sarebbero anche altre, ma la questione diventa più complicata, e pertanto proporrò la questione non appena saremo tutti un pochino più rilassati e disposti ad affrontare qualche discussione un po' più impegnativa. Intanto, ci diamo da fare? --[[Utente:Roberto Mura|<span style="color:#32cd32;">'''Roberto'''</span>]] <small>[[Discussioni utente:Roberto Mura|<span style="color:#ffa500; ">'''Segnali all'Indiano'''</span>]]</small> 23:00, 30 mag 2010 (CEST)

Sometimes, two clusters born at the same time will form a binary cluster. The best known example in the Milky Way is the [[Double Cluster]] of h Persei and χ Persei, but at least 10 more double clusters are known to exist.<ref>Subramaniam A., Gorti U., Sagar R., Bhatt H. C. (1995), ''Probable binary open star clusters in the Galaxy'', Astronomy and Astrophysics, v.302, p.86</ref> Many more are known in the [[Small Magellanic Cloud|Small]] and [[Large Magellanic Cloud]]s &mdash; they are easier to detect in external systems than in our own galaxy because [[projection effect]]s can cause unrelated clusters within the Milky Way to appear close to each other.

==Morphology and classification==

[[Image:NGC 2158 Digitized Sky Survey image.gif|thumb|right|250px|[[NGC 2158]] is a rich and concentrated cluster in [[Gemini (constellation)|Gemini]].]]

*gran parte dei criteri di enciclopedicità specifici, come [[Aiuto:Cosa mettere su Wikipedia/Scrittori e libri]], ma qui mi rendo conto che ci sarebbe molto da discutere...

Open clusters range from very sparse clusters with only a few members to large [[agglomeration]]s containing thousands of stars. They usually consist of quite a distinct dense core, surrounded by a more diffuse 'corona' of cluster members. The core is typically about 3&ndash;4&nbsp;[[light year]]s across, with the corona extending to about 20&nbsp;light years from the cluster centre. Typical star densities in the centre of a cluster are about 1.5 stars per [[cubic light year]] (the stellar density near the sun is about 0.003 star per cubic light year).<ref>Nilakshi S.R., Pandey A.K., Mohan V. (2002), ''A study of spatial structure of galactic open star clusters'', Astronomy and Astrophysics, v. 383, p. 153–162</ref>

Open clusters are often classified according to a scheme developed by [[Robert Trumpler]] in 1930. The Trumpler scheme gives a cluster a three part designation, with a [[Roman numeral]] from I-IV indicating its concentration and detachment from the surrounding star field (from strongly to weakly concentrated), an [[Arabic numeral]] from 1 to 3 indicating the range in brightness of members (from small to large range), and ''p'', ''m'' or ''r'' to indication whether the cluster is poor, medium or rich in stars. An 'n' is appended if the cluster lies within [[nebula|nebulosity]].<ref>Trumpler R.J. (1930), ''Preliminary results on the distances, dimensions and space distribution of open star clusters'', Lick Observatory bulletin no. 420, Berkeley : University of California Press, p. 154–188</ref>

Under the Trumpler scheme, the Pleiades are classified as I3rn (strongly concentrated and richly populated with nebulosity present), while the nearby Hyades are classified as II3m (more dispersed, and with fewer members).

==Numbers and distribution==

[[Image:Star cluster in the Small Magellanic Cloud.jpg|thumb|left|250px|NGC 346, an open cluster in the [[Small Magellanic Cloud]].]]

There are over 1,000 known open clusters in our galaxy, but the true total may be up to ten times higher than that.<ref>Dias W.S., Alessi B.S., Moitinho A., Lépine J.R.D. (2002), ''New catalogue of optically visible open clusters and candidates'', Astronomy and Astrophysics, v. 389, p. 871–873</ref> In [[spiral galaxy|spiral galaxies]], open clusters are invariably found in the spiral arms where gas densities are highest and so most star formation occurs, and clusters usually disperse before they have had time to travel beyond their spiral arm. Open clusters are strongly concentrated close to the galactic plane, with a [[scale height]] in our galaxy of about 180&nbsp;light years, compared to a galactic radius of approximately 100,000&nbsp;light years.<ref>Janes K.A., Phelps R.L. (1980), ''The galactic system of old star clusters: The development of the galactic disk'', The Astronomical Journal, v. 108, p. 1773–1785</ref>

In [[irregular galaxy|irregular galaxies]], open clusters may be found throughout the galaxy, although their concentration is highest where the gas density is highest. Open clusters are not seen in [[elliptical galaxy|elliptical galaxies]]: star formation ceased many millions of years ago in ellipticals, and so the open clusters which were originally present have long since dispersed.

In our galaxy, the distribution of clusters depends on age, with older clusters being preferentially found at greater distances from the [[galactic centre]]. [[Tidal force]]s are stronger nearer the centre of the galaxy, increasing the rate of disruption of clusters, and also the giant molecular clouds which cause the disruption of clusters are concentrated towards the inner regions of the galaxy, so clusters in the inner regions of the galaxy tend to get dispersed at a younger age than their counterparts in the outer regions.<ref>van den Bergh S., McClure R.D. (1980), ''Galactic distribution of the oldest open clusters'', Astronomy & Astrophysics, v.88, p.360</ref>

==Stellar composition==

[[Image:Tarantula nebula detail.jpg|thumb|right|250px|A cluster of stars a few million years old at the lower right illuminates the [[Tarantula Nebula]] in the [[Large Magellanic Cloud]].]]

Because open clusters tend to be dispersed before most of their stars reach the end of their lives, the light from them tends to be dominated by the young, hot blue stars. These stars are the most massive, and have the shortest lives of a few tens of millions of years. The older open clusters tend to contain more yellow stars.

Some open clusters contain hot blue stars which seem to be much younger than the rest of the cluster. These [[blue straggler]]s are also observed in globular clusters, and in the very dense cores of globulars they are believed to arise when stars collide, forming a much hotter, more massive star. However, the stellar density in open clusters is much lower than that in globular clusters, and stellar collisions cannot explain the numbers of blue stragglers observed. Instead, it is thought that most of them probably originate when dynamical interactions with other stars cause a binary system to coalesce into one star.<ref>Andronov N., Pinsonneault M., Terndrup D. (2003), ''Formation of Blue Stragglers in Open Clusters'', American Astronomical Society Meeting 203</ref>

Once they have exhausted their supply of [[hydrogen]] through [[nuclear fusion]], medium to low mass stars shed their outer layers to form a [[planetary nebula]] and evolve into [[white dwarf]]s. While most clusters become dispersed before a large proportion of their members have reached the white dwarf stage, the number of white dwarfs in open clusters is still generally much lower than would be expected, given the age of the cluster and the expected initial mass distribution of the stars. One possible explanation for the lack of white dwarfs is that when a [[red giant]] expels its outer layers to become a planetary nebula, a slight asymmetry in the loss of material could give the star a 'kick' of a few [[kilometre per second|kilometres per second]], enough to eject it from the cluster.<ref>Fellhauer M., Lin D.N.C., Bolte M., Aarseth S.J., Williams K.A. (2003), ''The White Dwarf Deficit in Open Clusters: Dynamical Processes'', The Astrophysical Journal, v. 595, pp. L53-L56</ref>

==Eventual fate==

[[Image:Triangulum.nebula.full.jpg|left|thumb|250px|[[NGC 604]] in the [[Triangulum Galaxy]] is a very massive open cluster surrounded by an [[H II region]].]]

Many open clusters are inherently unstable, with a small enough mass that the [[escape velocity]] of the system is lower than the average [[velocity]] of the constituent stars. These clusters will rapidly disperse within a few million years. In many cases, the stripping away of the gas from which the cluster formed by the radiation pressure of the hot young stars reduces the cluster mass enough to allow rapid dispersal.

Clusters which have enough mass to be gravitationally bound once the surrounding nebula has evaporated can remain distinct for many tens of millions of years, but over time internal and external processes tend also to disperse them. Internally, close encounters between members of the cluster will often result in the velocity of one being increased to beyond the escape velocity of the cluster, which results in the gradual 'evaporation' of cluster members.

Externally, about every half-billion years or so an open cluster tends to be disturbed by external factors such as passing close to or through a molecular cloud. The gravitational [[tidal force]]s generated by such an encounter tend to disrupt the cluster. Eventually, the cluster becomes a stream of stars, not close enough to be a cluster but all related and moving in similar directions at similar speeds. The timescale over which a cluster disrupts depends on its initial stellar density, with more tightly packed clusters persisting for longer. Estimated cluster [[half life|half lives]], after which half the original cluster members will have been lost, range from 150&ndash;800 million years, depending on the original density.<ref>de La Fuente M.R. (1998), ''Dynamical Evolution of Open Star Clusters'', Publications of the Astronomical Society of the Pacific, v. 110, pp. 1117–1117</ref>

After a cluster has become gravitationally unbound, many of its constituent stars will still be moving through space on similar trajectories, in what is known as a [[stellar association]], moving cluster or moving group. Several of the brightest stars in the 'Plough' of [[Ursa Major]] are former members of an open cluster which now form such an association, in this case, the [[Ursa Major moving group]]. Eventually their slightly different relative velocities will see them scattered throughout the galaxy. A larger cluster is then known as a stream, if we discover the similar velocities and ages of otherwise unrelated stars.

==Studying stellar evolution==

[[Image:Open cluster HR diagram ages.gif|right|thumb|250px|[[Hertzsprung-Russell diagram]]s for two open clusters. [[NGC 188]] is older, and shows a lower turn off from the [[main sequence]] than that seen in [[Messier 67|M67]].]]

When a [[Hertzsprung-Russell diagram]] is plotted for an open cluster, most stars lie on the [[main sequence]]. The most massive stars have begun to evolve away from the main sequence and are becoming [[red giant]]s; the position of the turn-off from the main sequence can be used to estimate the age of the cluster.

Because the stars in an open cluster are all at roughly the same distance from [[Earth]], and were born at roughly the same time from the same raw material, the differences in apparent brightness among cluster members is due only to their mass. This makes open clusters very useful in the study of stellar evolution, because when comparing one star to another, many of the variable parameters are fixed.

The study of the abundances of [[lithium]] and [[beryllium]] in open cluster stars can give important clues about the evolution of stars and their interior structures. While [[hydrogen]] nuclei cannot fuse to form [[helium]] until the temperature reaches about 10 million&nbsp;[[Kelvin|K]], lithium and beryllium are destroyed at temperatures of 2.5 million&nbsp;K and 3.5 million&nbsp;K respectively. This means that their abundances depend strongly on how much mixing occurs in stellar interiors. By studying their abundances in open cluster stars, variables such as age and chemical composition are fixed.

Studies have shown that the abundances of these light elements are much lower than models of stellar evolution predict. While the reason for this underabundance is not yet fully understood, one possibility is that [[convection]] in stellar interiors can 'overshoot' into regions where [[radiant energy|radiation]] is normally the dominant mode of energy transport.<ref>VandenBerg, D.A., Stetson P.B. (2004), ''On the Old Open Clusters M67 and NGC 188: Convective Core Overshooting, Color-Temperature Relations, Distances, and Ages'', Publications of the Astronomical Society of the Pacific, v. 116, pp. 997–1011</ref>

==Open clusters and the astronomical distance scale==

[[Image:M11, the Wild Duck Cluster.jpg|left|thumb|250px|[[Wild Duck Cluster|M11]], the Wild Duck Cluster is a very rich cluster located towards the center of the [[Milky Way]].]]

Determining the distances to astronomical objects is crucial to understanding them, but the vast majority of objects are too far away for their distances to be directly determined. Calibration of the [[cosmic distance ladder|astronomical distance scale]] relies on a sequence of indirect and sometimes uncertain measurements relating the closest objects, for which distances can be directly measured, to increasingly distant objects. Open clusters are a crucial step in this sequence.

The closest open clusters can have their distance measured directly by one of two methods. First, the [[parallax]] (the small change in apparent position over the course of a year caused by the Earth moving from one side of its orbit around the Sun to the other) of stars in close open clusters can be measured, like other individual stars. Clusters such as the Pleiades, Hyades and a few others within about 500&nbsp;light years are close enough for this method to be viable, and results from the [[Hipparcos]] position-measuring satellite yielded accurate distances for several clusters.<ref>Brown A.G.A. (2001), ''Open clusters and OB associations: a review'', Revista Mexicana de Astronomía y Astrofísica, v. 11, p89–96</ref>

The other direct method is the so-called [[moving cluster method]]. This relies on the fact that the stars of a cluster share a common motion through space. Measuring the proper motions of cluster members and plotting their apparent motions across the sky will reveal that they converge on a [[vanishing point]]. The radial velocity of cluster members can be determined from [[Doppler shift]] measurements of their [[electromagnetic spectrum|spectra]], and once the radial velocity, proper motion and angular distance from the cluster to its vanishing point are known, simple [[trigonometry]] will reveal the distance to the cluster. The [[Hyades (star cluster)|Hyades]] are the best known application of this method, which reveals their distance to be 46.3&nbsp;[[parsec]]s.<ref>Hanson R.B. (1975), ''A study of the motion, membership, and distance of the Hyades cluster'', Astronomical Journal, v. 80, p. 379–401</ref>

Once the distances to nearby clusters have been established, further techniques can extend the distance scale to more distant clusters. By matching the [[main sequence]] on the Hertzsprung-Russell diagram for a cluster at a known distance with that of a more distant cluster, the distance to the more distant cluster can be estimated. The nearest open cluster is the [[Hyades (star cluster)|Hyades]]: the stellar association consisting of most of the [[Ursa Major Moving Group|Plough stars]] is at about half the distance of the [[Hyades (star cluster)|Hyades]], but is a stellar association rather than an open cluster as the stars are not gravitationally bound to each other. The most distant known open cluster in our galaxy is [[Berkeley 29]], at a distance of about 15,000&nbsp;parsecs.<ref>Bragaglia A., Held E.V., Tosi M. (2005), ''Radial velocities and membership of stars in the old, distant open cluster Berkeley 29'', Astronomy and Astrophysics, v. 429, p. 881–886</ref> Open clusters are also easily detected in many of the galaxies of the [[Local Group]].

Accurate knowledge of open cluster distances is vital for calibrating the period-luminosity relationship shown by [[variable star]]s such as [[cepheid]] and [[RR Lyrae]] stars, which allows them to be used as [[standard candle]]s. These luminous stars can be detected at great distances, and are then used to extend the distance scale to nearby galaxies in the Local Group.

==See also==

* [[Stellar associations]]

* [[Moving groups]]

* [[Open cluster family]]

* [[Open cluster remnant]]

* [[Star clusters]]

==References==

{{reflist}}

==Further reading==

* {{cite book | author=W.J. Kaufmann | title=Universe | publisher=W H Freeman | year=1994 | id=ISBN 0-7167-2379-4}}

* {{cite book | author=E.V.P. Smith, K.C. Jacobs, M. Zeilik, S.A. Gregory | title=Introductory Astronomy and Astrophysics | publisher=Thomson Learning | year=1997 | id=ISBN 0-03-006228-4}}

==External links==

{{Spoken Wikipedia|En-Open cluster-article.ogg|2006-07-16}}

* [http://www.sky-map.org/?ra=12.895&de=-60.3667&zoom=10&img_source=IMG_all The Jewel Box (also known as NGC 4755 or Kappa Crucis Cluster) - open cluster in the Crux constellation @ SKY-MAP.ORG]

* [http://www.seds.org/messier/open.html Open Star Clusters @ SEDS Messier pages]

* [http://www.peripatus.gen.nz/Astronomy/OpeClu.html A general overview of open clusters]

* [http://www.armandocaussade.com/astronomy/open_and_globular.html Open and globular clusters overview]

* [http://zebu.uoregon.edu/~soper/Stars/movingcluster.html The moving cluster method]

* [http://www.nightskyinfo.com/open_clusters Open Clusters - Information and amateur observations]

* [http://www.space-and-telescope.com/MessierObjects.aspx Clickable table of Messier objects including open clusters]

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