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88-764: The Collinder catalogue is a catalogue of 471 open clusters compiled by Swedish astronomer Per Collinder . It was published in 1931 as an appendix to Collinder's paper On structural properties of open galactic clusters and their spatial distribution . The catalogue contains 452 open clusters , 11 globular clusters , 6 asterisms , 1 stellar moving group , and 1 stellar association . Catalogue objects are denoted by Collinder , e.g. "Collinder 399". Dated prefixes include as Col + catalogue number , or Cr + catalogue number , e.g. "Cr 399". There are some errors in Collinder's list or references to it. For example: Open cluster Young open clusters may be contained within

176-472: A dark star , would not be directly visible, but could be identified by the motions of a companion star if it was part of a binary system. The classical minimum radius for escape assuming light behaved like particles of matter is numerically equal to the Schwarzschild Radius in general relativity . Michell also suggested using a prism to measure what is now known as gravitational redshift ,

264-406: 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

352-401: A 30-inch aperture, was bought by the distinguished astronomer William Herschel after Michell's death. The two men had many interests in common, and exchanged letters at least twice, but only one record suggests that they ever met. Herschel recorded having visited and seen Michell's telescope while in the area in 1792; Michell was already frail, and his telescope was in disrepair. Herschel bought

440-577: A century later". The Society stated that while "he was one of the most brilliant and original scientists of his time, Michell remains virtually unknown today, in part because he did little to develop and promote his own path-breaking ideas". John Michell was born in 1724 in Eakring , in Nottinghamshire, the son of Gilbert Michell, a priest, and Obedience Gerrard. Gilbert was the son of William Michell and Mary Taylor of Kenwyn , Cornwall; Obedience

528-490: 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

616-466: 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

704-440: 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

792-428: A magnet decreases according to an inverse-square law , i.e. in proportion to the square of the distance between them. His 1750 paper Treatise of Artificial Magnets , which was written for seamen and instrument makers and intended as a practical manual on how to make magnets, included a list of the "Properties of Magnetical Bodies" that represented a major contribution to the understanding of magnetism. Michell devised

880-449: 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

968-465: 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

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1056-406: 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

1144-480: 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

1232-410: 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

1320-553: A success: the needle melted. Until the late 20th century Michell was considered important primarily because of his work on geology. His most important geological essay, written after the 1755 Lisbon earthquake , was entitled "Conjectures concerning the Cause and Observations upon the Phaenomena of Earthquakes" ( Philosophical Transactions , li. 1760). In this paper he introduced the idea that earthquakes spread out as waves through

1408-486: 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

1496-551: A torsion balance for measuring the mass of the Earth, but died before he could use it. His instrument passed into the hands of his lifelong friend Henry Cavendish , who first performed in 1798 the experiment now known as the Cavendish Experiment . Placing two 1-kg lead balls at the ends of a six-foot rod, he suspended the rod horizontally by a fibre attached to its centre. Then he placed a massive lead ball beside each of

1584-516: 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

1672-664: 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

1760-633: 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

1848-451: Is not less than that of the sun, and whose diameters are more than 500 times the diameter of the sun, since their light could not arrive at us; or if there should exist any other bodies of a somewhat smaller size, which are not naturally luminous; of the existence of bodies under either of these circumstances, we could have no information from sight; yet, if any other luminous bodies should happen to revolve about them we might still perhaps from

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1936-407: 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,

2024-411: 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

2112-404: 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

2200-488: The Beehive Cluster . John Michell John Michell ( / ˈ m ɪ tʃ əl / ; 25 December 1724 – 21 April 1793) was an English natural philosopher and clergyman who provided pioneering insights into a wide range of scientific fields including astronomy , geology , optics , and gravitation . Considered "one of the greatest unsung scientists of all time", he is the first person known to have proposed

2288-472: 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

2376-493: The Pleiades cluster, and calculated that the likelihood of finding such a close grouping of stars was about one in half a million. He concluded that the stars in these double or multiple star systems might be drawn to one another by gravitational pull, thus providing the first evidence for the existence of binary stars and star clusters . His work on double stars may have influenced his friend William Herschel 's research on

2464-560: 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

2552-502: 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

2640-726: 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

2728-474: 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

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2816-571: 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

2904-537: 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

2992-452: The Earth, and that they involve the offsets in geological strata now known as faults . He was able to estimate both the epicentre and the focus of the Lisbon earthquake, and may also have been the first to suggest that a tsunami is caused by a submarine earthquake. Michell's essay not only provided insights on earthquakes but also, more broadly, represented an advance in the understanding of

3080-859: 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

3168-403: 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

3256-514: 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

3344-412: 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

3432-405: The best natural magnets. Besides the description of the method of magnetization which still bears his name, this work contains a variety of accurate observations about magnetism, and features a lucid exposition of the nature of magnetic induction. At one point, Michell attempted to measure the radiation pressure of light by focusing sunlight onto one side of a compass needle. The experiment was not

3520-468: The care of St. Botolph's Church Cambridge, while he continued Fellow of Queens’ College, where he was esteemed a very ingenious Man, and an excellent Philosopher. He has published some things in that way, on the Magnet and Electricity." In 1910, Sir Edmund Whittaker observed that during the century after Isaac Newton 's death, "the only natural philosopher of distinction who lived and taught at Cambridge

3608-608: The cause of earthquakes was read before the Society beginning on 28 February 1760, leading to a recommendation by Savile and another member that Michell be invited to join the Society. He was elected a member on 12 June 1760. Michell followed his work in seismology with work in astronomy, and after publishing his findings in 1767 he served on an astronomical committee of the Royal Society. More recently, Michell has become known for his letter to Cavendish, published in 1784, on

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3696-450: 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

3784-572: 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

3872-654: The dozen candidate stellar black holes in our galaxy (the Milky Way) are in X-ray compact binary systems . Michell's ideas about gravity and light interested William Herschel, who tried to test them with his powerful telescopes. A few years after Michell came up with the concept of invisible, light-trapping stars, the French mathematician Pierre-Simon Laplace suggested essentially the same idea in his 1796 book, Exposition du Système du Monde . It has been written that Michell

3960-424: The effect of gravity on light. This paper was rediscovered in the 1970s and is now recognised as anticipating several astronomical ideas that had been considered to be 20th century innovations. Michell is now credited with being the first to study the case of a heavenly object massive enough to prevent light from escaping (the concept of escape velocity was well known at the time). Such an object, often referred to as

4048-482: 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

4136-415: The existence of stellar bodies comparable to black holes , and the first to have suggested that earthquakes travelled in (seismic) waves . Recognizing that double stars were a product of mutual gravitation, he was the first to apply statistics to the study of the cosmos . He invented an apparatus to measure the mass of the Earth , and explained how to manufacture an artificial magnet. He has been called

4224-415: The father both of seismology and of magnetometry . According to one science journalist, "a few specifics of Michell's work really do sound like they are ripped from the pages of a twentieth century astronomy textbook." The American Physical Society (APS) described Michell as being "so far ahead of his scientific contemporaries that his ideas languished in obscurity, until they were re-invented more than

4312-535: 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

4400-499: 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

4488-476: The geology of the Earth's crust. He recognized that the Earth is composed "of regular and uniform strata", some of which have been interrupted by upheavals. "The most important part of Michell's Earthquake paper", in the view of one commentator, "is the account which it contains of what is now known as 'the crust of the Earth.'" Exhibiting a remarkable knowledge of the geological strata in various parts of England and abroad, he drew on his own observations to advance

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4576-426: The gravitational weakening of starlight due to the surface gravity of the source. Michell acknowledged that some of these ideas were not technically practical at the time, but wrote that he hoped they would be useful to future generations. By the time that Michell's paper was rediscovered nearly two centuries later, these ideas had been reinvented by others. Michell was a man of "wide latitude in religious belief". He

4664-504: 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

4752-520: 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

4840-504: 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

4928-436: 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

5016-471: 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

5104-629: 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

5192-431: 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

5280-494: The motions of these revolving bodies infer the existence of the central ones with some degree of probability, as this might afford a clue to some of the apparent irregularities of the revolving bodies, which would not be easily explicable on any other hypothesis; but as the consequences of such a supposition are very obvious, and the consideration of them somewhat beside my present purpose, I shall not prosecute them any further. Michell suggested that there might be many such objects in

5368-468: 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

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5456-495: 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

5544-701: 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

5632-436: 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

5720-556: 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

5808-669: 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

5896-568: The same topic. In a paper for the Philosophical Transactions of the Royal Society of London , read on 27 November 1783, Michell was the first to propose the existence of celestial bodies similar to black holes . Having accepted Newton's corpuscular theory of light, which posited that light consists of minuscule particles, he reasoned that such particles, when emanated by a star, would be slowed down by its gravitational pull, and that it might therefore be possible to determine

5984-564: The small ones, causing a gravitational attraction that led the rod to turn clockwise. By measuring the rod's movement, Cavendish was able to calculate the force exerted by each of the large balls on the 1-kg balls. From these calculations, he was able to provide an accurate estimate of the gravitational constant and of the mass and average density of the Earth. Cavendish gave Michell full credit for his accomplishment. In 1987, gravity researcher A. H. Cook wrote: The most important advance in experiments on gravitation and other delicate measurements

6072-449: 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

6160-399: 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

6248-463: The star's mass based on the reduction in speed. This insight led in turn to the recognition that a star's gravitational pull might be so strong that the escape velocity would exceed the speed of light. Michell calculated that this would be the case with a star more than 500 times the size of the Sun. Since light would not be able to escape such a star, it would be invisible. In his own words: If there should really exist in nature any bodies, whose density

6336-454: 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

6424-750: 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

6512-501: 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

6600-407: The telescope the following year, for £30. Michell also wrote a paper on surveying that his biographer has described as "elegant" in theory. Michell was elected a member of the Royal Society. He was first invited to meetings of the Royal Society in 1751 as a guest of Sir George Savile, who would become his patron. He later attended meetings "one to four times a year", while at Cambridge. His paper on

6688-492: The understanding of sedimentary stratigraphy and was the first to define the Mesozoic stratigraphy in the U.K. In 1760, as a result of this work, he was elected a member of the Royal Society. A 1788 letter to Henry Cavendish indicated that Michell continued to be interested in geology several decades after his paper on earthquakes. Michell studied magnetism and discovered that the magnetic force exerted by each pole of

6776-434: The universe, and today astronomers believe that black holes do indeed exist at the centers of most galaxies. Similarly, Michell proposed that astronomers could detect them by looking for star systems which behaved gravitationally like two stars, but where only one star could be seen. Michell argued that this would show the presence of a star from which light was not escaping. It was an extraordinarily accurate prediction. All of

6864-472: 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,

6952-418: 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

7040-594: Was Michell", although his "researches seem to have attracted little or no attention among his collegiate contemporaries and successors, who silently acquiesced when his discoveries were attributed to others, and allowed his name to perish entirely from Cambridge tradition". Michell proceeded to take up clerical positions in Compton and then Havant, both in Hampshire. During this period he unsuccessfully sought positions at Cambridge, and as Astronomer Royal . In 1767, he

7128-605: Was Sarah Williamson (1727–1765), daughter of Luke Williamson and Sutton Holmes, "a young lady of considerable fortune", whom he married in 1764 and who unfortunately died only a year later, in 1765. On 13 February 1773, in Newark, Nottinghamshire, he married Ann Brecknock (1736-1805), daughter of Matthew and Ann Brecknock of Nottinghamshire. They had one child, Mary, who married Sir Thomas Turton of Leeds, son of William Turton Esq. of Kingston Lisle, Berkshire, and Jane Clarke of Hertford, Hertfordshire. Michell's younger brother Gilbert

7216-606: Was appointed rector of St. Michael's Church of Thornhill , near Leeds, Yorkshire, England, a post he held for the rest of his life. He did most of his important scientific work in Thornhill, where he died on 21 April 1793, aged 68. He is buried there. After local pressure, a blue plaque went up on the church wall to commemorate him. In 1750, Michell published at Cambridge a work of some eighty pages entitled "A Treatise of Artificial Magnets", in which he presented an easy and expeditious method of producing magnets that are superior to

7304-486: Was described by a contemporary as "a little short man, of black complexion, and fat", and was "esteemed a very ingenious Man, and an excellent Philosopher." During his years at Thornhill, he welcomed visitors including Benjamin Franklin , Joseph Priestley , Jan Ingenhousz , and Henry Cavendish (the discoverer of hydrogen). Michell wrote to Franklin in 1767 describing his first visit to Thornhill, "the place I told you I

7392-505: Was going to remove to". Priestley lived in nearby Birstall for a time. It was at Michell's rectory opposite the church that Priestley and Ingenhousz met for the first time. At the same meeting John Smeaton was introduced to Benjamin Franklin and together they viewed the canal that Smeaton had just finished constructing nearby. Michell also helped Smeaton revise his book on the Eddystone Lighthouse . Michell's first wife

7480-489: Was nominated Rector of St Botolph's, Cambridge, on 28 March 1760, and held this living until June 1763." From 1762 to 1764, he held the Woodwardian Chair of Geology until he was obliged to relinquish it on his marriage. There is no surviving portrait of Michell; he is said to have been "a little short Man, of a black Complexion, and fat but having no Acquaintance with him, can say little of him. I think he had

7568-478: Was so far ahead of his time in regard to black holes that the idea "made little impression" on his contemporaries. "He died in quiet obscurity", states the American Physical Society, "and his notion of a 'dark star' was forgotten until his writings re-surfaced in the 1970s." Michell constructed telescopes for his own use. One of them, a reflecting telescope with a 10-foot focal length and

7656-875: Was the daughter of Ralph and Hannah Gerrard of London. He was educated at Queens' College, Cambridge , and later became a Fellow of Queens'. The family was of Cornish origins. He obtained his M.A. degree in 1752, and his B.D. degree in 1761. He was Tutor of the college from 1751 to 1763; Praelector in Arithmetic in 1751; Censor in Theology in 1752; Praelector in Geometry in 1753; Praelector in Greek in 1755 and 1759; Senior Bursar in 1756; Praelector in Hebrew in 1759 and 1762; Censor in Philosophy and Examiner in 1760. "He

7744-437: Was the introduction of the torsion balance by Michell and its use by Cavendish. It has been the basis of all the most significant experiments on gravitation ever since. Michell was the first person to apply the new mathematics of statistics to the study of the stars, and demonstrated in a 1767 paper that many more stars occur in pairs or groups than a perfectly random distribution could account for. He focused his investigation on

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