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78-571: NGC 457 (also designated Caldwell 13 , and known as the Dragonfly Cluster , E.T. Cluster , Owl Cluster , Kachina Doll Cluster or Phi Cassiopeiae Cluster ) is an open star cluster in the constellation Cassiopeia . It was discovered by William Herschel on August 18, 1780, with a 6.2 inch reflector telescope, and catalogued as VII 42. It is an easy target for amateur astronomers, and can be seen even with small telescopes in light-polluted skies. It lies over 7,900 light years away from

156-406: 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

234-485: 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. Spectroscopic measurements revealed common radial velocities , thus showing that the clusters consist of stars bound together as

312-461: A great deal of intrinsic difference between a very sparse globular cluster such as Palomar 12 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. It

390-440: A group. 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

468-460: A lifetime contract for his work in building more powerful telescopes. He desired to return to Florence, and in hopes of gaining patronage there, he dedicated Sidereus Nuncius to his former pupil, now the Grand Duke of Tuscany, Cosimo II de' Medici . In addition, he named his discovered four moons of Jupiter the "Medicean Stars," in honor of the four royal Medici brothers. This helped him receive

546-449: 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 longer. Estimated cluster half lives , after which half

624-458: A more irregular shape. These were generally found in or near the galactic plane of the Milky Way. 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 . Micrometer measurements of

702-406: 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. In the Milky Way galaxy, the formation rate of open clusters is estimated to be one every few thousand years. The hottest and most massive of

780-432: A patent on one. By 1609 Galileo had heard about it and built his own improved version. He probably was not the first person to aim the new invention at the night sky but his was the first systematic (and published) study of celestial bodies using one. One of Galileo's first telescopes had 8x to 10x linear magnification and was made out of lenses that he had ground himself. This was increased to 20x linear magnification in

858-475: 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. In his 1610 treatise Sidereus Nuncius , Galileo Galilei wrote, "the galaxy is nothing else but a mass of innumerable stars planted together in clusters." Influenced by Galileo's work, the Sicilian astronomer Giovanni Hodierna became possibly

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936-410: 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

1014-481: A third of the original stars, with the remainder becoming unbound once the gas is expelled. The young stars so released from their natal cluster become part of the Galactic field population. Because most if not all stars form in clusters, star clusters are to be viewed as the fundamental building blocks of galaxies. The violent gas-expulsion events that shape and destroy many star clusters at birth leave their imprint in

1092-516: A younger age than their counterparts in the outer regions. 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, a few tens of millions of years. The older open clusters tend to contain more yellow stars. The frequency of binary star systems has been observed to be higher within open clusters than outside open clusters. This

1170-653: Is Berkeley 29 , at a distance of about 15,000 parsecs. Open clusters, especially super star clusters , are also easily detected in many of the galaxies of the Local Group and nearby: e.g., NGC 346 and the SSCs R136 and NGC 1569 A and B . 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

1248-611: 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 have formed 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. Sometimes, two clusters born at

1326-407: Is seen as evidence that single stars get ejected from open clusters due to dynamical interactions. 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,

1404-411: 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. 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

1482-404: 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

1560-494: Is usually translated into English as Sidereal Messenger , many of Galileo's early drafts of the book and later related writings indicate that the intended purpose of the book was "simply to report the news about recent developments in astronomy, not to pass himself off solemnly as an ambassador from heaven." The first telescopes appeared in the Netherlands in 1608 when Middelburg spectacle-maker Hans Lippershey tried to obtain

1638-535: The Beehive Cluster . Sidereus Nuncius Sidereus Nuncius (usually Sidereal Messenger , also Starry Messenger or Sidereal Message ) is a short astronomical treatise (or pamphlet ) published in Neo-Latin by Galileo Galilei on March 13, 1610. It was the first published scientific work based on observations made through a telescope, and it contains the results of Galileo's early observations of

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1716-465: The Galactic Center , generally at substantial distances above or below the galactic plane . Tidal forces are stronger nearer the center 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

1794-594: The Pleiades , the Hyades and 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. The prominent open cluster the Pleiades , in the constellation Taurus, has been recognized as a group of stars since antiquity, while

1872-532: The Sun . It has an estimated age of 21 million years. The cluster is sometimes referred by amateur astronomers as the Owl Cluster or the E.T. Cluster (due to its resemblance to the movie character). Two bright stars Phi Cassiopeiae (magnitude 5 and spectral type F0) and HD 7902 (magnitude 7) can be imagined as eyes. It is not yet clear if Phi Cassiopeiae is a member of the cluster, and if it is, then it would be one of

1950-492: 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 well-separated stars. 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

2028-712: 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

2106-471: The mass of the Sun . These clouds have densities that vary from 10 to 10 molecules of neutral hydrogen per cm , with star formation occurring in regions with densities above 10 molecules per cm . Typically, only 1–10% of the cloud by volume is above the latter density. Prior to collapse, these clouds maintain their mechanical equilibrium through magnetic fields, turbulence and rotation. Many factors may disrupt

2184-614: 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. The other direct method

2262-625: The Copernican system as fact rather than theory, it introduced "a more chaotic system, a less-than-godly lack of organization." In fact, the Copernican system that Galileo believed to be real challenged the Scripture, "which referred to the sun 'rising' and the earth as 'unmoving. ' " The conflict ended in 1633 with Galileo being sentenced to a form of house arrest by the Catholic Church. However, by 1633, Galileo had published other works in support of

2340-532: The Danish–Irish astronomer J. L. E. Dreyer , and the two supplemental Index Catalogues , published in 1896 and 1905. 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 center of the Milky Way . The other type consisted of a generally sparser population of stars in

2418-818: The Hyades (which also form part of Taurus ) is one of the oldest open clusters. Other open clusters were noted by early astronomers as unresolved fuzzy patches of light. In his Almagest , the Roman astronomer Ptolemy mentions the Praesepe cluster, the Double Cluster in Perseus , the Coma Star Cluster and the Ptolemy Cluster , while the Persian astronomer Al-Sufi wrote of the Omicron Velorum cluster . However, it would require

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2496-467: The Julian calendar (Marius claimed to have first observed Jupiter's moons on December 29, 1609). Although Galileo did indeed discover Jupiter's four moons before Marius, Io , Europa , Ganymede , and Callisto are now the names of Galileo's four moons. By 1626 knowledge of the telescope had spread to China when German Jesuit and astronomer Johann Adam Schall von Bell published Yuan jing shuo, (Explanation of

2574-517: The Medicean Stars after Jupiter became visible again in the autumn of 1610. Marius, a German astronomer who had studied with Tycho Brahe , was the first to publish a book of his observations. Marius attacked Galileo in Mundus Jovialis (published in 1614) by insisting that he had found Jupiter's four moons before Galileo and had been observing them since 1609. Marius believed that he therefore had

2652-404: The Medicean Stars was simply to say that the telescope had a lens defect and was producing illusory points of light and images; those saying this completely denied the existence of the moons. That only a few could initially see and verify what Galileo had observed supported the supposition that the optical theory during this period "could not clearly demonstrate that the instrument was not deceiving

2730-403: The Milky Way to appear close to each other. 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

2808-514: The Pleiades being the result of a chance alignment as seen from Earth was just 1 in 496,000. Between 1774 and 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. 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

2886-672: The Telescope) in Chinese and Latin. Galileo's drawings of an imperfect Moon directly contradicted Ptolemy 's and Aristotle 's cosmological descriptions of perfect and unchanging heavenly bodies made of quintessence (the fifth element in ancient and medieval philosophy of which the celestial bodies are composed). Before the publication of Sidereus Nuncius , the Catholic Church accepted the Copernican heliocentric system as strictly mathematical and hypothetical. However, once Galileo began to speak of

2964-545: The Virgin (1612), and Andrea Sacchi 's Divine Wisdom (1631). In addition, the discovery of the Medicean Stars fascinated other astronomers, and they wanted to view the moons for themselves. Their efforts "set the stage for the modern scientific requirement of experimental reproducibility by independent researchers. Verification versus falsifiability…saw their origins in the announcement of Sidereus Nuncius. " But many individuals and communities were sceptical. A common response to

3042-412: 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. Determining the distances to astronomical objects is crucial to understanding them, but

3120-451: The brightest stars known, surpassing Rigel in luminosity. For comparison, the Sun at the same distance as Phi Cassiopeiae would shine at just 17.3 magnitude. The next brightest star is the red supergiant variable star V466 Cassiopeiae. The cluster features a rich field of about 150 stars of magnitude 9-13. About 60 stars have been identified as true members of the cluster. Open cluster Young open clusters may be contained within

3198-450: The cluster center. Typical star densities in the center 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. 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 for little to very disparate, an Arabic numeral from 1 to 3 for

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3276-566: The distance scale to nearby galaxies in the Local Group. Indeed, the open cluster designated NGC 7790 hosts three classical Cepheids . RR Lyrae variables are too old to be associated with open clusters, and are instead found in globular clusters . The stars in open clusters can host exoplanets, just like stars outside open clusters. For example, the open cluster NGC 6811 contains two known planetary systems, Kepler-66 and Kepler-67 . Additionally, several hot Jupiters are known to exist in

3354-482: 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 and gravitational interactions. Even without external triggers, regions of the cloud can reach conditions where they become unstable against collapse. The collapsing cloud region will undergo hierarchical fragmentation into ever smaller clumps, including

3432-521: The first astronomer to use a telescope to find previously undiscovered open clusters. In 1654, he identified the objects now designated Messier 41 , Messier 47 , NGC 2362 and NGC 2451 . It was realized as early as 1767 that the stars in a cluster were physically related, when the English naturalist the Reverend John Michell calculated that the probability of even just one group of stars like

3510-494: The four stars. He made this distinction to show that there was in fact a difference between these two types of celestial bodies. It is important to note that Galileo used the terms planet and star interchangeably, and "both words were correct usage within the prevailing Aristotelian terminology." At the time of Sidereus Nuncius ' publication, Galileo was a mathematician at the University of Padua and had recently received

3588-491: The galaxy, although their concentration is highest where the gas density is highest. 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. In the Milky Way Galaxy, the distribution of clusters depends on age, with older clusters being preferentially found at greater distances from

3666-504: The host star. 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 that have enough mass to be gravitationally bound once

3744-511: The idea that stars were initially scattered across space, but later became clustered together as star systems because of gravitational attraction. He divided the nebulae into eight classes, with classes VI through VIII being used to classify clusters of stars. 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

3822-481: The imperfect and mountainous Moon, of hundreds of stars not visible to the naked eye in the Milky Way and in certain constellations , and of the Medicean Stars (later Galilean moons) that appeared to be circling Jupiter. The Latin word nuncius was typically used during this time period to denote messenger ; however, it was also (though less frequently) rendered as message . Though the title Sidereus Nuncius

3900-508: The improved telescope he used to make the observations in Sidereus Nuncius . Sidereus Nuncius contains more than seventy drawings and diagrams of the Moon, certain constellations such as Orion , the Pleiades , and Taurus , and the Medicean Stars of Jupiter. Galileo's text also includes descriptions, explanations, and theories of his observations. In observing the Moon, Galileo saw that

3978-499: The invention of the telescope to resolve these "nebulae" into their constituent stars. Indeed, in 1603 Johann Bayer gave three of these clusters designations as if they were single stars. 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

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4056-410: The line separating lunar day from night (the terminator ) was smooth where it crossed the darker regions of the Moon but quite irregular where it crossed the brighter areas. From this he deduced that the darker regions are flat, low-lying areas, and the brighter regions rough and mountainous. Basing his estimate on the distance of sunlit mountaintops from the terminator, he judged, quite accurately, that

4134-402: The lunar mountains were at least four miles high. Galileo's engravings of the lunar surface provided a new form of visual representation, besides shaping the field of selenography , the study of physical features on the Moon. Galileo reported that he saw at least ten times more stars through the telescope than are visible to the naked eye, and he published star charts of the belt of Orion and

4212-436: 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 are due only to their mass. This makes open clusters very useful in

4290-593: The major courts of Europe." The first astronomer to publicly support Galileo's findings was Johannes Kepler , who published an open letter in April 1610, enthusiastically endorsing Galileo's credibility. It was not until August 1610 that Kepler was able to publish his independent confirmation of Galileo's findings, due to the scarcity of sufficiently powerful telescopes. Several astronomers, such as Thomas Harriot , Joseph Gaultier de la Vatelle, Nicolas-Claude Fabri de Peiresc , and Simon Marius , published their confirmation of

4368-471: 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. 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

4446-629: The molecular cloud from which they formed, illuminating it to create an H II region . 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. A number of open clusters, such as

4524-431: The morphological and kinematical structures of galaxies. Most open clusters form with at least 100 stars and a mass of 50 or more solar masses. The largest clusters can have over 10 solar masses, with the massive cluster Westerlund 1 being estimated at 5 × 10 solar masses and R136 at almost 5 x 10 , typical of globular clusters. While open clusters and globular clusters form two fairly distinct groups, there may not be

4602-402: The naked eye. In the last part of Sidereus Nuncius , Galileo reported his discovery of four objects that appeared to form a straight line of stars near Jupiter. On the first night he detected a line of three little stars close to Jupiter parallel to the ecliptic; the following nights brought different arrangements and another star into his view, totalling four stars around Jupiter. Throughout

4680-468: 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

4758-495: The original cluster members will have been lost, range from 150–800 million years, depending on the original density. 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

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4836-717: The position of Chief Mathematician and Philosopher to the Medici at the University of Pisa. Ultimately, his effort at naming the moons failed, for they are now referred to as the " Galilean moons ". The reactions to Sidereus Nuncius , ranging from appraisal and hostility to disbelief, soon spread throughout Italy and England. Many poems and texts were published expressing love for the new form of astronomical science. Three works of art were even created in response to Galileo's book: Adam Elsheimer 's The Flight into Egypt (1610; contested by Keith Andrews ), Lodovico Cigoli 's Assumption of

4914-692: 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. 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. As astrometry became more accurate, cluster stars were found to share

4992-436: 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 . Once the distances to nearby clusters have been established, further techniques can extend the distance scale to more distant clusters. By matching

5070-551: 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 . Under the Trumpler scheme, the Pleiades are classified as I3rn, and the nearby Hyades are classified as II3m. There are over 1,100 known open clusters in our galaxy, but the true total may be up to ten times higher than that. In spiral galaxies , open clusters are largely found in

5148-628: The right to name them, which he did: he named them after Jupiter's love conquests: Io , Europa , Ganymede , and Callisto . But Galileo was not confounded; he pointed out that being outside the Church, Marius had not yet accepted the Gregorian calendar and was still using the Julian calendar . Therefore, the night Galileo first observed Jupiter's moons was January 7, 1610 on the Gregorian calendar—December 28, 1609 on

5226-660: 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 (also known as h and χ Persei), but at least 10 more double clusters are known to exist. New research indicates the Cepheid -hosting M25 may constitute a ternary star cluster together with NGC 6716 and Collinder 394. Many more binary clusters 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

5304-474: The senses." By naming the four moons after the Medici brothers and convincing the Grand Duke Cosimo II of his discoveries, the defence of Galileo's reports became a matter of State. Moran notes, “the court itself became actively involved in pursuing the confirmation of Galileo’s observations by paying Galileo out of its treasury to manufacture spyglasses that could be sent through ambassadorial channels to

5382-449: 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 with a galactic radius of approximately 50,000 light years. In irregular galaxies , open clusters may be found throughout

5460-543: The star cluster Pleiades showing some of the newly observed stars. With the naked eye observers could see only six stars in the Taurus cluster; through his telescope, however, Galileo was capable of seeing thirty-five – almost six times as many. When he turned his telescope on Orion, he was capable of seeing eighty stars, rather than the previously observed nine – almost nine times more. In Sidereus Nuncius , Galileo revised and reproduced these two star groups by distinguishing between

5538-399: 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 hold on to only

5616-458: The stars seen without the telescope and those seen with it. Also, when he observed some of the "nebulous" stars in the Ptolemaic star catalogue, he saw that rather than being cloudy, they were made of many small stars. From this he deduced that the nebulae and the Milky Way were "congeries of innumerable stars grouped together in clusters" too small and distant to be resolved into individual stars by

5694-454: 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. Once they have exhausted their supply of hydrogen through nuclear fusion , medium- to low-mass stars shed their outer layers to form

5772-750: The study of stellar evolution, because when comparing one star with 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. Through study of their abundances in open-cluster stars, variables such as age and chemical composition can be fixed. Studies have shown that

5850-501: 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. 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

5928-511: The text, Galileo gave illustrations of the relative positions of Jupiter and its apparent companion stars as they appeared nightly from late January through early March 1610. That they changed their positions relative to Jupiter from night to night and yet always appeared in the same straight line near it, persuaded Galileo that they were orbiting Jupiter. On January 11 after four nights of observation he wrote: In his drawings, Galileo used an open circle to represent Jupiter and asterisks to represent

6006-472: 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. 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,

6084-418: 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. As only 30 to 40 percent of the gas in the cloud core forms stars, the process of residual gas expulsion is highly damaging to

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