Open cluster

An open cluster is a group of up to a few thousand stars that were formed from the same giant molecular cloud and have roughly the same age. More than 1,100 open clusters have been discovered within the Milky Way Galaxy, and many more are thought to exist.[2] They are loosely bound by mutual gravitational attraction and become disrupted by close encounters with other clusters and clouds of gas as they orbit the galactic center. This can result in a migration to the main body of the galaxy and a loss of cluster members through internal close encounters.[3] Open clusters generally survive for a few hundred million years, with the most massive ones surviving for a few billion years. In contrast, the more massive globular clusters of stars exert a stronger gravitational attraction on their members, and can survive for longer. Open clusters have been found only in spiral and irregular galaxies, in which active star formation is occurring.[4]

Young open clusters may be contained within the molecular cloud from which they formed, illuminating it to create an H II region.[5] Over time, radiation pressure from the cluster will disperse the molecular cloud. Typically, about 10% of the mass of a gas cloud will coalesce into stars before radiation pressure drives the rest of the gas away.

Open clusters are key objects in the study of stellar evolution. Because the cluster members are of similar age and chemical composition, their properties (such as distance, age, metallicity, extinction, and velocity) are more easily determined than they are for isolated stars.[2] A number of open clusters, such as the Pleiades, Hyades or the Alpha Persei Cluster are visible with the naked eye. Some others, such as the Double Cluster, are barely perceptible without instruments, while many more can be seen using binoculars or telescopes. The Wild Duck Cluster, M11, is an example.[6]

The star cluster NGC 3572 and its dramatic surroundings
Star cluster NGC 3572 and its surroundings.[1]

Historical observations

VISTA Finds Star Clusters Galore
Mosaic of 30 open clusters discovered from VISTA's data. The open clusters were hidden by the dust in the Milky Way.[7] Credit ESO.

The prominent open cluster the Pleiades has been recognized as a group of stars since antiquity, while the Hyades forms part of Taurus, one of the oldest constellations. Other open clusters were noted by early astronomers as unresolved fuzzy patches of light. The Roman astronomer Ptolemy mentions the Praesepe, the Double Cluster in Perseus, and the Ptolemy Cluster, while the Persian astronomer Al-Sufi wrote of the Omicron Velorum cluster.[8] However, it would require the invention of the telescope to resolve these nebulae into their constituent stars.[9] Indeed, in 1603 Johann Bayer gave three of these clusters designations as if they were single stars.[10]

NGC 3590 open cluster Eso1416a
The colorful star cluster NGC 3590.[11]

The first person to use a telescope to observe the night sky and record his observations was the Italian scientist Galileo Galilei in 1609. When he turned the telescope toward some of the nebulous patches recorded by Ptolemy, he found they were not a single star, but groupings of many stars. For Praesepe, he found more than 40 stars. Where previously observers had noted only 6–7 stars in the Pleiades, he found almost 50.[12] In his 1610 treatise Sidereus Nuncius, Galileo Galilei wrote, "the galaxy is nothing else but a mass of innumerable stars planted together in clusters."[13] Influenced by Galileo's work, the Sicilian astronomer Giovanni Hodierna became possibly the first astronomer to use a telescope to find previously undiscovered open clusters.[14] In 1654, he identified the objects now designated Messier 41, Messier 47, NGC 2362 and NGC 2451.[15]

It was realised as early as 1767 that the stars in a cluster were physically related,[16] when the English naturalist Reverend John Michell calculated 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.[17] Between 1774–1781, French astronomer Charles Messier published a catalogue of celestial objects that had a nebulous appearance similar to comets. This catalogue included 26 open clusters.[10] In the 1790s, English astronomer William Herschel began an extensive study of nebulous celestial objects. He discovered that many of these features could be resolved into groupings of individual stars. Herschel conceived the idea that stars were initially scattered across space, but later became clustered together as star systems because of gravitational attraction.[18] He divided the nebulae into eight classes, with classes VI through VIII being used to classify clusters of stars.[19]

The number of clusters known continued to increase under the efforts of astronomers. Hundreds of open clusters were listed in the New General Catalogue, first published in 1888 by the Danish-Irish astronomer J. L. E. Dreyer, and the two supplemental Index Catalogues, published in 1896 and 1905.[10] Telescopic observations revealed two distinct types of clusters, one of which contained thousands of stars in a regular spherical distribution and was found all across the sky but preferentially towards the centre of the Milky Way.[20] The other type consisted of a generally sparser population of stars in a more irregular shape. These were generally found in or near the galactic plane of the Milky Way.[21][22] Astronomers dubbed the former globular clusters, and the latter open clusters. Because of their location, open clusters are occasionally referred to as galactic clusters, a term that was introduced in 1925 by the Swiss-American astronomer Robert Julius Trumpler.[23]

Micrometer measurements of the positions of stars in clusters were made as early as 1877 by the German astronomer E. Schönfeld and further pursued by the American astronomer E. E. Barnard prior to his death in 1923. No indication of stellar motion was detected by these efforts.[24] However, in 1918 the Dutch-American astronomer Adriaan van Maanen was able to measure the proper motion of stars in part of the Pleiades cluster by comparing photographic plates taken at different times.[25] As astrometry became more accurate, cluster stars were found to share a common proper motion through space. By comparing the photographic plates of the Pleiades cluster taken in 1918 with images taken in 1943, van Maanen was able to identify those stars that had a proper motion similar to the mean motion of the cluster, and were therefore more likely to be members.[26] Spectroscopic measurements revealed common radial velocities, thus showing that the clusters consist of stars bound together as a group.[2]

The first color-magnitude diagrams of open clusters were published by Ejnar Hertzsprung in 1911, giving the plot for the Pleiades and Hyades star clusters. He continued this work on open clusters for the next twenty years. From spectroscopic data, he was able to determine the upper limit of internal motions for open clusters, and could estimate that the total mass of these objects did not exceed several hundred times the mass of the Sun. He demonstrated a relationship between the star colors and their magnitudes, and in 1929 noticed that the Hyades and Praesepe clusters had different stellar populations than the Pleiades. This would subsequently be interpreted as a difference in ages of the three clusters.[27]


Trapezium cluster optical and infrared comparison
Infrared light reveals the dense open cluster forming at the heart of the Orion nebula.

The formation of an open cluster begins with the collapse of part of a giant molecular cloud, a cold dense cloud of gas and dust containing up to many thousands of times the mass of the Sun. These clouds have densities that vary from 102 to 106 molecules of neutral hydrogen per cm3, with star formation occurring in regions with densities above 104 molecules per cm3. Typically, only 1–10% of the cloud by volume is above the latter density.[28] Prior to collapse, these clouds maintain their mechanical equilibrium through magnetic fields, turbulence, and rotation.[29]

Many factors may disrupt the equilibrium of a giant molecular cloud, triggering a collapse and initiating the burst of star formation that can result in an open cluster. These include shock waves from a nearby supernova, collisions with other clouds, or gravitational interactions. Even without external triggers, regions of the cloud can reach conditions where they become unstable against collapse.[29] The collapsing cloud region will undergo hierarchical fragmentation into ever smaller clumps, including a particularly dense form known as infrared dark clouds, eventually leading to the formation of up to several thousand stars. This star formation begins enshrouded in the collapsing cloud, blocking the protostars from sight but allowing infrared observation.[28] In the Milky Way galaxy, the formation rate of open clusters is estimated to be one every few thousand years.[30]

Eagle nebula pillars
The so-called "Pillars of Creation", a region of the Eagle Nebula where the molecular cloud is being evaporated by young, massive stars

The hottest and most massive of the newly formed stars (known as OB stars) will emit intense ultraviolet radiation, which steadily ionizes the surrounding gas of the giant molecular cloud, forming an H II region. Stellar winds and radiation pressure from the massive stars begins to drive away the hot ionized gas at a velocity matching the speed of sound in the gas. After a few million years the cluster will experience its first core-collapse supernovae, which will also expel gas from the vicinity. In most cases these processes will strip the cluster of gas within ten million years and no further star formation will take place. Still, about half of the resulting protostellar objects will be left surrounded by circumstellar disks, many of which form accretion disks.[28]

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 star formation process. All clusters thus suffer significant infant weight loss, while a large fraction undergo infant mortality. At this point, the formation of an open cluster will depend on whether the newly formed stars are gravitationally bound to each other; otherwise an unbound stellar association will result. Even when a cluster such as the Pleiades does form, it may only hold on to a third of the original stars, with the remainder becoming unbound once the gas is expelled.[31] 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.[32] Most open clusters form with at least 100 stars and a mass of 50 or more solar masses. The largest clusters can have 104 solar masses, with the massive cluster Westerlund 1 being estimated at 5 × 104 solar masses; close to that of a globular cluster.[28] 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 clusters form via the same basic mechanism, with the difference being that the conditions that allowed the formation of the very rich globular clusters containing hundreds of thousands of stars no longer prevail in the Milky Way.[33]

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 and Praesepe, two prominent nearby open clusters, suggests that they formed in the same cloud about 600 million years ago.[34] 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 NGC 869 and NGC 884 (sometimes mistakenly called h and χ Persei; h refers to a neighboring star and χ to both clusters), but at least 10 more double clusters are known to exist.[35] Many more are known in the Small and Large Magellanic Clouds—they are easier to detect in external systems than in our own galaxy because projection effects can cause unrelated clusters within the Milky Way to appear close to each other.

Morphology and classification

The colourful star cluster NGC 2367
NGC 2367 is an infant stellar grouping that lies at the center of an immense and ancient structure on the margins of the Milky Way.[36]

Open clusters range from very sparse clusters with only a few members to large agglomerations 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 3–4 light years across, with the corona extending to about 20 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 stars per cubic light year.[37]

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 nebulosity.[38]

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

NGC 346 in Small magellanic cloud
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.[39] In spiral galaxies, open clusters are largely 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 light years, compared to a galactic radius of approximately 50,000 light years.[40]

In irregular galaxies, open clusters may be found throughout the galaxy, although their concentration is highest where the gas density is highest.[41] Open clusters are not seen in 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.[42]

In our galaxy, the distribution of clusters depends on age, with older clusters being preferentially found at greater distances from the galactic centre, generally at substantial distances above or below the galactic plane.[43] Tidal forces 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.[44]

Stellar composition

Tarantula nebula detail
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 stragglers 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.[45]

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 dwarfs. 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 kilometres per second, enough to eject it from the cluster.[46]

Because of their high density, close encounters between stars in an open cluster are common. For a typical cluster with 1,000 stars with a 0.5 parsec half-mass radius, on average a star will have an encounter with another member every 10 million years. The rate is even higher in denser clusters. These encounters can have a significant impact on the extended circumstellar disks of material that surround many young stars. Tidal perturbations of large disks may result in the formation of massive planets and brown dwarfs, producing companions at distances of 100 AU or more from the host star.[47]

Eventual fate

Nursery of New Stars - GPN-2000-000972
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.[48]

Clusters that 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 stars can increase the velocity of a member beyond the escape velocity of the cluster. This results in the gradual 'evaporation' of cluster members.[49]

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 forces 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 lives, after which half the original cluster members will have been lost, range from 150–800 million years, depending on the original density.[49]

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.[50] 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.[51][52]

Studying stellar evolution

Open cluster HR diagram ages
Hertzsprung-Russell diagrams for two open clusters. NGC 188 is older, and shows a lower turn off from the main sequence than that seen in M67.

When a Hertzsprung-Russell diagram is plotted for an open cluster, most stars lie on the main sequence.[53] The most massive stars have begun to evolve away from the main sequence and are becoming red giants; 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.[53] 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 K, lithium and beryllium are destroyed at temperatures of 2.5 million K and 3.5 million 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.[54]

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 radiation is normally the dominant mode of energy transport.[54]

Astronomical distance scale

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 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.[55] 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 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.[56][57]

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 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 are the best known application of this method, which reveals their distance to be 46.3 parsecs.[58]

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: the stellar association consisting of most of the Plough stars is at about half the distance of the 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 parsecs.[59] 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 stars such as cepheid stars, which allows them to be used as standard candles. 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.[60] Indeed, the open cluster designated NGC 7790 hosts three classical Cepheids.[61][62] RR Lyrae variables are too old to be associated with open clusters, and are instead found in globular clusters.


The open cluster NGC 6811 contains two known planetary systems Kepler 66 and Kepler 67.

See also


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Further reading

  • Kaufmann, W. J. (1994). Universe. W H Freeman. ISBN 0-7167-2379-4.
  • Smith, E.V.P.; Jacobs, K.C.; Zeilik, M.; Gregory, S.A. (1997). Introductory Astronomy and Astrophysics. Thomson Learning. ISBN 0-03-006228-4.

External links

Herschel 400 Catalogue

The Herschel 400 catalogue is a subset of William Herschel's original Catalogue of Nebulae and Clusters of Stars, selected by Brenda F. Guzman (Branchett), Lydel Guzman, Paul Jones, James Morrison, Peggy Taylor and Sara Saey of the Ancient City Astronomy Club in St. Augustine, Florida, United States c. 1980. They decided to generate the list after reading a letter published in Sky & Telescope by James Mullaney of Pittsburgh, Pennsylvania, USA.In this letter Mr. Mullaney suggested that William Herschel's original catalogue of 2,500 objects would be an excellent basis for deep sky object selection for amateur astronomers looking for a challenge after completing the Messier Catalogue.

The Herschel 400 is a subset of John Herschel's General Catalogue of Nebulae and Clusters published in 1864 of 5,000 objects, and hence also of the New General Catalogue.

The catalogue forms the basis of the Astronomical League's Herschel 400 club. In 1997, another subset of 400 Herschel objects was selected by the Rose City Astronomers of Portland, Oregon as the Herschel II list, which forms the basis of the Astronomical League's Herschel II Program.

Messier 103

Messier 103 (also known as M103, or NGC 581) is an open cluster where a few thousand stars formed in the constellation Cassiopeia. This open cluster was discovered in 1781 by Charles Messier's friend and collaborator Pierre Méchain. It is one of the most distant open clusters known, with distances of 8,000 to 9,500 light-years from the earth and ranging about 15 light-years apart. There are about 40 member stars within M103, two of which have magnitudes 10.5, and a 10.8 red giant, which is the brightest within the cluster. Observation of M103 is generally dominated by the appearance of Struve 131, though the star is not a member of the 172-star cluster. M103 is about 25 million years old.

Messier 34

Messier 34 (also known as M34 or NGC 1039) is an open cluster in the constellation Perseus. It was probably discovered by Giovanni Batista Hodierna before 1654 and included by Charles Messier in his catalog of comet-like objects in 1764. Messier described it as, "A cluster of small stars a little below the parallel of γ (Andromedae). In an ordinary telescope of 3 feet one can distinguish the stars."Based on the distance modulus of 8.38, this cluster is located at a distance of about 470 parsecs, or 1,500 light years. For stars in the range from 0.12 to 1.0 solar masses, M34 contains an estimated 400 members. It spans about 35' on the sky which translates to a true radius of 7.5 light years. The cluster is just visible to the naked eye in very dark conditions, well away from city lights. It is possible to see it in binoculars when light pollution is limited.The age of this cluster lies between the respective ages of the Pleiades open cluster at 100 million years and the Hyades open cluster at 800 million years. Comparisons between the observed stellar spectra and the values predicted by stellar evolutionary models gives an age estimate of 200–250 million years for M34. This is roughly the age at which stars with 0.5 solar masses enter the main sequence. By comparison, stars like the Sun enter the main sequence after 30 million years.The average proportion of elements with higher atomic numbers than helium is termed the metallicity by astronomers. This is expressed by the logarithm of the ratio of iron to hydrogen, compared to the same proportion in the Sun. For M34, the metallicity has a value of [Fe/H] = +0.07 ± 0.04. This is equivalent to a 17% higher proportion of iron compared to the Sun. Other elements show a similar abundance, with the exception of nickel which is underabundant.At least 19 members of this cluster are white dwarfs. These are stellar remnants of progenitor stars of up to eight solar masses that have evolved through the main sequence and are no longer engaged in thermonuclear fusion to generate energy. Seventeen of the white dwarfs are of spectral type DA or DAZ, while one is a type DB and the last is a type DC.

Messier 35

Messier 35 or M35, also known as NGC 2168, is an open cluster of stars in the northern constellation of Gemini. It was discovered by Philippe Loys de Chéseaux around 1745 and independently discovered by John Bevis before 1750. The cluster is scattered over an area of the sky almost the size of the full moon and is located 3,870 light-years (1,186 parsecs) from Earth. The compact open cluster NGC 2158 lies directly southwest of M35.

Leonard & Merritt (1989) computed the mass of M35 using a statistical technique based on proper motion velocities of its stars. The mass within the central 3.75 parsecs was found to be between 1600 and 3200 solar masses (95 percent confidence), consistent with the mass of a realistic stellar population within the same radius. Bouy et al. (2015) found a mass of around 1,600 M☉ within the central 27.5′ × 27.5′. There are 305 candidate members with a probability of 95% or higher, and up to 4,349 with a 50% membership probability. The cluster metallicity is given by [Fe/H] = −0.21±0.10.Of 418 probable cluster members, Leiner et al. (2015) found 64 that have variable radial velocities and thus are binary star systems. Four probable cluster members are chemically peculiar stars, while HD 41995, which lies within the cluster area, shows emission lines. Hu et al. (2005) found 13 variable stars in the cluster field, although at least three are suspect as cluster members.

Messier 36

Messier 36 or M36, also known as NGC 1960, is an open cluster of stars in the Auriga constellation. It was discovered by Giovanni Batista Hodierna before 1654, who described it as a nebulous patch. The cluster was independently re-discovered by Guillaume Le Gentil in 1749, then Charles Messier observed it in 1764 and added it to his catalogue. M36 is at a distance of about 1,330 pc (4,340 light years) away from Earth. The cluster is very similar to the Pleiades cluster (M45), and if it were the same distance from Earth it would be of similar magnitude.This cluster has an angular diameter of 10′ and a core radius of 3.2′. It has a mass of roughly 746 M☉ and a linear tidal radius of 10.6±1.6 pc. Based upon photometry, the age of the cluster has been estimated by Wu et al. (2009) as 25.1 Myr and 26.3+3.2−5.2 Myr by Bell et al. (2013). The luminosity of the stars that have not yet consumed their lithium implies an age of 22±4 Myr, in good agreement these previous estimates.M36 includes ten stars with a visual magnitude brighter than 10, and 178 down to magnitude 14. 38 members display an infrared excess, with one being particularly high. There is one candidate B-type variable star, which is 9th magnitude.

Messier 37

Messier 37 (also known as M37 or NGC 2099) is the richest open cluster in the constellation Auriga. It is the brightest of three open clusters in Auriga and was discovered by the Italian astronomer Giovanni Battista Hodierna before 1654. M37 was missed by French astronomer Guillaume Le Gentil when he rediscovered M36 and M38 in 1749. French astronomer Charles Messier independently rediscovered M37 in September 1764 but all three clusters were recorded by Hodierna. It is classified as Trumpler type I,1,r or I,2,r.

M37 is located in the antipodal direction, opposite from the Galactic Center as seen from Earth. Estimates of its age range from 347 million to 550 million years. It has 1,500 times the mass of the Sun and contains over 500 identified stars, with roughly 150 stars brighter than magnitude 12.5. M37 has at least a dozen red giants and its hottest surviving main sequence star is of stellar classification B9 V. The abundance of elements other than hydrogen and helium, what astronomers term metallicity, is similar to, if not slightly higher than, the abundance in the Sun.At its estimated distance of around 4,500 light-years (1,400 parsecs) from Earth, the cluster's angular diameter of 24 arcminutes corresponds to a physical extent of about 20–25 ly (6.1–7.7 pc). The tidal radius of the cluster, where external gravitational perturbations begin to have a significant influence on the orbits of its member stars, is about 46–59 ly (14–18 pc). This cluster is following an orbit through the Milky Way with a period of 219.3 Ma and an eccentricity of 0.22. This will bring it as close as 19.6 kly (6.0 kpc) to, and as distant as 30.7 kly (9.4 kpc) from, the Galactic Center. It reaches a peak distance above the galactic plane of 0.29 kly (0.089 kpc) and will cross the plane with a period of 31.7 Ma.

Messier 39

Messier 39 or M39, also known as NGC 7092, is an open cluster of stars in the constellation of Cygnus, positioned two degrees to the south of the star Pi Cygni and around 9° east-northeast of Deneb. The cluster was discovered by Guillaume Le Gentil in 1749, then Charles Messier added it to his catalogue in 1764. When observed in a small telescope at low power the cluster shows around two dozen members, but it is best observed with binoculars. It has a total integrated magnitude (brightness) of 5.5 and spans an angular diameter of 29 arcminutes – about the size of the full Moon. M39 is at a distance of about 1,010 light-years (311 parsecs) from the Sun.

This cluster has an estimated mass of 232 M☉ and a linear tidal radius of 8.6±1.8 pc. Of the 15 brightest components, six form binary star systems with one more suspected. HD 205117 is a probable eclipsing binary system with a period of 113.2 days that varies by 0.051 in visual magnitude. Both members appear to be subgiant stars. There are at least five chemically peculiar stars in the cluster and ten suspected short-period variable stars.

Messier 41

Messier 41 (also known as M41 or NGC 2287) is an open cluster in the constellation Canis Major. It was discovered by Giovanni Batista Hodierna before 1654 and was perhaps known to Aristotle about 325 BC. M41 lies about four degrees almost exactly south of Sirius, and forms a triangle with it and Nu2 Canis Majoris—all three can be seen in the same field in binoculars. The cluster itself covers an area around the size of the full moon. It contains about 100 stars including several red giants, the brightest being a spectral type K3 giant of apparent magnitude 6.3 near the cluster's center, and a number of white dwarfs. The cluster is estimated to be moving away from us at 23.3 km/s. The diameter of the cluster is between 25 and 26 light years. It is estimated to be 190 million years old, and cluster properties and dynamics suggest a total life expectancy of 500 million years for this cluster, before it will have disintegrated.Walter Scott Houston describes the appearance of the cluster in small telescopes:

Many visual observers speak of seeing curved lines of stars in M41. Although they seem inconspicuous on photographs, the curves stand out strongly in my 10-inch [reflecting telescope], and the bright red star near the center of the cluster is prominent.

Messier 46

Messier 46 or M46, also known as NGC 2437, is an open cluster of stars in the constellation of Puppis. It was discovered by Charles Messier in 1771. Dreyer described it as "very bright, very rich, very large." M46 is about 4,920 light-years away. There are an estimated 500 stars in the cluster with a combined mass of 453 M☉, and it is thought to be some 251.2 million years old.The cluster has a tidal radius of 37.8 ± 4.6 ly (11.6 ± 1.4 pc) and a core radius of 8.5 ± 1.3 ly (2.6 ± 0.4 pc). It has a greater spatial extend in the infrared than in visible light, suggesting that the cluster is undergoing some mass segregation with the fainter (redder) stars migrating to a coma region. The fainter stars that extend out to the south and west may form a tidal tail due to a past interaction.The planetary nebula NGC 2438 appears to lie within the cluster near its northern edge (the faint smudge at the top center of the image), but it is most likely unrelated since it does not share the cluster's radial velocity. It is an example of a superimposed pair possibly similar to that of NGC 2818.

On the other hand, the illuminating star of the bipolar Calabash Nebula shares the radial velocity and proper motion of Messier 46, and is at the same distance, so is a bona fide member of the open cluster.M46 is located close by to another open cluster, Messier 47. M46 is about a degree east of M47 in the sky, so the two fit well in a binocular or wide-angle telescope field.

Messier 48

Messier 48 or M48, also known as NGC 2548, is an open cluster of stars in the constellation of Hydra. It was discovered by Charles Messier in 1771. There is no cluster in the position indicated by Messier. The value that he gave for the right ascension matches that of NGC 2548, however, his declination is off by five degrees. Credit for discovery is sometimes given instead to Caroline Herschel in 1783. Her nephew John Herschel described it as, "a superb cluster which fills the whole field; stars of 9th and 10th to the 13th magnitude – and none below, but the whole ground of the sky on which it stands is singularly dotted over with infinitely minute points". The brightest component is HIP 40348 at visual magnitude 8.3.M48 is visible to the naked eye under good atmospheric conditions. It is located some 2,500 light-years from the Sun. The age estimated from isochrones is 400±100 Myr, while gyrochronology age estimate is 450±50 Myr – in good agreement. The cluster has a tidal radius of 63.3 ± 7.8 ly (19.4 ± 2.4 pc) with at least 438 members and a mass of 2,366 M☉. The general structure of the cluster is fragmented and lumpy, which may be due to interactions with the galactic disk. The cluster is now subdivided into three groups, each of which has its own collective proper motion.

Messier 52

Messier 52 or M52, also known as NGC 7654, is an open cluster of stars in the northern constellation of Cassiopeia. It was discovered by Charles Messier on September 7, 1774. M52 can be seen from Earth with binoculars. The brightness of the cluster is influenced by extinction, which is stronger in the southern half.R. J. Trumpler classified the cluster appearance as II2r, indicating a rich cluster with little central concentration and a medium range in the brightness of the stars. This was later revised to I2r, denoting a dense core. The cluster has a core radius of 2.97 ± 0.46 ly (0.91 ± 0.14 pc) and a tidal radius of 42.7 ± 7.2 ly (13.1 ± 2.2 pc). It has an estimated age of 158.5 million years and a mass of 1,200 M☉.The magnitude 8.3 supergiant star BD +60°2532 is a probable member of M52. The stellar population includes 18 candidate slowly pulsating B stars, one of which is a δ Scuti variable, and three candidate γ Dor variables. There may also be three Be stars. The core of the cluster shows a lack of interstellar matter, which may be the result of supernovae explosions early in the cluster's history.

Messier 67

Messier 67 (also known as M67 or NGC 2682) is an open cluster in the constellation of Cancer. M67's Trumpler class is variously given as II 2 r, II 2 m, or II 3 r. It was discovered by Johann Gottfried Koehler in 1779. Age estimates for the cluster range between 3.2 and 5 billion years, with the most recent estimate (4 Gyr) implying stars in M67 are younger than the Sun. Distance estimates are likewise varied and typically range between 800–900 pc. Recent estimates of 855, 840, and 815 pc were established via binary star modelling and infrared color-magnitude diagram fitting, accordingly.

M67 is not the oldest known open cluster, but Galactic clusters known to be older are few, and none of those is closer than M67. The latter is an important laboratory for studying stellar evolution, since the cluster is well populated, negligible amounts of dust obscuration, and all its stars are at the same distance and age, except for approximately 30 anomalous blue stragglers, whose origins are not fully understood.

M67 is probably the second best observed open cluster after the Hyades cluster, which is amongst the nearest open clusters and younger than M67. M67 is one of the most-studied open clusters, yet estimates of its physical parameters such as age, mass, and number of stars of a given type, vary substantially. Richer et al. estimate its age to be 4 billion years, its mass to be 1080 solar masses, and the number of white dwarfs to be 150. Hurley et al. estimate its current mass to be 1400 solar masses and its initial mass to be approximately 10 times as great.

M67 has more than 100 stars similar to the Sun, and numerous red giants. The total star count has been estimated at well over 500. The ages and prevalence of Sun-like stars contained within the cluster had led astronomers to consider M67 as the possible parent cluster of the Sun. However, computer simulations have suggested that this is highly unlikely to be the case.The cluster contains no main sequence stars bluer than spectral type F, other than perhaps some of the blue stragglers, since the brighter stars of that age have already left the main sequence. In fact, when the stars of the cluster are plotted on the Hertzsprung-Russell diagram, there is a distinct "turn-off" representing the stars which have terminated hydrogen fusion in the core and are destined to become red giants. As the cluster ages, the turn-off moves progressively down the main sequence.

It appears that M67 does not contain an unbiased sample of stars. One cause of this is mass segregation, the process by which lighter stars (actually, systems) gain speed at the expense of more massive stars during close encounters, which causes the lighter stars to be at a greater average distance from the center of the cluster or to escape altogetherA March 2016 joint AIP/JHU study by Barnes at al. on rotational periods of 20 Sun-like stars, measured by the effects of moving starspots on light curves, suggests that these approximately 4 Gyr old stars spin for about 26 days - much like our Sun, which has a period at the equator of 25.38 days. Measurements were carried out as part of the extended K2 mission of Kepler space telescope. This discovery strengthens the solar-stellar connection, a fundamental principle of modern solar and Stellar astrophysics. Sydney Barnes (first author of the study) commented: "We had predicted this would occur, but it has been a real privilege to have been able to actually make the measurements." Co-author Jörg Weingrill adds: "With the measured rotational periods for stars up to the age of our Sun we can now confidently trace back the evolution of our home star."

Messier 93

Messier 93 or M93, also known as NGC 2447, is an open cluster in the constellation Puppis. It was discovered by Charles Messier then added to his catalogue of comet-like objects on March 20, 1781. Caroline Herschel, the younger sister of William Herschel, independently discovered M93 in 1783, thinking it had not yet been catalogued by Messier. Walter Scott Houston described its appearance as follows:

Some observers mention the cluster as having the shape of a starfish. With a fair-sized telescope, this is its appearance on a dull night, but [a four-inch refractor] shows it as a typical star-studded galactic cluster.

It has a Trumpler class of I 3 r, indicating it is strongly concentrated (I) with a large range in brightness (3) and is rich in stars (r).M93 is at a distance of about 3,380 light years from Earth and has a spatial radius of some 5 light years, a tidal radius of 13.1±2.3 ly, and a core radius of 4.2 ly. Its age is estimated at 387.3 million years. The cluster is positioned nearly on the galactic plane and it is following an orbit that varies between 28–29 kly (8.5–8.9 kpc) from the Galactic Center over a period of 242.7±7.9 Myr.54 variable stars have been found in M93, including one slowly pulsating B-type star, one rotating ellipsoidal variable, seven Delta Scuti variables, six Gamma Doradus variables, and one hybrid δ Sct/γ Dor pulsator. Four spectroscopic binary systems in M93 include a yellow straggler component.

NGC 188

NGC 188 is an open cluster in the constellation Cepheus. It was discovered by John Herschel in 1825.

Unlike most open clusters that drift apart after a few million years because of the gravitational interaction of our Milky Way galaxy, NGC 188 lies far above the plane of the galaxy and is one of the most ancient of open clusters known, at approximately 6.8 billion years old. NGC 188 is very close to the North Celestial Pole, under five degrees away, and in the constellation of Cepheus at an estimated 5,000 light-years' distance, this puts it slightly above the Milky Way's disc and further from the center of the galaxy than the Sun.

NGC 225

NGC 225 is an open cluster in the constellation Cassiopeia. It is located roughly 2100 light-years from Earth.

NGC 381

NGC 381 is an open cluster in the Cassiopeia constellation. It was discovered by Caroline Herschel in 1787.

NGC 752

NGC 752 (also known as Caldwell 28) is an open cluster in the constellation Andromeda. The cluster was discovered by Caroline Herschel in 1783 and cataloged by her brother William Herschel in 1786, although an object that may have been NGC 752 was described by Giovanni Batista Hodierna before 1654.The large cluster lies 1,300 light-years away from the Earth and is easily seen through binoculars, although it may approach naked eye visibility under good observing conditions. A telescope reveals about 60 stars no brighter than 9th magnitude within NGC 752.

Orion Arm

The Orion Arm is a minor spiral arm of the Milky Way some 3,500 light-years (1,100 parsecs) across and approximately 10,000 light-years (3,100 parsecs) in length, containing the Solar System, including the Earth. It is also referred to by its full name, the Orion–Cygnus Arm, as well as Local Arm, Orion Bridge, and formerly, the Local Spur and Orion Spur.

The arm is named for the Orion constellation, which is one of the most prominent constellations of Northern Hemisphere winter (Southern Hemisphere summer). Some of the brightest stars and most famous celestial objects of the constellation (e.g. Betelgeuse, Rigel, the three stars of Orion's Belt, the Orion Nebula) are within it as shown on the interactive map below.

The arm is between the Carina–Sagittarius Arm (the local portions of which are toward the Galactic Center) and the Perseus Arm (the local portion of which is the main outer-most arm and one of two major arms of the galaxy).

Long thought to be a minor structure, namely a "spur" between the two arms mentioned, evidence was presented in mid 2013 that the Orion Arm might be a branch of the Perseus Arm, or possibly an independent arm segment.Within the arm, the Solar System is close to its inner rim, in a relative cavity in the arm's Interstellar Medium known as the Local Bubble, about halfway along the arm's length, approximately 8,000 parsecs (26,000 light-years) from the Galactic Center.


The Pleiades (), also known as the Seven Sisters and Messier 45, are an open star cluster containing middle-aged, hot B-type stars located in the constellation of Taurus. It is among the nearest star clusters to Earth and is the cluster most obvious to the naked eye in the night sky.

The cluster is dominated by hot blue and luminous stars that have formed within the last 100 million years. Reflection nebulae around the brightest stars were once thought to be left over material from the formation of the cluster, but are now considered likely to be an unrelated dust cloud in the interstellar medium through which the stars are currently passing.Computer simulations have shown that the Pleiades were probably formed from a compact configuration that resembled the Orion Nebula. Astronomers estimate that the cluster will survive for about another 250 million years, after which it will disperse due to gravitational interactions with its galactic neighborhood.

Visual grouping

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