Naked eye called bare eye or unaided eye, is the practice of engaging in visual perception unaided by a magnifying or light-collecting optical instrument, such as a telescope or microscope. Vision corrected to normal acuity using corrective lenses is still considered "naked". In astronomy, the naked eye may be used to observe celestial events and objects visible without equipment, such as conjunctions, passing comets, meteor showers, the brightest asteroids, including 4 Vesta. Sky lore and various tests demonstrate an impressive variety of phenomena visible to the unaided eye; the basic accuracies of the human eye are: Quick autofocus from distances of 25 cm to 50 cm to infinity. Angular resolution: about 1 arcminute 0.02° or 0.0003 radians, which corresponds to 0.3 m at a 1 km distance. Field of view: simultaneous visual perception in an area of about 160° × 175°. Faint stars up to +8 magnitude under a dark sky. Photometry to ±10% or 1% of intensity – in a range between night and day of 1:10,000,000,000.
Symmetries of 10–20', see the measurements of Tycho Brahe. Interval estimations to 3–5%. Unconscious recognizing of movement. Visual perception allows a person to gain much information about their surroundings: the distances and 3-dimensional position of things and persons the vertical and the slope of plain objects luminosities and colors and their changes by time and direction The visibility of astronomical objects is affected by light pollution. A few hundred kilometers away from a metropolitan area where the sky can appear to be dark, it is still the residual light pollution that sets the limit on the visibility of faint objects. For most people, these are to be the best observing conditions within their reach. Under such "typical" dark sky conditions, the naked eye can see stars with an apparent magnitude up to +6m. Under perfect dark sky conditions where all light pollution is absent, stars as faint as +8m might be visible; the angular resolution of the naked eye is about 1′. There is anecdotal evidence that people had seen the Galilean moons of Jupiter before telescopes were invented.
Uranus and Vesta had most been seen but could not be recognized as planets because they appear so faint at maximum brightness. Uranus, when discovered in 1781, was the first planet discovered using technology rather than being spotted by the naked eye. Theoretically, in a typical dark sky, the dark adapted human eye would see the about 5,600 stars brighter than +6m while in perfect dark sky conditions about 45,000 stars brighter than +8m might be visible. In practice, the atmospheric extinction and dust reduces this number somewhat. In the center of a city, where the naked-eye limiting magnitude due to extreme amounts of light pollution can be as low as 2m, with as few as 50 stars visible. Colors can be seen but this is limited by the fact that the eye uses rods instead of cones to view fainter stars; the visibility of diffuse objects such as star clusters and galaxies is much more affected by light pollution than is that of planets and stars. Under typical dark conditions only a few such objects are visible.
These include the Pleiades, h/χ Persei, the Andromeda galaxy, the Carina Nebula, the Orion Nebula, Omega Centauri, 47 Tucanae, the Ptolemy Cluster Messier 7 near the tail of Scorpius and the globular cluster M13 in Hercules. The Triangulum Galaxy is a difficult averted vision object and only visible at all if it is higher than 50° in the sky; the globular clusters M 3 in Canes Venatici and M 92 in Hercules are visible with the naked eye under such conditions. Under dark sky conditions, however, M33 is easy to see in direct vision. Many other Messier objects are visible under such conditions; the most distant objects that have been seen by the naked eye are nearby bright galaxies such as Centaurus A, Bode's Galaxy, Sculptor Galaxy, Messier 83. Five planets can be recognized as planets from Earth with the naked eye: Mercury, Mars and Saturn. Under typical dark sky conditions Uranus can be seen as well with averted vision, as can the asteroid Vesta at its brighter oppositions; the Sun and the Moon—the remaining noticeable naked-eye objects of the solar system—are sometimes added to make seven "planets."
During daylight only the Moon and Sun are obvious naked eye objects, but in many cases Venus can be spotted in daylight and in rarer cases Jupiter. Close to sunset and sunrise bright stars like Sirius or Canopus can be spotted with the naked eye as long as one knows the exact position in which to look; the zenith of naked-eye astronomy was the work of Tycho Brahe. He built an extensive observatory to make precise measurements of the heavens without any instruments for magnification. In 1610, Galileo Galilei pointed a telescope towards the sky, he discovered the moons of Jupiter and the phases of Venus, among other things. Meteor showers are better observed by naked eye than with binoculars; such showers include the December Geminids. Some 100 satellites per night, the International Space Station and the Milky Way are other popular objects visible to the naked eye. Many other things can be estimated without an instrument. If an arm is stretched the span of the hand corresponds to an angle of 18 to 20°.
The distance of a person, just covered up by the outstretched thumbnail, is about 100 meters. The vertical can be esti
Scorpius is one of the constellations of the zodiac. Its name is Latin for scorpion, its symbol is. Scorpius is one of the 48 constellations identified by the Greek astronomer Ptolemy in the second century, it is an ancient constellation. It lies between Libra to Sagittarius to the east, it is a large constellation located in the southern hemisphere near the center of the Milky Way. Scorpius contains many bright stars, including Antares, "rival of Mars," so named because of its distinct reddish hue. Given their proximity to one another, λ Sco and υ Sco are sometimes referred to as the Cat's Eyes; the constellation's bright stars form a pattern like a longshoreman's hook. Most of them are massive members of the nearest OB association: Scorpius-Centaurus; the star δ Sco, after having been a stable 2.3 magnitude star, flared in July 2000 to 1.9 in a matter of weeks. It has since become a variable star fluctuating between 2.0 and 1.6. This means. U Scorpii is the fastest known nova with a period of about 10 years.
The close pair of stars ω¹ Scorpii and ω² Scorpii are an optical double, which can be resolved by the unaided eye. They have contrasting yellow colours; the star once designated γ Sco is today known as σ Lib. Moreover, the entire constellation of Libra was considered to be claws of Scorpius in Ancient Greek times, with a set of scales held aloft by Astraea being formed from these western-most stars during Greek times; the division into Libra was formalised during Roman times. Due to its location straddling the Milky Way, this constellation contains many deep-sky objects such as the open clusters Messier 6 and Messier 7, NGC 6231, the globular clusters Messier 4 and Messier 80. Messier 80 is a globular cluster of 33,000 light-years from Earth, it is a compact Shapley class II cluster. M80 was discovered in 1781 by Charles Messier, it was the site of a rare discovery in 1860. NGC 6302 called the Bug Nebula, is a bipolar planetary nebula. NGC 6334 known as the Cat's Paw Nebula, is an emission nebula and star-forming region.
In Greek mythology, the myths associated with Scorpio invariably contain a reference to Orion. According to one of these myths it is written that Orion boasted to goddess Artemis and her mother, that he would kill every animal on the Earth. Although Artemis was known to be a hunter herself she offered protection to all creatures. Artemis and her mother Leto sent a scorpion to deal with Orion; the pair battled and the scorpion killed Orion. However, the contest was a lively one that caught the attention of the king of the gods Zeus, who raised the scorpion to heaven and afterwards, at the request of Artemis, did the same for Orion to serve as a reminder for mortals to curb their excessive pride. There is a version that Orion was better than the goddess Artemis but said that Artemis was better than he and so Artemis took a liking to Orion; the god Apollo, Artemis's twin brother, sent a scorpion to attack Orion. After Orion was killed, Artemis asked Zeus to put Orion up in the sky. So every winter Orion hunts in the sky, but every summer he flees as the constellation of the scorpion comes.
In another Greek story involving Scorpio without Orion, Phaeton went to his father, who had earlier sworn by the River Styx to give Phaeton anything he should ask for. Phaeton wanted to drive his father's Sun Chariot for a day. Although Helios tried to dissuade his son, Phaeton was adamant. However, when the day arrived, Phaeton panicked and lost control of the white horses that drew the chariot. First, the Earth grew chill as Phaeton flew too high and encountered the celestial scorpion, its deadly sting raised to strike. Alarmed, he dipped the chariot too close. By accident, Phaeton turned most of Africa into desert and darkened the skin of the Ethiopian nation until it was black. Zeus was forced to intervene by striking the runaway chariot and Phaeton with a lightning bolt to put an end to its rampage and Phaeton plunged into the River Eridanos; the Babylonians called this constellation MUL. GIR. TAB - the'Scorpion', the signs can be read as'the a burning sting'. In some old descriptions the constellation of Libra is treated as the Scorpion's claws.
Libra was known as the Claws of the Scorpion in Greek. The Western astrological sign Scorpio differs from the astronomical constellation. Astronomically, the sun is in Scorpius for just six days, from November 23 to November 28. Much of the difference is due to the constellation Ophiuchus, used by few astrologers. Scorpius corresponds to the Hindu nakshatras Anuradha and Mula; the Javanese people of Indonesia call this constellation Banyakangrem or Kalapa Doyong due to the shape similarity. In Hawaii, Scorpius is known as the demigod Maui's Fishhook or Ka Makau Nui o Māui and the nam
The Pleiades known as the Seven Sisters and Messier 45, are an open star cluster containing middle-aged, hot B-type stars located in the constellation of Taurus. It is among the nearest star clusters to Earth and is the cluster most obvious to the naked eye in the night sky; the cluster is dominated by hot blue and luminous stars that have formed within the last 100 million years. Reflection nebulae around the brightest stars were once thought to be left over material from the formation of the cluster, but are now considered to be an unrelated dust cloud in the interstellar medium through which the stars are passing. Computer simulations have shown that the Pleiades were formed from a compact configuration that resembled the Orion Nebula. Astronomers estimate that the cluster will survive for about another 250 million years, after which it will disperse due to gravitational interactions with its galactic neighborhood; the name of the Pleiades comes from Ancient Greek. It derives from plein because of the cluster's importance in delimiting the sailing season in the Mediterranean Sea: "the season of navigation began with their heliacal rising".
However, in mythology the name was used for the Pleiades, seven divine sisters, the name deriving from that of their mother Pleione and meaning "daughters of Pleione". In reality, the name of the star cluster certainly came first, Pleione was invented to explain it; the Pleiades are a prominent sight in winter in the Northern Hemisphere, are visible out to mid-Southern latitudes. They have been known since antiquity to cultures all around the world, including the Celts, Hawaiians, Māori, Aboriginal Australians, the Persians, the Arabs, the Chinese, the Quechua, the Japanese, the Maya, the Aztec, the Sioux, the Kiowa, the Cherokee. In Hinduism, the Pleiades are associated with the war-god Kartikeya, they are mentioned three times in the Bible. The earliest known depiction of the Pleiades is a Northern German bronze age artifact known as the Nebra sky disk, dated to 1600 BC; the Babylonian star catalogues name the Pleiades MULMUL, meaning "stars", they head the list of stars along the ecliptic, reflecting the fact that they were close to the point of vernal equinox around the 23rd century BC.
The Ancient Egyptians may have used the names "Followers" and "Ennead" in the prognosis texts of the Calendar of Lucky and Unlucky Days of papyrus Cairo 86637. Some Greek astronomers considered them to be a distinct constellation, they are mentioned by Hesiod's Works and Days, Homer's Iliad and Odyssey, the Geoponica; some scholars of Islam suggested that the Pleiades are the "star" mentioned in Sura An-Najm of the Quran. In Japan, the constellation is mentioned under the name Mutsuraboshi in the 8th century Kojiki; the constellation is now known in Japan as Subaru. It was chosen as the brand name of Subaru automobiles to reflect the origins of the firm as the joining of five companies, is depicted in the firm's six-star logo. Galileo Galilei was the first astronomer to view the Pleiades through a telescope, he thereby discovered. He published his observations, including a sketch of the Pleiades showing 36 stars, in his treatise Sidereus Nuncius in March 1610; the Pleiades have long been known to be a physically related group of stars rather than any chance alignment.
John Michell calculated in 1767 that the probability of a chance alignment of so many bright stars was only 1 in 500,000, so surmised that the Pleiades and many other clusters of stars must be physically related. When studies were first made of the stars' proper motions, it was found that they are all moving in the same direction across the sky, at the same rate, further demonstrating that they were related. Charles Messier measured the position of the cluster and included it as M45 in his catalogue of comet-like objects, published in 1771. Along with the Orion Nebula and the Praesepe cluster, Messier's inclusion of the Pleiades has been noted as curious, as most of Messier's objects were much fainter and more confused with comets—something that seems scarcely possible for the Pleiades. One possibility is that Messier wanted to have a larger catalogue than his scientific rival Lacaille, whose 1755 catalogue contained 42 objects, so he added some bright, well-known objects to boost his list.
Edme-Sébastien Jeaurat drew in 1782 a map of 64 stars of the Pleiades from his observations in 1779, which he published in 1786. The distance to the Pleiades can be used as an important first step to calibrate the cosmic distance ladder; as the cluster is so close to the Earth, its distance is easy to measure and has been estimated by many methods. Accurate knowledge of the distance allows astronomers to plot a Hertzsprung-Russell diagram for the cluster, when compared to those plotted for clusters whose distance is not known, allows their distances to be estimated. Other methods can extend the distance scale from open clusters to galaxies and clusters of galaxies, a cosmic distance ladder can be constructed. Astronomers' understanding of the age and future evolution of the universe is influenced by their knowledge of the distance to the Pleiades, yet some authors argue that the controversy over the distance to the Pleiades discussed below is a red herring, since the cosmic distance ladder can rely on a suite of other nearby clusters where consensus exists regarding the distances as esta
Messier 22 or M22 known as NGC 6656, is an elliptical globular cluster of stars in the constellation Sagittarius, near the Galactic bulge region. It is one of the brightest globulars, visible in the night sky; the brightest stars are 11th magnitude, with hundreds of stars bright enough to resolve with an 8" telescope. M22 is located just south of the Ecliptic, northwest of Lambda Sagittarii, the northernmost star of the "Teapot" asterism. M22 was one of the first globulars to be discovered, on August 26, 1665 by Abraham Ihle and it was included in Charles Messier's catalog of comet-like objects on June 5, 1764, it was one of the first globular clusters to be studied first by Harlow Shapley in 1930. He discovered 70,000 stars and found it had a dense core. Halton Arp and William G. Melbourne continued studies in 1959; because of the large color spread of its red giant branch sequence, similar to that observed in Omega Centauri, it became the object of intense scrutiny starting in 1977 with James E. Hesser et al.
M22 is one of the nearer globular clusters to Earth at a distance of about 10,600 light-years away. It spans 32' on the sky. 32 variable stars have been recorded in M22. It is projected in front of the galactic bulge and is therefore useful for its microlensing effect on the background stars in the bulge. Despite its relative proximity to us, this metal-poor cluster's light is limited by dust extinction, giving it an apparent magnitude of 5.5 making it the brightest globular cluster visible from mid-northern latitudes. However, due to its southerly declination, M22 never rises high in the sky and so appears less impressive to northern hemisphere observers than other summer sky globulars such as M13 and M5. M22 is unusual in that it is one of only four globulars that are known to contain a planetary nebula, it was discovered using the IRAS satellite by Fred Gillett et al.in 1986 as a pointlike source and subsequently identified as a planetary nebula in 1989 by Gillett et al. The planetary nebula's central star is a blue star.
The planetary nebula is estimated to be a mere ~6,000 years old. Two black holes of between 10 and 20 solar masses each have been discovered with the Very Large Array radio telescope in New Mexico, corroborated by the Chandra X-ray telescope, in 2012, their detection implies that gravitational ejection of black holes from clusters is not as efficient as was thought, leads to estimates of a total 5 to 100 black holes within M22. Interactions between stars and black holes could explain the unusually large core of the cluster. New General Catalogue Messier object List of Messier objects List of globular clusters Messier 22, SEDS Messier pages Messier 22, Galactic Globular Clusters Database page NASA Astronomy Picture of the Day: Globular Cluster M22 from CFHT NASA Astronomy Picture of the Day: M22 and the Wanderers Merriefield, Mike. "M22 – Globular Cluster". Deep Sky Videos. Brady Haran. Messier 22 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Sky Map and images
An astronomical object or celestial object is a occurring physical entity, association, or structures that exists in the observable universe. In astronomy, the terms object and body are used interchangeably. However, an astronomical body or celestial body is a single bound, contiguous entity, while an astronomical or celestial object is a complex, less cohesively bound structure, which may consist of multiple bodies or other objects with substructures. Examples of astronomical objects include planetary systems, star clusters and galaxies, while asteroids, moons and stars are astronomical bodies. A comet may be identified as both body and object: It is a body when referring to the frozen nucleus of ice and dust, an object when describing the entire comet with its diffuse coma and tail; the universe can be viewed as having a hierarchical structure. At the largest scales, the fundamental component of assembly is the galaxy. Galaxies are organized into groups and clusters within larger superclusters, that are strung along great filaments between nearly empty voids, forming a web that spans the observable universe.
The universe has a variety of morphologies, with irregular and disk-like shapes, depending on their formation and evolutionary histories, including interaction with other galaxies, which may lead to a merger. Disc galaxies encompass lenticular and spiral galaxies with features, such as spiral arms and a distinct halo. At the core, most galaxies have a supermassive black hole, which may result in an active galactic nucleus. Galaxies can have satellites in the form of dwarf galaxies and globular clusters; the constituents of a galaxy are formed out of gaseous matter that assembles through gravitational self-attraction in a hierarchical manner. At this level, the resulting fundamental components are the stars, which are assembled in clusters from the various condensing nebulae; the great variety of stellar forms are determined entirely by the mass and evolutionary state of these stars. Stars may be found in multi-star systems. A planetary system and various minor objects such as asteroids and debris, can form in a hierarchical process of accretion from the protoplanetary disks that surrounds newly formed stars.
The various distinctive types of stars are shown by the Hertzsprung–Russell diagram —a plot of absolute stellar luminosity versus surface temperature. Each star follows an evolutionary track across this diagram. If this track takes the star through a region containing an intrinsic variable type its physical properties can cause it to become a variable star. An example of this is the instability strip, a region of the H-R diagram that includes Delta Scuti, RR Lyrae and Cepheid variables. Depending on the initial mass of the star and the presence or absence of a companion, a star may spend the last part of its life as a compact object; the table below lists the general categories of bodies and objects by their structure. List of light sources List of Solar System objects List of Solar System objects by size Lists of astronomical objects SkyChart, Sky & Telescope at the Library of Congress Web Archives Monthly skymaps for every location on Earth
The Moon is an astronomical body that orbits planet Earth and is Earth's only permanent natural satellite. It is the fifth-largest natural satellite in the Solar System, the largest among planetary satellites relative to the size of the planet that it orbits; the Moon is after Jupiter's satellite Io the second-densest satellite in the Solar System among those whose densities are known. The Moon is thought to have formed not long after Earth; the most accepted explanation is that the Moon formed from the debris left over after a giant impact between Earth and a Mars-sized body called Theia. The Moon is in synchronous rotation with Earth, thus always shows the same side to Earth, the near side; the near side is marked by dark volcanic maria that fill the spaces between the bright ancient crustal highlands and the prominent impact craters. After the Sun, the Moon is the second-brightest visible celestial object in Earth's sky, its surface is dark, although compared to the night sky it appears bright, with a reflectance just higher than that of worn asphalt.
Its gravitational influence produces the ocean tides, body tides, the slight lengthening of the day. The Moon's average orbital distance is 1.28 light-seconds. This is about thirty times the diameter of Earth; the Moon's apparent size in the sky is the same as that of the Sun, since the star is about 400 times the lunar distance and diameter. Therefore, the Moon covers the Sun nearly during a total solar eclipse; this matching of apparent visual size will not continue in the far future because the Moon's distance from Earth is increasing. The Moon was first reached in September 1959 by an unmanned spacecraft; the United States' NASA Apollo program achieved the only manned lunar missions to date, beginning with the first manned orbital mission by Apollo 8 in 1968, six manned landings between 1969 and 1972, with the first being Apollo 11. These missions returned lunar rocks which have been used to develop a geological understanding of the Moon's origin, internal structure, the Moon's history. Since the Apollo 17 mission in 1972, the Moon has been visited only by unmanned spacecraft.
Both the Moon's natural prominence in the earthly sky and its regular cycle of phases as seen from Earth have provided cultural references and influences for human societies and cultures since time immemorial. Such cultural influences can be found in language, lunar calendar systems and mythology; the usual English proper name for Earth's natural satellite is "the Moon", which in nonscientific texts is not capitalized. The noun moon is derived from Old English mōna, which stems from Proto-Germanic *mēnô, which comes from Proto-Indo-European *mḗh₁n̥s "moon", "month", which comes from the Proto-Indo-European root *meh₁- "to measure", the month being the ancient unit of time measured by the Moon; the name "Luna" is used. In literature science fiction, "Luna" is used to distinguish it from other moons, while in poetry, the name has been used to denote personification of Earth's moon; the modern English adjective pertaining to the Moon is lunar, derived from the Latin word for the Moon, luna. The adjective selenic is so used to refer to the Moon that this meaning is not recorded in most major dictionaries.
It is derived from the Ancient Greek word for the Moon, σελήνη, from, however derived the prefix "seleno-", as in selenography, the study of the physical features of the Moon, as well as the element name selenium. Both the Greek goddess Selene and the Roman goddess Diana were alternatively called Cynthia; the names Luna and Selene are reflected in terminology for lunar orbits in words such as apolune and selenocentric. The name Diana comes from the Proto-Indo-European *diw-yo, "heavenly", which comes from the PIE root *dyeu- "to shine," which in many derivatives means "sky and god" and is the origin of Latin dies, "day"; the Moon formed 4.51 billion years ago, some 60 million years after the origin of the Solar System. Several forming mechanisms have been proposed, including the fission of the Moon from Earth's crust through centrifugal force, the gravitational capture of a pre-formed Moon, the co-formation of Earth and the Moon together in the primordial accretion disk; these hypotheses cannot account for the high angular momentum of the Earth–Moon system.
The prevailing hypothesis is that the Earth–Moon system formed after an impact of a Mars-sized body with the proto-Earth. The impact blasted material into Earth's orbit and the material accreted and formed the Moon; the Moon's far side has a crust, 30 mi thicker than that of the near side. This is thought to be; this hypothesis, although not perfect best explains the evidence. Eighteen months prior to an October 1984 conference on lunar origins, Bill Hartmann, Roger Phillips, Jeff Taylor challenged fellow lunar scientists: "You have eighteen months. Go back to your Apollo data, go back to your computer, do whatever you have to, but make up your mind. Don't come to our conference unless you have something to say about the Moon's birth." At the 1984 conference at Kona, the giant impact hypothesis emerged as the most consensual theory. Before the conference, there were parti
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