SN 1572, or B Cassiopeiae, was a supernova of Type Ia in the constellation Cassiopeia, one of eight supernovae visible to the naked eye in historical records. It was independently discovered by many individuals; the remnant of the supernova has been observed optically but was first detected at radio wavelengths. The appearance of the Milky Way supernova of 1572 belongs among the more important observation events in the history of astronomy; the appearance of the "new star" helped to revise ancient models of the heavens and to speed on a revolution in astronomy that began with the realisation of the need to produce better astrometric star catalogues. It challenged the Aristotelian dogma of the unchangeability of the realm of stars; the supernova of 1572 is called "Tycho's supernova", because of Tycho Brahe's extensive work De nova et nullius aevi memoria prius visa stella, a work containing both Tycho Brahe's own observations and the analysis of sightings from many other observers. Tycho was not the first to observe the 1572 supernova, although he was the most accurate observer of the object.
As accurate were his European colleagues, such as Wolfgang Schuler, Thomas Digges, John Dee, Francesco Maurolico, Jerónimo Muñoz, Tadeáš Hájek, or Bartholomäus Reisacher. In England, Queen Elizabeth had the mathematician and astrologer Thomas Allen and visit "to have his advice about the new Star that appeared in the Cassiopeia to which he gave his Judgement learnedly", as the antiquary John Aubrey recorded in his memoranda a century later. In Ming dynasty China, the star became an issue between Zhang Juzheng and the young Wanli Emperor: in accordance with the cosmological tradition, the emperor was warned to consider his misbehavior, since the new star was interpreted as an evil omen; the more reliable contemporary reports state that the new star itself burst forth soon after November 2, by November 11 it was brighter than Jupiter. Around November 16, 1572, it reached its peak brightness at about magnitude −4.0, with some descriptions giving it as equal to Venus when that planet was at its brightest.
The supernova remained visible to the naked eye into early 1574 fading until it disappeared from view. The supernova was classified as type I on the basis of its historical light curve soon after type I and type II supernovae were first defined on the basis of their spectra; the X-ray spectrum of the remnant showed that it was certainly of type Ia, but its exact classification continued to be debated until the detection of a light echo in 2008 gave final confirmation that it is a normal type Ia. The classification as a type Ia supernova of normal luminosity allows an accurate measure of the distance to SN 1572; the peak absolute magnitude can be calculated from the B-band decline rate to be −19.0±0.3. Given estimates of the peak apparent magnitude and the known extinction of 1.86±0.2 magnitudes, the distance is 3.8+1.5−0.9 kpc. The distance to the supernova remnant has been estimated to between 2 and 5 kpc, with recent studies suggesting a narrower range of 2.5 and 3 kpc. The search for a supernova remnant was negative until 1952, when Hanbury Brown and Cyril Hazard reported a radio detection at 158.5 MHz, obtained at the Jodrell Bank Observatory.
This was confirmed, its position more measured in 1957 by Baldwin and Edge using the Cambridge Radio Telescope working at a wavelength of 1.9 m. The remnant was identified tentatively in the second Cambridge Catalogue of Radio Sources as object "2C 34," and more as "3C 10" in the third Cambridge list. There is no dispute that 3C 10 is the remnant of the supernova observed in 1572–1573. Following a 1964 review article by Minkowski, the designation 3C 10 appears to be that most used in the literature when referring to the radio remnant of B Cas, although some authors use the tabulated Galactic designation G120.7+2.1 and many authors refer to it as Tycho's supernova remnant. Because the radio remnant was reported before the optical supernova-remnant wisps were discovered, the designation 3C 10 is used by some to signify the remnant at all wavelengths. An X-ray source designated Cepheus X-1 was detected by the Uhuru X-ray observatory at 4U 0022+63. Earlier catalog designations are X120+2 and XRS 00224+638.
Cepheus X-1 is in the constellation Cassiopeia, it is SN 1572, the Tycho SNR. The supernova remnant of B Cas was discovered in the 1960s by scientists with a Palomar Mountain telescope as a faint nebula, it was photographed by a telescope on the international ROSAT spacecraft. The supernova has been confirmed as Type Ia, in which a white dwarf star has accreted matter from a companion until it approaches the Chandrasekhar limit and explodes; this type of supernova does not create the spectacular nebula more typical of Type II supernovas, such as SN 1054 which created the Crab Nebula. A shell of gas is still expanding from its center at about 9,000 km/s. A recent study indicates a rate of expansion below 5,000 km/s. In October 2004, a letter in Nature reported the discovery of a G2 star, similar in type to our own Sun and named Tycho G, it is thought to be the companion star that contributed mass to the white dwarf that resulted in the supernova. A subsequent study, published in Marc
Edward Charles Pickering
Prof Edward Charles Pickering FRS HFRSE was an American astronomer and physicist and the older brother of William Henry Pickering. Along with Carl Vogel, Pickering discovered, he wrote Elements of Physical Manipulations. Pickering was born in Boston on 19 July 1846 the son of Edward Pickering and his wife, Charlotte Hammond, he was educated at Boston Latin School, studied Science at Harvard, where he received his BS degree in 1865. Soon after graduating from Harvard, Pickering taught physics at the Massachusetts Institute of Technology, he served as director of Harvard College Observatory from 1877 to his death in 1919, where he made great leaps forward in the gathering of stellar spectra through the use of photography. At Harvard, he recruited over 80 women to work for him, including Annie Jump Cannon, Henrietta Swan Leavitt, Antonia Maury; these women, the Harvard Computers, made several important discoveries at HCO. Leavitt's discovery of the period-luminosity relationship for Cepheids, published by Pickering, would prove the foundation for the modern understanding of cosmological distances.
In 1864 he married. In 1876 he co-founded the Appalachian Mountain Club. In 1882, Pickering developed a method to photograph the spectra of multiple stars by putting a large prism in front of the photographic plate, he along with Williamina Fleming and Annie Jump Cannon designed a stellar classification system based on an alphabetic system for spectral classes, first known as the Harvard Stellar Classification and became the basis for the Henry Draper Catalog. In 1896, Pickering published observations of unknown lines in the spectra of the star ζ-Puppis; these lines became known as the Pickering series and Pickering attributed them to hydrogen in 1897. Alfred Fowler gave the same attribution to similar lines that he observed in a hydrogen-helium mixture in 1912. Analysis by Niels Bohr included in his'trilogy' on atomic structure argued that the spectral lines arose from ionised helium, He+, not from hydrogen. Fowler was initially-skeptical but was convinced that Bohr was correct, by 1915 "spectroscopists had transferred definitively to helium."
Bohr's theoretical work on the Pickering series had demonstrated the need for "a re-examination of problems that seemed to have been solved within classical theories" and provided important confirmation for his atomic theory. Pickering is credited for making the Harvard College Observatory known and respected around the world, it continues today to be a well-respected observatory and program. Awards and honors Fellow of the American Academy of Arts and Sciences Gold Medal of the Royal Astronomical Society Valz Prize of the French Academy of Sciences Henry Draper Medal from the National Academy of Sciences Bruce Medal Prix Jules Janssen, the highest award of the Société astronomique de France, the French astronomical society Named after him The crater Pickering on the Moon The crater Pickering Mars. Asteroid 784 Pickeringia Elements of physical manipulation New York: Hurd & Houghton OCLC 16078533 A plan for securing observations of the variable stars Cambridge: J. Wilson and Son OCLC 260332440 An investigation in stellar photography Cambridge: J. Wilson and Son OCLC 15790725 Preparation and discussion of the Draper catalogue Cambridge: J. Wilson and Son OCLC 3492105 Plan for the endowment of astronomical research Cambridge: Astronomical observatory of Harvard College OCLC 30005226 Pickering, EC.
"The Allegheny Observatory In Its Relation To Astronomy". Science. 36. Pp. 417–421. Bibcode:1912Sci....36..417P. Doi:10.1126/science.36.927.417. PMID 17788756. Works by Edward Charles Pickering at Project Gutenberg Works by or about Edward Charles Pickering at Internet Archive Works by Edward Charles Pickering at LibriVox Edward Charles Pickering — Biographical Memoirs of the National Academy of Sciences Women Astronomers at Harvard at the Turn of the CenturyObituaries AN 208 133/134 JRASC 13 160 MNRAS 80 360 PASP 31 73
A star catalogue or star catalog, is an astronomical catalogue that lists stars. In astronomy, many stars are referred to by catalogue numbers. There are a great many different star catalogues which have been produced for different purposes over the years, this article covers only some of the more quoted ones. Star catalogues were compiled by many different ancient people, including the Babylonians, Chinese and Arabs, they were sometimes accompanied by a star chart for illustration. Most modern catalogues are available in electronic format and can be downloaded from space agencies data centres. Completeness and accuracy is described by the weakest apparent magnitude V and the accuracy of the positions. From their existing records, it is known that the ancient Egyptians recorded the names of only a few identifiable constellations and a list of thirty-six decans that were used as a star clock; the Egyptians called the circumpolar star "the star that cannot perish" and, although they made no known formal star catalogues, they nonetheless created extensive star charts of the night sky which adorn the coffins and ceilings of tomb chambers.
Although the ancient Sumerians were the first to record the names of constellations on clay tablets, the earliest known star catalogues were compiled by the ancient Babylonians of Mesopotamia in the late 2nd millennium BC, during the Kassite Period. They are better known by their Assyrian-era name'Three Stars Each'; these star catalogues, written on clay tablets, listed thirty-six stars: twelve for "Anu" along the celestial equator, twelve for "Ea" south of that, twelve for "Enlil" to the north. The Mul. Apin lists, dated to sometime before the Neo-Babylonian Empire, are direct textual descendants of the "Three Stars Each" lists and their constellation patterns show similarities to those of Greek civilization. In Ancient Greece, the astronomer and mathematician Eudoxus laid down a full set of the classical constellations around 370 BC, his catalogue Phaenomena, rewritten by Aratus of Soli between 275 and 250 BC as a didactic poem, became one of the most consulted astronomical texts in antiquity and beyond.
It contains descriptions of the positions of the stars, the shapes of the constellations and provided information on their relative times of rising and setting. In the 3rd century BC, the Greek astronomers Timocharis of Alexandria and Aristillus created another star catalogue. Hipparchus completed his star catalogue in 129 BC, which he compared to Timocharis' and discovered that the longitude of the stars had changed over time; this led him to determine the first value of the precession of the equinoxes. In the 2nd century, Ptolemy of Roman Egypt published a star catalogue as part of his Almagest, which listed 1,022 stars visible from Alexandria. Ptolemy's catalogue was based entirely on an earlier one by Hipparchus, it remained the standard star catalogue in the Arab worlds for over eight centuries. The Islamic astronomer al-Sufi updated it in 964, the star positions were redetermined by Ulugh Beg in 1437, but it was not superseded until the appearance of the thousand-star catalogue of Tycho Brahe in 1598.
Although the ancient Vedas of India specified how the ecliptic was to be divided into twenty-eight nakshatra, Indian constellation patterns were borrowed from Greek ones sometime after Alexander's conquests in Asia in the 4th century BC. The earliest known inscriptions for Chinese star names were written on oracle bones and date to the Shang Dynasty. Sources dating from the Zhou Dynasty which provide star names include the Zuo Zhuan, the Shi Jing, the "Canon of Yao" in the Book of Documents; the Lüshi Chunqiu written by the Qin statesman Lü Buwei provides most of the names for the twenty-eight mansions. An earlier lacquerware chest found in the Tomb of Marquis Yi of Zeng contains a complete list of the names of the twenty-eight mansions. Star catalogues are traditionally attributed to Shi Shen and Gan De, two rather obscure Chinese astronomers who may have been active in the 4th century BC of the Warring States period; the Shi Shen astronomy is attributed to Shi Shen, the Astronomic star observation to Gan De.
It was not until the Han Dynasty that astronomers started to observe and record names for all the stars that were apparent in the night sky, not just those around the ecliptic. A star catalogue is featured in one of the chapters of the late 2nd-century-BC history work Records of the Grand Historian by Sima Qian and contains the "schools" of Shi Shen and Gan De's work. Sima's catalogue—the Book of Celestial Offices —includes some 90 constellations, the stars therein named after temples, ideas in philosophy, locations such as markets and shops, different people such as farmers and soldiers. For his Spiritual Constitution of the Universe of 120 AD, the astronomer Zhang Heng compiled a star catalogue comprising 124 constellations. Chinese constellation names were adopted by the Koreans and Japanese. A large number of star catalogues were published by Muslim astronomers in the medieval Islamic world; these were Zij treatises, including Arzachel's Tables of Toledo, the Maragheh observatory's Zij-i Ilkhani and Ulugh Beg's Zij-i-Sultani.
Sky & Telescope
Sky & Telescope is a monthly American magazine covering all aspects of amateur astronomy, including the following: current events in astronomy and space exploration. The articles are intended for the informed lay reader and include detailed discussions of current discoveries by participating scientists; the magazine is illustrated in full color, with both amateur and professional photography of celestial sights, as well as tables and charts of upcoming celestial events. Sky & Telescope was founded by Charles A Federer and his wife Helen Spence Federer and began publication at Harvard College Observatory in November 1941, as a result of the merger of the separate magazines, The Sky and The Telescope. In 2005, Sky Publishing Corporation was acquired by New Track Media, a portfolio company of the private equity firm Boston Ventures. In 2014, New Track was sold to F+W Media; the magazine played an important role in the dissemination of knowledge about telescope making, through the column "Gleanings for ATMs" that ran from 1933 to 1990.
Its main competitor is Astronomy. Amateur astronomy Amateur telescope making Official website
A nova or classical nova is a transient astronomical event that causes the sudden appearance of a bright "new" star, that fades over several weeks or many months. Novae involve an interaction between two stars that cause the flareup, perceived as a new entity, much brighter than the stars involved. Causes of the dramatic appearance of a nova vary, depending on the circumstances of the two progenitor stars. All observed novae involve located binary stars, either a pair of red dwarfs in the process of merging, or a white dwarf and another star; the main sub-classes of novae are classical novae, recurrent novae, dwarf novae. They are all considered to be cataclysmic variable stars. Luminous red novae share the name and are cataclysmic variables, but are a different type of event caused by a stellar merger. With similar names are the much more energetic supernovae and kilonovae. Classical nova eruptions are the most common type of nova, they are created in a close binary star system consisting of a white dwarf and either a main sequence, sub-giant, or red giant star.
When the orbital period falls in the range of several days to one day, the white dwarf is close enough to its companion star to start drawing accreted matter onto the surface of the white dwarf, which creates a dense but shallow atmosphere. This atmosphere is hydrogen and is thermally heated by the hot white dwarf, which reaches a critical temperature causing rapid runaway ignition by fusion. From the dramatic and sudden energies created, the now hydrogen-burnt atmosphere is dramatically expelled into interstellar space, its brightened envelope is seen as the visible light created from the nova event, was mistaken as a "new" star. A few novae produce short-lived nova remnants, lasting for several centuries. Recurrent nova processes are the same as the classical nova, except that the fusion ignition may be repetitive because the companion star can again feed the dense atmosphere of the white dwarf. Novae most occur in the sky along the path of the Milky Way near the observed galactic centre in Sagittarius.
They occur far more than galactic supernovae, averaging about ten per year. Most are found telescopically only one every year to eighteen months reaching naked-eye visibility. Novae reaching first or second magnitude occur only several times per century; the last bright nova was V1369 Centauri reaching 3.3 magnitude on 14 December 2013. During the sixteenth century, astronomer Tycho Brahe observed the supernova SN 1572 in the constellation Cassiopeia, he described it in his book De nova stella. In this work he argued that a nearby object should be seen to move relative to the fixed stars, that the nova had to be far away. Although this event was a supernova and not a nova, the terms were considered interchangeable until the 1930s. After this, novae were classified as classical novae to distinguish them from supernovae, as their causes and energies were thought to be different, based in the observational evidence. Despite the term "stella nova" meaning "new star", novae most take place as a result of white dwarfs: remnants of old stars.
Evolution of potential novae begins with two main sequence stars in a binary system. One of the two evolves into a red giant, leaving its remnant white dwarf core in orbit with the remaining star; the second star—which may be either a main sequence star or an aging giant—begins to shed its envelope onto its white dwarf companion when it overflows its Roche lobe. As a result, the white dwarf captures matter from the companion's outer atmosphere in an accretion disk, in turn, the accreted matter falls into the atmosphere; as the white dwarf consists of degenerate matter, the accreted hydrogen does not inflate, but its temperature increases. Runaway fusion occurs when the temperature of this atmospheric layer reaches ~20 million K, initiating nuclear burning, via the CNO cycle. Hydrogen fusion may occur in a stable manner on the surface of the white dwarf for a narrow range of accretion rates, giving rise to a super soft X-ray source, but for most binary system parameters, the hydrogen burning is unstable thermally and converts a large amount of the hydrogen into other, heavier chemical elements in a runaway reaction, liberating an enormous amount of energy.
This blows the remaining gases away from the surface of the white dwarf surface and produces an bright outburst of light. The rise to peak brightness may be rapid, or gradual; this is related to the speed class of the nova. The time taken for a nova to decay by around 2 or 3 magnitudes from maximum optical brightness is used for classification, via its speed class. Fast novae will take fewer than 25 days to decay by 2 magnitudes, while slow novae will take more than 80 days. In spite of their violence the amount of material ejected in novae is only about 1⁄10,000 of a solar mass, quite small relative to the mass of the white dwarf. Furthermore, only five percent of the accreted mass is fused during the power outburst. Nonetheless, this is enough energy to accelerate nova ejecta to velocities as high as several thousand kilometers per second—higher for fast novae than slow ones—with a concurrent rise in luminosity from a few times solar to 50,000–100,000 times solar. In 2010 scientists using NASA's Fermi Gamma-ray Space Telescope discovered that a nova can emit gamma-rays.
A white dwarf can generate multiple novae over t
Ida Barney was an American astronomer, best known for her 22 volumes of astrometric measurements on 150,000 stars. She was educated at Smith College and Yale University and spent most of her career at the Yale University Observatory, she was the 1952 recipient of the Annie J. Cannon Award in Astronomy. Barney was born on 6 November 1886 in Connecticut, her mother was Ida Bushnell Barney and her father was Samuel Eben Barney. She was the New Haven Bird Club President. After her retirement from Yale, she continued to live in New Haven, where she died on 7 March 1982, 95 years old. In 1908, Barney graduated from Smith College with a Bachelor of Arts degree. There, she was a member of national honor societies for students. Three years she received her Ph. D. in mathematics from Yale University. From 1911–1912, just after receiving her Ph. D. Barney was a mathematics professor at Rollins College. At the conclusion of that year, she moved to her alma mater to Smith College, where she was an instructor of mathematics.
In 1917, she was hired as a professor at Lake Erie College, where she stayed until 1919. In 1920, she returned to Smith College as an assistant professor. In 1922, the Yale University Observatory appointed Barney a Research Assistant, a title she held until 1949, when she was promoted to Research Associate; the Observatory, like many other university observatories, was allocating significant resources to astrometry, thanks to the development of telescope-mounted cameras. At the beginning of her career in astronomy, Barney worked under Frank Schlesinger; the work was tedious, which Schlesinger thought to be suitable for women incapable of theoretical research. Despite this, she developed several methods that increased both the accuracy and speed of her measurements, including the use of a machine that automatically centered the photographic plates, her life's work, completed over 23 years, contributed to the Yale Observatory Zone Catalog, a series of star catalogs published by the Yale Observatory for 1939 to 1983, containing around 400,000 stars, influenced the Bright Star Catalogue.
In 1941, when Schlesinger retired, Barney took over full supervision of the cataloguing. Under her direction, the measurements of the photographic plates were completed at the IBM Watson Scientific Laboratory using a new electronic device that further reduced eye strain and increased accuracy, her individual contribution to these star catalogues recorded the position and proper motion of 150,000 stars. Due to its high accuracy, the catalogue is still used today in proper motion studies, she retired from academic life in 1955. She was succeeded by Ellen Dorrit Hoffleit. While a Research Associate at the Yale University Observatory, in 1952, Barney was awarded the triennial Annie J. Cannon Award in Astronomy, a prestigious award for women astronomers given by the American Astronomical Society, her remains are interred at Grove Street Cemetery in Connecticut. Asteroid 5655 Barney, discovered by Ingrid van Houten-Groeneveld, Cornelis Johannes van Houten and Tom Gehrels at Palomar Observatory in 1973, was named it in her memory.
Barney, Ida. "Discussion of the proper motions in the equatorial Zone". Astronomical Journal. 37: 181. Bibcode:1927AJ.....37..181B. Doi:10.1086/104785. Barney, Ida. "Analysis of the Yale proper motions in the zones between +50 degrees and +55 degrees and between +55 degrees and +60". Astronomical Journal. 40: 168. Bibcode:1930AJ.....40..168B. Doi:10.1086/105000. Barney, Ida. "An effect of a star's color upon its apparent photographic position". Astronomical Journal. 47: 86. Bibcode:1938AJ.....47...86B. Doi:10.1086/105478. Barney, Ida. "On the accuracy of the proper motions in the General Catalogue Albany". Astronomical Journal. 48: 51. Bibcode:1939AJ.....48...51B. Doi:10.1086/105546. Barney, Ida. "New reductions of astrographic plates with the help of the Yale photographic Catalogues". Astronomical Journal. 49: 39. Bibcode:1940AJ.....49...39B. Doi:10.1086/105625. List of Minor Planets 5001–6000, #5655 List of minor planets named after people Meanings of minor planet names: 5501–6000 Citations ReferencesAnnie J. Cannon Award in Astronomy, American Astronomical Society, 2012, retrieved 20 November 2012 "General Notes", Publications of the Astronomical Society of the Pacific, 65: 98–100, April 1953, Bibcode:1953PASP...65...98.
Doi:10.1086/126550 Hockey, Thomas, " Barney1159 T-2", The Biographical Encyclopedia of Astronomers, Springer Publishing, ISBN 978-0-387-31022-0, retrieved November 19, 2012, Hoffleit, E. Dorrit, "Ida M. Barney, Ace Astrometrist", STATUS: The Committee on the Status of Women in Astronomy, American Astronomical Society, retrieved 17 November 2012 Ida Barney, Find A Grave, retrieved 19 November 2012 Milite, George A. Pamela Proffitt, ed. "Ida Barney", Notable Women Scientists, Farmington Hills, Michigan: Gale Group, Inc. p. 27, ISBN 978-0-7876-3900-6 Ogilvie, Marilyn. "Ida Barney", Notable Women in the Physical Sciences: A Biographical Dictionary, Connecticut: Greenwood Press, pp. 1–4, ISBN 978-0-313-29303-0CS1 maint: Uses editors parameter "Bibliography: Ida Smith Barney". Women in Astronomy. Library of Congress. Retrieved 18 November 2012. Hockey, Thomas. Doritt E. Hoffleit
A globular cluster is a spherical collection of stars that orbit a galactic core, as a satellite. Globular clusters are tightly bound by gravity, which gives them their spherical shapes, high stellar densities toward their centers; the name of this category of star cluster is derived from globulus -- a small sphere. A globular cluster is sometimes known, more as a globular. Globular clusters are found in the halo of a galaxy and contain more stars, are much older, than the less dense, open clusters which are found in the disk of a galaxy. Globular clusters are common. Larger galaxies can have more: The Andromeda Galaxy, for instance, may have as many as 500; some giant elliptical galaxies, such as M87, have as many as 13,000 globular clusters. Every galaxy of sufficient mass in the Local Group has an associated group of globular clusters, every large galaxy surveyed, has been found to possess a system of globular clusters; the Sagittarius Dwarf galaxy, the disputed Canis Major Dwarf galaxy appear to be in the process of donating their associated globular clusters to the Milky Way.
This demonstrates. Although it appears that globular clusters contain some of the first stars to be produced in the galaxy, their origins and their role in galactic evolution are still unclear, it does appear clear that globular clusters are different from dwarf elliptical galaxies and were formed as part of the star formation of the parent galaxy, rather than as a separate galaxy. The first known globular cluster, now called M22, was discovered in 1665 by Abraham Ihle, a German amateur astronomer. However, given the small aperture of early telescopes, individual stars within a globular cluster were not resolved until Charles Messier observed M4 in 1764; the first eight globular clusters discovered are shown in the table. Subsequently, Abbé Lacaille would list NGC 104, NGC 4833, M55, M69, NGC 6397 in his 1751–52 catalogue; the M before a number refers to Charles Messier's catalogue, while NGC is from the New General Catalogue by John Dreyer. When William Herschel began his comprehensive survey of the sky using large telescopes in 1782 there were 34 known globular clusters.
Herschel discovered another 36 himself and was the first to resolve all of them into stars. He coined the term "globular cluster" in his Catalogue of a Second Thousand New Nebulae and Clusters of Stars published in 1789; the number of globular clusters discovered continued to increase, reaching 83 in 1915, 93 in 1930 and 97 by 1947. A total of 152 globular clusters have now been discovered in the Milky Way galaxy, out of an estimated total of 180 ± 20; these additional, undiscovered globular clusters are believed to be hidden behind the gas and dust of the Milky Way. Beginning in 1914, Harlow Shapley began a series of studies of globular clusters, published in about 40 scientific papers, he examined the RR Lyrae variables in the clusters and used their period–luminosity relationship for distance estimates. It was found that RR Lyrae variables are fainter than Cepheid variables, which caused Shapley to overestimate the distances of the clusters. Of the globular clusters within the Milky Way, the majority are found in a halo around the galactic core, the large majority are located in the celestial sky centered on the core.
In 1918, this asymmetrical distribution was used by Shapley to make a determination of the overall dimensions of the galaxy. By assuming a spherical distribution of globular clusters around the galaxy's center, he used the positions of the clusters to estimate the position of the Sun relative to the galactic center. While his distance estimate was in significant error, it did demonstrate that the dimensions of the galaxy were much greater than had been thought, his error was due to interstellar dust in the Milky Way, which absorbs and diminishes the amount of light from distant objects, such as globular clusters, that reaches the Earth, thus making them appear to be more distant than they are. Shapley's measurements indicated that the Sun is far from the center of the galaxy contrary to what had been inferred from the nearly distribution of ordinary stars. In reality, most ordinary stars lie within the galaxy's disk and those stars that lie in the direction of the galactic centre and beyond are thus obscured by gas and dust, whereas globular clusters lie outside the disk and can be seen at much further distances.
Shapley was subsequently assisted in his studies of clusters by Henrietta Swope and Helen Battles Sawyer. In 1927–29, Shapley and Sawyer categorized clusters according to the degree of concentration each system has toward its core; the most concentrated clusters were identified as Class I, with successively diminishing concentrations ranging to Class XII. This became known as the Shapley–Sawyer Concentration Class In 2015, a new type of globular cluster was proposed on the basis of observational data, the dark globular clusters; the formation of globular clusters remains a poorly understood phenomenon and it remains uncertain whether the stars in a globular cluster form in a single generation or are spawned across multiple generations over a period of several hundred million years. In many globular clusters, most of the stars are at approxima