Magellanic Clouds

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The Large and Small Magellanic Clouds Magellanic Clouds -- Irregular Dwarf Galaxies.jpg
The Large and Small Magellanic Clouds
Small and Large Magellanic Clouds over Paranal Observatory A Starry Combination.jpg
Small and Large Magellanic Clouds over Paranal Observatory
ALMA antennae bathed in red light. In the background are the southern Milky Way on the left and the Magellanic Clouds at the top. Red alert.jpg
ALMA antennae bathed in red light. In the background are the southern Milky Way on the left and the Magellanic Clouds at the top.

The Magellanic Clouds (Magellanic system [2] [3] or Nubeculae Magellani [4] ) are two irregular dwarf galaxies in the southern celestial hemisphere. Orbiting the Milky Way galaxy, these satellite galaxies are members of the Local Group. Because both show signs of a bar structure, they are often reclassified as Magellanic spiral galaxies.

Contents

The two galaxies are the following:

The Magellanic clouds are visible to the unaided eye from the Southern Hemisphere, but cannot be observed from the most northern latitudes.

History

An early possible mention of the Large Magellanic Cloud is in petroglyphs and rock drawings found in Chile. They may be the objects mentioned by the polymath Ibn Qutaybah (d. 889 CE), in his book on Al-Anwā̵’ (the stations of the Moon in pre-Islamic Arabian culture):

وأسفل من سهيل قدما سهيل . وفى مجرى قدمى سهيل، من خلفهما كواكب زهر كبار، لا ترى بالعراق، يسميها أهل تهامة الأعبار

And below Canopus, there are the feet of Canopus, and on their extension, behind them bright big stars, not seen in Iraq, the people of Tihama call them al-a‘bār. [5]

Later Al Sufi, a professional astronomer, [6] in 964 CE, in his Book of Fixed Stars , mentioned the same quote, but with a different spelling. Under Argo Navis, he quoted that "unnamed others have claimed that beneath Canopus there are two stars known as the 'feet of Canopus', and beneath those there are bright white stars that are unseen in Iraq nor Najd, and that the inhabitants of Tihama call them al-Baqar [cows], and Ptolemy did not mention any of this so we [Al-Sufi] do not know if this is true or false." [7] Both Ibn Qutaybah and Al-Sufi were probably quoting from the former's contemporary (and compatriot) and famed scientist Abu Hanifa Dinawari's mostly lost work on Anwaa. Abu Hanifa was probably quoting earlier sources, which may be just travelers stories, and hence Al-Sufi's comments about their veracity.

In Europe, the Clouds were reported by 16th century Italian authors Peter Martyr d'Anghiera and Andrea Corsali, both based on Portuguese voyages. [8] [9] Subsequently, they were reported by Antonio Pigafetta, who accompanied the expedition of Ferdinand Magellan on its circumnavigation of the world in 1519–1522. [10] [4] However, naming the clouds after Magellan did not become widespread until much later. In Bayer's Uranometria they are designated as nubecula major and nubecula minor. [11] [12] In the 1756 star map of the French astronomer Lacaille, they are designated as le Grand Nuage and le Petit Nuage ("the Large Cloud" and "the Small Cloud"). [13] [14] John Herschel studied the Magellanic Clouds from South Africa, writing an 1847 report detailing 919 objects in the Large Magellanic Cloud and 244 objects in the Small Magellanic Cloud. [15] In 1867 Cleveland Abbe suggested that they were separate satellites of the Milky Way. [16] Distances were first estimated by Ejnar Hertzsprung in 1913 using 1912 measurements of Cepheid variables in the SMC by Henrietta Leavitt. [17] [18] Recalibration of the Cepheid scales allowed Harlow Shapley to refine the measurement, [19] and these were again revised in 1952 following further research. [20] As of 2023, some astronomers believe the Magellanic Clouds should be renamed, alleging that Magellan was a murderer and neither an astronomer nor the discoverer of the dwarf galaxies. [21] [22] [23]

Characteristics

The Large Magellanic Cloud and its neighbour and relative, the Small Magellanic Cloud, are conspicuous objects in the southern hemisphere, looking like separated pieces of the Milky Way to the naked eye. Roughly 21° apart in the night sky, the true distance between them is roughly 75,000 light-years. Until the discovery of the Sagittarius Dwarf Elliptical Galaxy in 1994, they were the closest known galaxies to our own. The LMC lies about 160,000 light years away, [24] [25] [26] [27] while the SMC is around 200,000. [28] The LMC is about 70% larger than the diameter of the SMC (32,200 ly and 18,900 ly respectively). For comparison, the Milky Way is about 87,400 ly across.

The total mass of these two galaxies is uncertain. Only a fraction of their gas seems to have coalesced into stars and they probably both have large dark matter halos. One recent estimate of the total mass of the LMC is about 1/10 that of the Milky Way. That would make the LMC rather a large galaxy in the current observable universe. Since the sizes of relatively nearby galaxies are highly skewed, the average mass can be a misleading statistic. In terms of rank, the LMC appears to be the fourth most massive member of over 50 galaxies in the local group. Suggesting that the Magellanic cloud system is historically not a part of the Milky Way is evidence that the SMC has been in orbit about the LMC for a very long time. The Magellanic system seems most similar to the distinct NGC 3109 system, which is on the edge of the Local Group.

Astronomers have long assumed that the Magellanic Clouds have orbited the Milky Way at approximately their current distances, but evidence suggests that it is rare for them to come as close to the Milky Way as they are now. [29] Observation and theoretical evidence suggest that the Magellanic Clouds have both been greatly distorted by tidal interaction with the Milky Way as they travel close to it. The LMC maintains a very clear spiral structure in radio-telescope images of neutral hydrogen. Streams of neutral hydrogen connect them to the Milky Way and to each other, and both resemble disrupted barred spiral galaxies. [30] Their gravity has affected the Milky Way as well, distorting the outer parts of the galactic disk.

Aside from their different structure and lower mass, they differ from our galaxy in two major ways. They are gas-rich; a higher fraction of their mass is hydrogen and helium compared to the Milky Way. [31] They are also more metal-poor than the Milky Way; the youngest stars in the LMC and SMC have a metallicity of 0.5 and 0.25 times solar, respectively. [32] Both are noted for their nebulae and young stellar populations, but as in our own galaxy their stars range from the very young to the very old, indicating a long stellar formation history. [33]

The Large Magellanic Cloud was the host galaxy to a supernova (SN 1987A), the brightest observed in over four centuries.

Measurements with the Hubble Space Telescope, announced in 2006, suggest the Magellanic Clouds may be moving too fast to be long term companions of the Milky Way. [34] If they are in orbit, that orbit takes at least 4 billion years. They are possibly on a first approach and we are witnessing the start of a galactic merger that may overlap with the Milky Way's expected merger with the Andromeda Galaxy (and perhaps the Triangulum Galaxy) in the future.

In 2019, astronomers discovered the young star cluster Price-Whelan 1 using Gaia data. The star cluster has a low metallicity and belongs to the leading arm of the Magellanic Clouds. The existence of this star cluster suggests that the leading arm of the Magellanic Clouds is 90,000 light-years away from the Milky Way—closer than previously thought. [35]

Mini Magellanic Cloud (MMC)

Astrophysicists D. S. Mathewson, V. L. Ford and N. Visvanathan proposed that the SMC may in fact be split in two, with a smaller section of this galaxy behind the main part of the SMC (as seen from Earth's perspective), and separated by about 30,000 light years. They suggest the reason for this is due to a past interaction with the LMC splitting the SMC, and that the two sections are still moving apart. They have dubbed this smaller remnant the Mini Magellanic Cloud. [36] [37] This hypothesis was confirmed in 2023. The part of the SMC which is closer to Earth lies 196,000 light-years (60 kiloparsecs) away, whereas the farther part lies 215,000 light-years (66 kiloparsecs) away. [38]

See also

Related Research Articles

<span class="mw-page-title-main">Dorado</span> Constellation in the southern sky

Dorado is a constellation in the Southern Sky. It was named in the late 16th century and is now one of the 88 modern constellations. Its name refers to the mahi-mahi, which is known as dorado ("golden") in Spanish, although it has also been depicted as a swordfish. Dorado contains most of the Large Magellanic Cloud, the remainder being in the constellation Mensa. The South Ecliptic pole also lies within this constellation.

<span class="mw-page-title-main">Star cluster</span> Group of stars

Star clusters are large groups of stars held together by self-gravitation. Two main types of star clusters can be distinguished. Globular clusters are tight groups of ten thousand to millions of old stars which are gravitationally bound. Open clusters are more loosely clustered groups of stars, generally containing fewer than a few hundred members, that are often very young. As they move through the galaxy, over time, open clusters become disrupted by the gravitational influence of giant molecular clouds. Even though they are no longer gravitationally bound, they will continue to move in broadly the same direction through space and are then known as stellar associations, sometimes referred to as moving groups.

<span class="mw-page-title-main">Large Magellanic Cloud</span> Satellite galaxy of the Milky Way

The Large Magellanic Cloud (LMC) is a dwarf galaxy and satellite galaxy of the Milky Way. At a distance of around 50 kiloparsecs (163,000 light-years), the LMC is the second- or third-closest galaxy to the Milky Way, after the Sagittarius Dwarf Spheroidal (c. 16 kiloparsecs (52,000 light-years) away) and the possible dwarf irregular galaxy called the Canis Major Overdensity. Based on the D25 isophote at the B-band (445 nm wavelength of light), the Large Magellanic Cloud is about 9.86 kiloparsecs (32,200 light-years) across. It is roughly one-hundredth the mass of the Milky Way and is the fourth-largest galaxy in the Local Group, after the Andromeda Galaxy (M31), the Milky Way, and the Triangulum Galaxy (M33).

<span class="mw-page-title-main">Celestial pole</span> Imaginary sky rotation points

The north and south celestial poles are the two points in the sky where Earth's axis of rotation, indefinitely extended, intersects the celestial sphere. The north and south celestial poles appear permanently directly overhead to observers at Earth's North Pole and South Pole, respectively. As Earth spins on its axis, the two celestial poles remain fixed in the sky, and all other celestial points appear to rotate around them, completing one circuit per day.

Timeline of stellar astronomy

<span class="mw-page-title-main">Timeline of knowledge about galaxies, clusters of galaxies, and large-scale structure</span>

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<span class="mw-page-title-main">Cepheid variable</span> Type of variable star that pulsates radially

A Cepheid variable is a type of variable star that pulsates radially, varying in both diameter and temperature. It changes in brightness, with a well-defined stable period and amplitude.

<span class="mw-page-title-main">Small Magellanic Cloud</span> Dwarf irregular galaxy, satellite galaxy of the Milky Way

The Small Magellanic Cloud (SMC) is a dwarf galaxy near the Milky Way. Classified as a dwarf irregular galaxy, the SMC has a D25 isophotal diameter of about 5.78 kiloparsecs (18,900 light-years), and contains several hundred million stars. It has a total mass of approximately 7 billion solar masses. At a distance of about 200,000 light-years, the SMC is among the nearest intergalactic neighbors of the Milky Way and is one of the most distant objects visible to the naked eye.

<span class="mw-page-title-main">Henrietta Swan Leavitt</span> American astronomer (1868–1921)

Henrietta Swan Leavitt was an American astronomer. Her discovery of how to effectively measure vast distances to remote galaxies led to a shift in the scale and understanding of the scale and the nature of the universe. Nomination of Leavitt for the Nobel Prize had to be halted because of her death.

<span class="mw-page-title-main">Extinction (astronomy)</span> Interstellar absorption and scattering of light

In astronomy, extinction is the absorption and scattering of electromagnetic radiation by dust and gas between an emitting astronomical object and the observer. Interstellar extinction was first documented as such in 1930 by Robert Julius Trumpler. However, its effects had been noted in 1847 by Friedrich Georg Wilhelm von Struve, and its effect on the colors of stars had been observed by a number of individuals who did not connect it with the general presence of galactic dust. For stars lying near the plane of the Milky Way which are within a few thousand parsecs of the Earth, extinction in the visual band of frequencies is roughly 1.8 magnitudes per kiloparsec.

<span class="mw-page-title-main">NGC 6822</span> Galaxy in the constellation Sagittarius

NGC 6822 is a barred irregular galaxy approximately 1.6 million light-years away in the constellation Sagittarius. Part of the Local Group of galaxies, it was discovered by E. E. Barnard in 1884, with a six-inch refractor telescope. It is the closest non-satellite galaxy to the Milky Way, but lies just outside its virial radius. It is similar in structure and composition to the Small Magellanic Cloud. It is about 7,000 light-years in diameter.

<span class="mw-page-title-main">Magellanic Bridge</span> Stream of neutral hydrogen gas linking the two Magellanic clouds

The Magellanic Bridge (MBR) is a stream of neutral hydrogen that links the two Magellanic Clouds, with a few known stars inside it. It should not be confused with the Magellanic Stream, which links the Magellanic Clouds to the Milky Way. It was discovered in 1963 by J. V. Hindman et al.

<span class="mw-page-title-main">Jeremy Mould</span> Australian astronomer

Jeremy Richard Mould is an Australian astronomer currently at the Centre for Astrophysics and Supercomputing at Swinburne University of Technology. Mould was previously Director of the Research School of Astronomy and Astrophysics at the Australian National University and the American National Optical Astronomy Observatory. He is an Honorary Professorial Fellow, at the University of Melbourne.

<span class="mw-page-title-main">HV 2112</span> Small Magellanic Cloud star in the constellation Tucana

HV 2112 is a cool luminous variable star in the Small Magellanic Cloud. Until 2018, it was considered to be the most likely candidate for a Thorne–Żytkow object, but it is now thought to be an asymptotic giant branch star.

Galaxy X is a postulated dark satellite dwarf galaxy of the Milky Way Galaxy. If it exists, it would be composed mostly of dark matter and interstellar gas with few stars. Its proposed location is some 90 kpc (290 kly) from the Sun, behind the disk of the Milky Way, and some 12 kpc (39 kly) in extent. Galactic coordinates would be (l= -27.4°,b=-1.08°).

<span class="mw-page-title-main">OGLE-LMC-CEP0227</span> Variable star in the Large Magellanic Cloud

OGLE-LMC-CEP0227 is an eclipsing binary and Cepheid variable star, pulsating every 3.8 days. The star, in the Large Magellanic Cloud, was the first Cepheid star system found to be orbiting exactly edge on.

<span class="mw-page-title-main">NGC 121</span> Globular cluster in the constellation Tucana

NGC 121 is a globular cluster of stars in the southern constellation of Tucana. It is the oldest globular cluster in the Small Magellanic Cloud (SMC), which is a dwarf satellite galaxy of the Milky Way. This cluster was first discovered by English astronomer John Herschel on September 20, 1835. The compiler of the New General Catalogue, Danish astronomer John Louis Emil Dreyer, described this object as "pretty bright, pretty small, little extended, very gradually brighter middle". The cluster is located at a distance of around 200,000 light-years (60 kpc) from the Sun.

<span class="mw-page-title-main">N119</span> Spiral shaped H II region in the constellation Dorado

N119 is a spiral-shaped H II region in the Large Magellanic Cloud. Its dimensions are large, at 131 x 175 pc. It contains several luminous stars including S Doradus, LH41-1042, and LMC195-1. Its peculiar S-shaped structure is difficult to explain with classical models.

<span class="mw-page-title-main">NGC 1466</span> Globular star cluster in the constrellation Hydrus

NGC 1466 is the New General Catalogue designation for a globular cluster in the deep southern constellation of Hydrus. It is located in the outskirts of the Large Magellanic Cloud, which is a satellite galaxy of the Milky Way. The object was discovered November 26, 1834 by English astronomer John Herschel. John Dreyer described it as "pF, pS, iR, glbM, *7 f", meaning "pretty faint, pretty small, irregular round, gradually a little brighter middle, with a 7th magnitude star nearby". When using a small telescope, this is a "faint, small, unresolved and difficult" target with an angular size of 1.9 arc minutes. It has an integrated visual magnitude of 11.4.

<span class="mw-page-title-main">Period-luminosity relation</span> Astronomical principle

In astronomy, a period-luminosity relation is a relationship linking the luminosity of pulsating variable stars with their pulsation period. The best-known relation is the direct proportionality law holding for Classical Cepheid variables, sometimes called the Leavitt Law. Discovered in 1908 by Henrietta Swan Leavitt, the relation established Cepheids as foundational indicators of cosmic benchmarks for scaling galactic and extragalactic distances. The physical model explaining the Leavitt's law for classical cepheids is called kappa mechanism.

References

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  2. Tepper-García, Thor; Bland-Hawthorn, Joss; Pawlowski, Marcel S.; Fritz, Tobias K. (2019-09-01). "The Magellanic System: the puzzle of the leading gas stream". Monthly Notices of the Royal Astronomical Society . 488 (1): 918–938. arXiv: 1901.05636 . doi: 10.1093/mnras/stz1659 . ISSN   0035-8711.
  3. "Magellanic System | UW-Madison Astronomy". astro.wisc.edu. Retrieved 2022-06-14.
  4. 1 2 Allen, R. H. (1963). Star Names: Their Lore and Meaning (Reprint  ed.). New York: Dover Publications Inc. pp.  294–295.
  5. "Al-Anwaa, Ibn Qutaybah". Archived from the original on 24 July 2017. Retrieved 4 Sep 2017.
  6. "Observatoire de Paris (Abd-al-Rahman Al Sufi)" . Retrieved 22 July 2011.
  7. "Book of Fixed Stars, Al-Sufi (manuscript written and illustrated by his son)" . Retrieved 22 Feb 2017.
  8. For Peter Martyr d'Anghiera's mention of the Magellanic clouds, see:
    • Petrus Martyr de Anghiera (1574) De rebus Oceanicis et Orbe Novo [Concerning the ocean and the new world] (Cologne, (Germany): Geruinum Calenium (Gerwin Calenius), 1574), decade 3, book 1, p. 217. (in Latin) From p. 217: "Assecuti sunt Portugallenses alterius poli gradum quintum & quinquagesimum amplius, ubi punctum, circumeuntes quasdam nubeculas licet intueri, veluti in lactea via sparsos fulgores per universum coeli globum intra eius spatii latitudinem." ("The Portuguese reached beyond the 55th degree of the other pole, where one may observe certain nebulae revolving around the point [i.e., the southern celestial pole], scattered in the Milky Way like luminous patches throughout the whole sphere of the sky, within the breadth of its extent." [That is, nebulae appear in or beside the Milky Way throughout its entire length in the southern sky.])
    • Humboldt, Alexander von, with E.C. Otte and B.H. Paul, trans., Cosmos: A Sketch of a Physical Description of the Universe (London, England: Henry G. Bohn, 1852), vol. 4, pp. 340–341.
    • For further details of – and other editions of – Peter Martyr d'Anghiera's book De Orbe Novo, see Wikipedia's article: Decades of the New World
  9. From 1515 to 1517, Andrea Corsali sailed to the East Indies and China in a Portuguese ship. In 1516, Andrea Corsali sent a letter to Giuliano de' Medici, Duke of Nemours, mentioning the Magellanic clouds. This letter was translated into English by Richard Eden (c.1520–1576) and published in 1555. The relevant part of Corsali's letter (translated by Eden) appears in:
    • Richard Eden, with Edward Arber, ed., The First Three English Books on America ... (Birmingham, England: 1885), "Of the pole antarike and the starres abowt the same ... ", p. 279. Corsali said that his ship had passed the Cape of Good Hope ("the cape of Bona Speranza") and was at 37 degrees south latitude when he observed the Magellanic clouds: "Here we sawe a marueylous order of starres, so that in the parte of heauen contrary to owre northe pole, to know in what place and degree the south pole was, we tooke the day with the soonne, and obserued the nyght with the Astrolabie, and sawe manifestly twoo clowdes of reasonable bygnesse mouynge abowt the place of the pole continually now rysynge and nowe faulynge, so keepynge theyr continuall course in circular mouying, with a starre euer in the myddest which is turned abowt with them abowte xi degrees from the pole." ("Here we saw a marvelous arrangement of stars, so that in the part of heaven [that is] opposite our north [celestial] pole, in order to know in what place and degree [of latitude] the south [celestial] pole was, we [measured our position during] the day using the sun, and observed [our position during] the night using an astrolabe, and saw clearly two clouds of reasonable bigness revolving around the location of the [southern celestial] pole, continually now rising and now falling, thus maintaining their continual course of circular motion, with a star always in the middle [between them], which revolves with them about 11 degrees from the [south celestial] pole.")
    • See also: Kanas, Nick, Star Maps: History, Artistry, and Cartography, 2nd ed. (New York, New York: Springer Science + Business Media, 2012), § 4.3.2.2 Andreas Corsali, p. 118.
  10. Pigafetta et al., with Lord Stanley of Alderley, trans., The First Voyage Round the World, by Magellan (London, England: Hakluyt Society, 1874), p. 66. From p. 66: "The antarctic pole is not so covered with stars as the arctic, for there are to be seen there many small stars congregated together, which are like to two clouds a little separated from one another, and a little dimmed..."
  11. Bayer Johann (1603) Uranometria. Augsburg, (Germany): Christoph Mang. Star chart 49. The Large Magellanic Cloud (Nubecula major) appears below the chart's center and just above the fish Dorado; the Small Magellanic Cloud (Nubecula minor) appears to the left and below the chart's center and touches the right side of Hydrus the water snake.
  12. Bayer, J., (1661) Uranometria, pl. Aaa (49) Archived 2011-07-22 at the Wayback Machine U.S. Naval Observatory; retrieved on 2009-09-05
  13. de la Caille (1752). "Table des ascensions droites et des déclinaisons apparentes des étoiles australes renfermées dans le tropique cu Capricorne; observées au cap de Bonne-espérance, dans l'intervalle du 6 Août 1751, au 18 Juillet 1752" [Table of the apparent right ascensions and declinations of the southern stars contained within the Tropic of Capricorn; observed at the Cape of Good Hope during the period from 6 August 1751 to 18 July 1752]. Histoire de l'Académie Royale des Sciences, avec les Mémoires de Mathématique & de Physique de l'Académie Royale des Sciences (in French): 539–592. See the plate (Planisphere contenant des constellations celestes comprises entre le pole austral et le Tropique du Capricorne [Planisphere containing the celestial constellations included between the south [celestial] pole and the Tropic of Capricorn]) following p. 592. (The Large Magellanic Cloud (Le Grand Nuage) and the Small Magellanic Cloud (Le Petit Nuage) appear just below the center of the diagram.)
  14. de Lacaille, N. L., (1756) Planisphere contenant les Constellations Celestes, Memoires Academie Royale des Sciences pour 1752. Archived 2009-05-09 at the Wayback Machine Linda Hall Library; retrieved on 2009-09-05
  15. Herschel, John F.W. (1847). Results of Astronomical Observations Made During the Years 1834, 5, 6, 7, 8 at the Cape of Good Hope. London, England: Smith, Elder and Co. pp. 151–165.
  16. Abbe, Cleveland (1867). "On the distribution of the nebulae in space". Monthly Notices of the Royal Astronomical Society. 27 (7): 257–264. doi: 10.1093/mnras/27.7.257a . From p. 262: "2. The Nebulae resolved and unresolved lie in general without [i.e., outside of] the Via Lactea [i.e., Milky Way], which is therefore essentially stellar. 3. The visible universe is composed of systems, of which the Via Lactea, the two Nubeculae [sic] [i.e., Magellanic Clouds], and the Nebulae, are individuals, and which are themselves composed of stars (either simple, multiple, or in clusters) and of gaseous bodies of both regular and irregular outlines."
  17. Hertzsprung, E. (1913). "Über die räumliche Verteilung der Veränderlichen vom δ Cephei-Typus" [On the spatial distribution of variable [stars] of the δ Cepheid type]. Astronomische Nachrichten (in German). 196 (4692): 201–208. Bibcode:1913AN....196..201H. From p. 204: "Zunächst ergibt sich eine Parallaxe der kleinen Magellanschen Wolke. ... und als außerhalb der Milchstraße liegend zu betrachten sein." (First, a parallax of the Small Magellanic Cloud follows. According to the 13 δ Cepheid variable [stars] that are treated above, the absolute brightness (the mean between the maximum and the minimum) of -7.3 m corresponds to a period of 6.6 days. Variable [stars] of the period 6.6 days have in the Small Magellanic Cloud a mean photographic star size of 14.5 m. If one assumes — according to the universal yellow color of the δ Cepheid variables — a color index of + 1.5 m, then the corresponding visual star size will equal 13.0 m. This consideration thus leads to a parallax p of the Small Magellanic Cloud, which is given by 5 log p = -7.3 - 13.0 = -20.3. One obtains p = 0.0001", corresponding to a distance of about 3000 light-years. Since the galactic latitude of the Small Magellanic Cloud amounts to about - 45°, then it would lie — according to the foregoing — about 2000 light-years from a plane [passing] through our Sun [and] lying parallel to the Milky Way and [it] would have to be regarded as lying outside the Milky Way.)
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  19. Shapley, Harlow (1918). "Studies on the colors and magnitudes in stellar clusters. Seventh paper: The distances, distributions in space, and dimensions of 69 globular clusters". The Astrophysical Journal. 48: 154–181. doi: 10.1086/142423 . See p. 155.
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