In "Meteorologica" (DK 59 A80), Aristotle (384–322 BC) wrote that the Greek philosophers Anaxagoras (–428 BC) and Democritus (460–370 BC) proposed that the Milky Way might consist of distant stars. However, Aristotle himself believed the Milky Way to be caused by "the ignition of the fiery exhalation of some stars which were large, numerous and close together" and that the "ignition takes place in the upper part of the atmosphere, in the region of the world which is continuous with the heavenly motions." The Neoplatonist philosopher Olympiodorus the Younger (–570 ) criticized this view, arguing that if the Milky Way were sublunary, it should appear different at different times and places on Earth, and that it should have parallax, which it does not.
Astronomical history
Milky Way
Milky Way | Astronomical history
Aristotle, Anaxagoras, Democritus Edwin Hubble, Harlow Shapley and Heber Curtis, to name a few. Sorry. I cannot find the answer based on the context. Galileo Galilei, Harlow Shapley and Heber Curtis
where is the solar system located in our galaxy
user
The Solar System, is located in the Milky Way's galactic habitable zone.
assistant
what is the milky way?
user
It is the galaxy that contains our Solar System.
assistant
can you tell me the names of the stars that form a part of this galaxy?
user
The Milky Way may contain ten billion white dwarfs, a billion neutron stars, and a hundred million stellar black holes.
assistant
can this galaxy be seen from earth in the northern hemisphere?
user
The Milky Way is visible from Earth as a hazy band of white light, some 30° wide, arching across the night sky.  As viewed from Earth, the visible region of the Milky Way's galactic plane occupies an area of the sky that includes 30 constellations.
assistant
how would you describe its size?
user
The Milky Way is the second-largest galaxy in the Local Group with its stellar disk approximately in diameter and, on average, approximately thick. The Milky Way is approximately 890 billion times the mass of the Sun.
assistant
name the structural parts of this.
user
Sorry. I cannot find the answer based on the context.
assistant
what is the estimated age of this galaxy?
user
Several individual stars have been found in the Milky Way's halo with measured ages very close to the 13.80-billion-year age of the Universe.
assistant
what are the names of the intellects who studied and observed this cluster of stars?
user
This was explained by the interstellar dust scattered through the galaxy, resulting in the absorption (extinction) of light or interstellar extinction of light. Trumpler further studied and catalogued open clusters in order to determine the size of the Milky Way galaxy. At first he thought his analysis placed an upper limit on the Milky Way's diameter of about 10,000 parsecs with the Sun located somewhat near the center although he later revised this. While cataloguing open clusters, he also devised a system for their classification according to the number of stars observed within them, how concentrated these stars are in the center of the cluster and the range of their apparent brightness.
Robert Julius Trumpler | Career
After graduation, Shapley received a fellowship to Princeton University for graduate work, where he studied under Henry Norris Russell and used the period-luminosity relation for Cepheid variable stars (discovered by Henrietta Swan Leavitt) to determine distances to globular clusters. He was instrumental in moving astronomy away from the idea that Cepheids were spectroscopic binaries, and toward the concept that they were pulsators. He realized that the Milky Way Galaxy was far larger than previously believed, and that the Sun's place in the galaxy was in a nondescript location. This discovery supports the Copernican principle, according to which the Earth is not at the center of our Solar System, our galaxy, or our Universe.
Harlow Shapley | Early years
In 2004, a team of astronomers focused on the cluster to estimate the age of the Milky Way Galaxy. The team consisted of Luca Pasquini, Piercarlo Bonifacio, Sofia Randich, Daniele Galli, and Raffaele G. Gratton. They used the UV-Visual Echelle Spectrograph of the Very Large Telescope at Cerro Paranal to measure the beryllium content of two stars in the cluster. This allowed them to deduce the time elapsed between the rise of the first generation of stars in the entire Galaxy and the first generation of stars in the cluster. This, added to the estimated age of the stars in the cluster, gives an estimate for the age for the Galaxy: about 13.6 billion years, which is nearly as old as the universe itself.
NGC 6397 | Estimating the age of the Milky Way
A different method by Harlow Shapley based on the cataloguing of globular clusters led to a radically different picture: a flat disk with diameter approximately 70 kiloparsecs and the Sun far from the center. Both analyses failed to take into account the absorption of light by interstellar dust present in the galactic plane, but after Robert Julius Trumpler quantified this effect in 1930 by studying open clusters, the present picture of our host galaxy, the Milky Way, emerged.
Galaxy | Milky Way
Using the Hubble Space Telescope, Figer observed approximately one thousand stars in a young star cluster, the Arches Cluster, which is near the Galactic center. By measuring the initial mass function, he identified an upper limit of 150 solar masses to the masses of stars. Figer led a team to identify the Pistol star as one of the most massive stars known in the Galaxy using data that the team obtained using the Hubble Space Telescope. His teams performed the first census of massive stars in both the Quintuplet Cluster and the Arches Cluster, finding that massive star clusters form in the present-day Galaxy, as opposed to the previous paradigm that suggested such clusters only formed at the time that the Galaxy formed.
Donald Figer | Scientific Research
In 1654, he identified the objects now designated Messier 41, Messier 47, NGC 2362 and NGC 2451. It was realised as early as 1767 that the stars in a cluster were physically related, when the English naturalist Reverend John Michell calculated that the probability of even just one group of stars like the Pleiades being the result of a chance alignment as seen from Earth was just 1 in 496,000. Between 1774–1781, 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.
Open cluster | Historical observations
In our Galaxy, globular clusters are distributed roughly spherically in the galactic halo, around the Galactic Centre, orbiting the centre in highly elliptical orbits. In 1917, the astronomer Harlow Shapley made the first reliable estimate of the Sun's distance from the galactic centre based on the distribution of globular clusters. Until the mid-1990s, globular clusters were the cause of a great mystery in astronomy, as theories of stellar evolution gave ages for the oldest members of globular clusters that were greater than the estimated age of the universe. However, greatly improved distance measurements to globular clusters using the Hipparcos satellite and increasingly accurate measurements of the Hubble constant resolved the paradox, giving an age for the universe of about 13 billion years and an age for the oldest stars of a few hundred million years less.
Star cluster | Globular cluster
He observed the variation of star movements based on their distance from the core of a galaxy. He also found that there were globular clusters in the galaxy orbiting in a retrograde direction. He also discovered an important error in the period-luminosity law of Cepheid variables, that had resulted in a significant underestimation of the size of the universe.
Henri Mineur | Introduction
The Greek philosopher Democritus (450–370 BCE) proposed that the bright band on the night sky known as the Milky Way might consist of distant stars. Aristotle (384–322 BCE), however, believed the Milky Way to be caused by "the ignition of the fiery exhalation of some stars that were large, numerous and close together" and that the "ignition takes place in the upper part of the atmosphere, in the region of the World that is continuous with the heavenly motions." The Neoplatonist philosopher Olympiodorus the Younger (–570 CE) was critical of this view, arguing that if the Milky Way is sublunary (situated between Earth and the Moon) it should appear different at different times and places on Earth, and that it should have parallax, which it does not.
Galaxy | Milky Way
He also postulated the existence of the mysterious invisible dark matter in 1932, which is believed to make up roughly 84.5% of the total matter in the Universe and whose gravitational pull causes "the clustering of stars into galaxies and galaxies into connecting strings of galaxies". He discovered the galactic halo, a group of stars orbiting the Milky Way but outside the main disk. Additionally Oort is responsible for a number of important insights about comets, including the realization that their orbits "implied there was a lot more solar system than the region occupied by the planets." The Oort cloud, the Oort constants, and the asteroid, 1691 Oort, were all named after him.
Jan Oort | Introduction
They co-authored a number of academic papers on star clusters, stellar magnitudes, and the structure of the Milky Way. Bok was fiercely interested in the mechanical processes which form star clusters and galaxies, and his paper "The Apparent Clustering of External Galaxies" ("Nature", vol. 133, p. 578) was the first to demonstrate how dissipation of a star cluster relates to its density, and so by examining the clusters in our galaxy he concluded that it cannot be older than about twenty billion years.
Bart Bok | Harvard 1929–1957
With this technique, the age of the globular cluster M4 was estimated as . Age estimates of the oldest of these clusters gives a best fit estimate of 12.6 billion years, and a 95% confidence upper limit of 16 billion years. In November 2018, astronomers reported the discovery of one of the oldest stars in the universe. About 13.5 billion-years-old, 2MASS J18082002-5104378 B is a tiny ultra metal-poor (UMP) star made almost entirely of materials released from the Big Bang, and is possibly one of the first stars. The discovery of the star in the Milky Way galaxy suggests that the galaxy may be at least 3 billion years older than previously thought.
Milky Way | Age and cosmological history
As an undergraduate at Harvard College, Bidelman received an Honorary Harvard College Scholarship for academic excellence in 1939. He graduated in 1940. Bidelman entered the graduate program at the University of Chicago affiliated with Yerkes Observatory. His doctoral advisor was William W.Morgan, who discovered the first definite evidence that our Milky Way Galaxy is a spiral galaxy, and, with Philip Keenan, the Morgan-Keener (MK) system of stellar classification. As a graduate student, Bidelman assisted Morgan and Keenan by taking some of the spectrograms for their book, "An Atlas of Stellar Spectra". For his 1943 dissertation, Bidelman reported the Double Cluster in the I Persei association is physically associated with neighborhood supergiant stars, and is part of an association of O- and B-type stars, and designated 47 stars as its members.
William P. Bidelman | Education
Other astronomers such as Simon Marius, Giovanni Battista Riccioli, and Martinus Hortensius made similar measurements of stars, and Marius and Riccioli concluded the smaller sizes were not small enough to answer Tycho's argument.
Galileo Galilei | Milky Way and stars
Hubble had posited the earlier value; he had considered only the weaker Population II Cepheid variables as standard candles. After Baade's pronouncements, Sandage showed that astronomers' previous assumption, that the brightest stars in galaxies were of approximately equal inherent intensity, was mistaken in the case of H II regions which he found not to be stars and inherently brighter than the brightest stars in distant galaxies. This resulted in another 1.5-fold increase in the calculated age of the universe, to approximately 5.5 billion years. Throughout the 1950s and well into the 1980s Sandage was regarded as the pre-eminent observational cosmologist, making contributions to all aspects of the cosmological distance scale, ranging from calibrators within our own Milky Way Galaxy, to cosmologically distant galaxies.
Allan Sandage | Career
Matter continues to draw together under the influence of gravity, to form galaxies. The stars from this time period, known as Population II stars, are formed early on in this process, with more recent Population I stars formed later. Gravitational attraction also gradually pulls galaxies towards each other to form groups, clusters and superclusters. Hubble Ultra Deep Field observations has identified a number of small galaxies merging to form larger ones, at 800 million years of cosmic time (13 billion years ago). (This age estimate is now believed to be slightly overstated). Using the 10-metre Keck II telescope on Mauna Kea, Richard Ellis of the California Institute of Technology at Pasadena and his team found six star forming galaxies about 13.2 billion light-years away and therefore created when the universe was only 500 million years old.
Chronology of the universe | Galaxies, clusters and superclusters
In 1750, Thomas Wright speculated (correctly) that the Milky Way is a flattened disk of stars, and that some of the nebulae visible in the night sky might be separate Milky Ways. Toward the end of the 18th century, Charles Messier compiled a catalog containing the 109 brightest celestial objects having nebulous appearance. Subsequently, William Herschel assembled a catalog of 5,000 nebulae. In 1845, Lord Rosse constructed a new telescope and was able to distinguish between elliptical and spiral nebulae. He also managed to make out individual point sources in some of these nebulae, lending credence to Kant's earlier conjecture.
Galaxy | Distinction from other nebulae
According to Jamil Ragep, the Persian astronomer Naṣīr al-Dīn al-Ṭūsī (1201–1274) in his "Tadhkira" writes: "The Milky Way, i.e. the Galaxy, is made up of a very large number of small, tightly clustered stars, which, on account of their concentration and smallness, seem to be cloudy patches. Because of this, it was likened to milk in color."
Milky Way | Astronomical history
The galactic center was determined to be in the direction of the Sagittarius constellation, approximately 50,000 light-years from us. In 1920 Heber Doust Curtis and Harlow Shapley participated in the Great Debate on the nature of nebulae and galaxies, and the size of the universe. Shapley believed that distant nebulae were relatively small and lay within the Milky Way galaxy. Curtis advocated the now-accepted view that nebulae were farther away, and that other galaxies apart from the Milky Way therefore existed. By 1925, Edwin Hubble had confirmed that many objects previously thought to be clouds of dust and gas and classified as "nebulae" were actually galaxies beyond the Milky Way.
Galactocentrism | Harlow Shapley and Galactocentrism
This work demonstrated the inner solid-body rotation and the outer Keplerian motion. In 1953, together with O.J. Eggen, Mayall identified six likely globular clusters (including Mayall II) around the Andromeda Galaxy in a Palomar 48-inch schmidt plate exposed in 1948 that was provided to them by Hubble. Gerry Kron marveled at the sensitivity of Mayall's eyes that could reach down to +17 visual magnitude using the telescope. Mayall's eyesight later deteriorated to the point that he could no longer read.
Nicholas Mayall | Postwar Lick research
In the 14th century, the Syrian-born Ibn Qayyim proposed the Milky Way galaxy to be "a myriad of tiny stars packed together in the sphere of the fixed stars." Actual proof of the Milky Way consisting of many stars came in 1610 when the Italian astronomer Galileo Galilei used a telescope to study the Milky Way and discovered that it is composed of a huge number of faint stars. In 1750 the English astronomer Thomas Wright, in his "An Original Theory or New Hypothesis of the Universe", speculated (correctly) that the galaxy might be a rotating body of a huge number of stars held together by gravitational forces, akin to the Solar System but on a much larger scale.
Galaxy | Milky Way
The Arabian astronomer Alhazen (965–1037) made the first attempt at observing and measuring the Milky Way's parallax, and he thus "determined that because the Milky Way had no parallax, it was very remote from the earth and did not belong to the atmosphere." The Persian astronomer Abū Rayhān al-Bīrūnī (973–1048) proposed the Milky Way galaxy to be "a collection of countless fragments of the nature of nebulous stars." The Andalusian astronomer Ibn Bajjah ("Avempace", d. 1138) proposed that the Milky Way was made up of many stars which almost touched one another and appeared to be a continuous image due to the effect of refraction from sublunary material, citing his observation of the conjunction of Jupiter and Mars on 500 AH (1106/1107 AD) as evidence.
Cosmology in medieval Islam | Galaxy observation
During her tenure at the Harvard College Observatory, she worked together with Harlow Shapley on the Shapley-Ames catalog, which lists galaxies beyond the 13th magnitude. From their observations of approximately 1250 galaxies, they found evidence of clustering near the north pole of the Milky Way that differs from the south pole. These results were significant because their finding of "general unevenness in distribution" of galaxies deviated from the assumption of isotropy.
Adelaide Ames | Shapley-Ames Catalog
In his view, the Milky Way is celestial. According to Mohani Mohamed, the Arabian astronomer Alhazen (965–1037) made the first attempt at observing and measuring the Milky Way's parallax, and he thus "determined that because the Milky Way had no parallax, it must be remote from the Earth, not belonging to the atmosphere." The Persian astronomer al-Bīrūnī (973–1048) proposed the Milky Way galaxy to be "a collection of countless fragments of the nature of nebulous stars." The Andalusian astronomer Ibn Bâjjah ("Avempace", 1138) proposed that the Milky Way is made up of many stars that almost touch one another and appear to be a continuous image due to the effect of refraction from sublunary material, citing his observation of the conjunction of Jupiter and Mars as evidence of this occurring when two objects are near.
Galaxy | Milky Way
Abell's first professional astronomical occupation came as a Caltech grad student when he was an observer on the National Geographic Society – Palomar Observatory Sky Survey. Several scientific advances came out of this work including, An extended version of the clusters of galaxies catalogue was published after Abell's death in 1987 under the authorship of Abell, Harold G. Corwin and Ronald P. Olowin. This extended catalog includes clusters seen from the southern hemisphere, lists approximately 4,000 clusters of galaxies and includes thirty members with a redshift up to z = 0.2. (See List of Abell clusters.)
George O. Abell | Palomar sky survey
Moore and collaborators identified the cuspy halo problem and dwarf galaxy problem that persist in the cold dark matter paradigm. He also formulated a new mechanism for the origin of dwarf galaxies in clusters named galaxy harassment. Using numerical simulations he determined that Earth-Moon planetary systems are not rare. Moore and collaborators predicted that the first structures to form in the Universe were Earth-sized dark matter haloes which collapsed just 20 million years after the Big Bang.
Ben Moore (astrophysicist) | Research
In 1962, he published research with Olin Eggen and Allan Sandage arguing that the Milky Way originated through the dynamic collapse of a single large gas cloud. In 1969 he published his theory that quasars are powered by massive black holes accreting material. From counting dead quasars, he deduced that most massive galaxies have black holes at their centres. Lynden-Bell developed a theory for the relaxation of a system of particles in changing potential field known as "violent relaxation." Violent relaxation has many applications in dynamical astronomy, affecting the orbits of stars within star clusters and galaxies. Lynden-Bell is also known for the development of the theory of the "gravothermal catastrophe," a phenomenon in star clusters that is the result of the negative heat capacity of gravitational systems.
Donald Lynden-Bell | Biography
In 1918, as Kapteyn was refining his model, Herschel's heliocentric theory was overthrown by astronomer Harlow Shapley's work on globular clusters. Shapley's observational cosmology marked a transition from Heliocentrism to Galactocentrism. Shapley had been studying the asymmetrical distribution of globular clusters, estimating the distance and location of individual objects by using variable stars as standard candles. Globular clusters contain many cepheid variable stars, whose precise relationship between luminosity and variability period was established by Henrietta Leavitt in 1908. Using cepheid and RR Lyrae variables to systematically chart the distribution of globular clusters, Shapley discovered that the stars in the Milky Way orbited a common center thousands of light years away from the Sun.
Galactocentrism | Harlow Shapley and Galactocentrism
In the astronomical literature, the capitalized word "Galaxy" is often used to refer to our galaxy, the Milky Way, to distinguish it from the other galaxies in our universe. The English term "Milky Way" can be traced back to a story by Chaucer : Galaxies were initially discovered telescopically and were known as "spiral nebulae". Most 18th to 19th century astronomers considered them as either unresolved star clusters or anagalactic nebulae, and were just thought as a part of the Milky Way, but their true composition and natures remained a mystery. Observations using larger telescopes of a few nearby bright galaxies, like the Andromeda Galaxy, began resolving them into huge conglomerations of stars, but based simply on the apparent faintness and sheer population of stars, the true distances of these objects placed them well beyond the Milky Way.
Galaxy | Etymology
On the other hand, Curtis argued that the Milky way was much smaller and that the observed nebulae were in fact galaxies similar to our own Milky Way. This debate was not settled until late 1923 when the astronomer Edwin Hubble measured the distance to M31 (currently known as the Andromeda galaxy) using Cepheid Variable stars. By measuring the period of these stars, Hubble was able to estimate their intrinsic luminosity and upon combining this with their measured apparent magnitude he estimated a distance of 300 kpc, which was an order-of-magnitude larger than the estimated size of the universe made by Shapley.
Satellite galaxy | Early 20th century
Shapley and Ames began their study of all nearby galaxies in 1926. An important finding from this research was that the galaxies were not evenly distributed (they violated the isotropy assumption) in that the northern hemisphere contained more galaxies than the southern hemisphere. It also found that the Virgo cloud extended further than previously believed. From this data, Shapley and Ames therefore created a new hierarchy of clusters called a supercluster, which is a cluster of galaxy clusters, and called this Virgo cloud in the northern hemisphere a "Local Supercluster".
Shapley-Ames Catalog | History
Thomas Wright and Kant first speculated that fuzzy patches of light called nebulae were actually distant "island universes" consisting of many stellar systems. The shape of our own galaxy was expected to resemble such "islands universes." But "scientific arguments were marshalled against such a possibility," and this view was rejected by almost all scientists until Edwin Hubble's measurements in 1924. In 1783, amateur astronomer William Herschel attempted to determine the shape of the galaxy by examining stars through his handmade telescopes. Herschel was the first to propose a model of the galaxy based on observation and measurement. He concluded that it was in the shape of a disk, but incorrectly assumed that the sun was in the center of the disk.
Galactocentrism | William Herschel's Heliocentrism
In 1935 Hogg moved to the University of Toronto, after her husband had received a job offer to work at the David Dunlap Observatory. For her first year there, Hogg continued her work photographing globular clusters, amassing thousands of photographs which she used to identify many thousands of variable stars. She published "Catalogue of 1116 Variable Stars in Globular Clusters" in 1939, the first of three catalogues she completed, with a fourth in the works at the time of her death. In addition to her work on variable stars in globular clusters, Hogg used the period-luminosity relationship of Cepheid variable stars (discovered by Henrietta Swan Leavitt in 1908) to enhance the understanding of the Milky Way Galaxy's age, size and structure.
Helen Sawyer Hogg | Scientific work
Also, when he observed some of the "nebulous" stars in the Ptolemaic star catalogue, he saw that rather than being cloudy, they were made of many small stars. From this he deduced that the nebulae and the Milky Way were "congeries of innumerable stars grouped together in clusters" too small and distant to be resolved into individual stars by the naked eye.
Sidereus Nuncius | Stars
English astronomer John Herschel described it as "coarsely scattered clusters of stars". Telescopic observations of the cluster reveal about 80 stars within a field of view of 1.3° across. At the cluster's estimated distance of 980 light years this corresponds to an actual diameter of 25 light years. The tidal radius of the cluster is and it has a combined mass of about 735 times the mass of the Sun. The age of the cluster is around 200 million years while the brightest member star is of magnitude 5.6. In terms of composition, the cluster contains a similar abundance of elements other than hydrogen and helium as the Sun.
Messier 7 | Introduction
The star (and the Milky Way) must have formed soon after the Big Bang. Another result is the first measurement of the beryllium content of two stars in a Milky Way globular cluster. With this measurement, astronomers found that the first generation of stars in our galaxy must have formed soon after the end of the 200-million-year "Dark Age" following the Big Bang.
European Southern Observatory | Age of the universe
The Andromeda Galaxy was later independently noted by Simon Marius in 1612. In 1734, philosopher Emanuel Swedenborg in his "Principia" speculated that there may be galaxies outside our own that are formed into galactic clusters that are minuscule parts of the universe which extends far beyond what we can see. These views "are remarkably close to the present-day views of the cosmos." In 1745, Pierre Louis Maupertuis conjectured that some nebula-like objects are collections of stars with unique properties, including a glow exceeding the light its stars produce on their own, and repeated Johannes Hevelius's view that the bright spots are massive and flattened due to their rotation.
Galaxy | Distinction from other nebulae
Odenkirchen and Soubiran demonstrated that the distances from the Earth to the six stars were very different from each other, and the stars were moving in different directions. Therefore, they concluded that the stars were only an asterism. Although M73 was determined to be only a chance alignment of stars, further analysis of asterisms is still important for the identification of sparsely populated open clusters. Such clusters can be important for demonstrating how open clusters are ripped apart by the gravitational forces in the Milky Way.
Messier 73 | Scientific research: asterism or open cluster?
Johann Gottfried Koehler (15 December 1745 – 19 September 1801) was a German astronomer who discovered a number of nebulae, star clusters, and galaxies. Koehler is best remembered for his discovery of Open Cluster M67, Elliptical Galaxy M59, and Elliptical Galaxy M60. The latter two were discovered on the same day, 11 April 1779. He worked with the noted astronomer Johann Elert Bode, who refined and published Koehler's proposal for the symbol of Uranus. From 1784 he was the director of Mathematisch-Physikalischer Salon.
Johann Gottfried Koehler | Introduction
Among the dimmest stars in the cluster are groups of white dwarfs that are 6 billion years old and another group that appear to be 4 billion years old. The ages are out of sync with those of the cluster's normal stars, which are 8 billion years old. This seeming contradiction in age for this cluster has been studied and a solution proposed with age of about 8 billion years.
NGC 6791 | Age studies
Wright's publication "An original theory or new hypothesis of the Universe" (1750) explained the appearance of the Milky Way as "an optical effect due to our immersion in what locally approximates to a flat layer of stars." His idea was taken up and elaborated by Immanuel Kant in his "Universal Natural History and Theory of the Heavens", and later empirically advanced by William Herschel in 1785, leading to galactocentrism (a form of heliocentrism, with the Sun at the center of the Milky Way). Another of his ideas, which is also often attributed to Kant, was that many faint nebulae are actually incredibly distant galaxies.
Thomas Wright (astronomer) | Astronomy
The structure and size of the galaxy was of great interest to many astronomers during Kapteyn's time. Around 1900, Hugo von Seegler made an attempt at the structure of the galaxy by making counts of stars between successive magnitudes. Ultimately, von Seegler was able to determine the rates at which the galaxy was diminishing in multiple different areas of the sky. In 1901, Kapteyn employed the proper motions technique and derived a statistical approach that allowed him to estimate the average distance to stars between successive magnitudes, effectively providing a scale for von Seegler's discoveries. Conclusive results from von Seegler and Kapteyn estimated the galaxy to be an oblate star system approximately 10 kpc in width and 2 kpc in thickness, the sun being relatively close (0.6 kpc) to the center.
Jacobus Kapteyn | The Kapteyn Universe
Hubble's finding settled the question raised in the "Great Debate" of whether the Milky Way represented the entire Universe or was merely one of numerous galaxies in the Universe.
Cepheid variable | History
It is 13 arc minutes in diameter. By the western edge of the cluster is the unrelated star, 5.5-magnitude HD 56405. American astronomer Olin J. Eggen surveyed the cluster in 1968, concluding that the brightest star in the field, magnitude-8.96 HD 56847, is likely to lie in the field and not a true member of the cluster. He also identified one or possibly two blue stragglers. These are unexpectedly hot and luminous stars that appear younger than surrounding stars, and have likely developed by sucking matter off companion stars. Four are now recognised to be in the cluster. By analysing the masses of the smallest stars that have evolved into red giants—namely, stars of 1.8 or 1.9 solar masses—Swiss astronomers Jean-Claude Mermilliod and Michel Mayor were able to date the age of the cluster at 2.2 billion years.
NGC 2360 | Introduction
Because of interstellar dust along the line of sight, the Galactic Center cannot be studied at visible, ultraviolet, or soft (low-energy) X-ray wavelengths. The available information about the Galactic Center comes from observations at gamma ray, hard (high-energy) X-ray, infrared, submillimetre, and radio wavelengths. Immanuel Kant stated in "General Natural History and Theory of the Heavens" (1755) that a large star was at the center of the Milky Way Galaxy, and that Sirius might be the star. Harlow Shapley stated in 1918 that the halo of globular clusters surrounding the Milky Way seemed to be centered on the star swarms in the constellation of Sagittarius, but the dark molecular clouds in the area blocked the view for optical astronomy.
Galactic Center | Discovery
A team of astronomers Stockholm University, Sweden, and the Geneva Observatory, Switzerland, identified 200 separate clusters of very young, massive stars, many of which are less than 10 million years old. The observations have led the astronomers to conclude that H11 is most likely the result of a merger between a galaxy that is rich in stars and a younger, gas-rich galaxy.
Haro 11 | Background
It was first recognized as a globular cluster by Scottish astronomer James Dunlop in 1826, who described it as a "beautiful globe of stars very gradually and moderately compressed to the centre".
Omega Centauri | Observation history
He recognized that the stars are much larger than the planets, and thus argued: Al-Jawziyya also recognized the Milky Way galaxy as "a myriad of tiny stars packed together in the sphere of the fixed stars" and thus argued that "it is certainly impossible to have knowledge of their influences."
Astrology in medieval Islam | Medieval understanding
This episode provides an overview of the composition of stars, and their fate in billions of years. Tyson describes how early man would identify stars via the use of constellations that tied in with various myths and beliefs, such as the Pleiades. Tyson describes the work of Edward Charles Pickering to capture the spectra of multiple stars simultaneously, and the work of the Harvard Computers or "Pickering's Harem", a team of women researchers under Pickering's mentorship, to catalog the spectra. This team included Annie Jump Cannon, who developed the stellar classification system, and Henrietta Swan Leavitt, who discovered the means to measure the distance from a star to the Earth by its spectra, later used to identify other galaxies in the universe.
Sisters of the Sun | Episode summary
In 1927, the Belgian Roman Catholic priest Georges Lemaître independently derived the Friedmann–Lemaître–Robertson–Walker equations and proposed, on the basis of the recession of spiral nebulae, that the universe began with the "explosion" of a "primeval atom"—which was later called the Big Bang. In 1929, Edwin Hubble provided an observational basis for Lemaître's theory. Hubble showed that the spiral nebulae were galaxies by determining their distances using measurements of the brightness of Cepheid variable stars. He discovered a relationship between the redshift of a galaxy and its distance. He interpreted this as evidence that the galaxies are receding from Earth in every direction at speeds proportional to their distance.
Physical cosmology | Subject history
Edwin Hubble, in the paper "N.G.C. 6822, A Remote Stellar System", identified 15 variable stars (11 of which were Cepheids) of this galaxy. He also surveyed the galaxy's stars distribution down to magnitude 19.4. He provided spectral characteristics, luminosities and dimensions for the five brightest "diffuse nebulae" (giant H II regions) that included the Bubble Nebula and the Ring Nebula. He also computed the absolute magnitude of the entire galaxy. Hubble's detection of eleven Cepheid variable stars was a milestone in astronomy. Utilizing the Cepheid Period-Luminosity relationship, Hubble determined a distance of 214 kiloparsecs or 698,000 light-years. This was the first system beyond the Magellanic Clouds to have its distance determined.
NGC 6822 | Observational history
Jean L. Turner is an astrophysicist and distinguished professor in the Department of Physics and Astronomy at the University of California, Los Angeles. She was lead author on research and discovery of a particular star cluster in the dwarf galaxy NGC 5253, considered 'remarkable' for being an extremely dusty gas cloud and having highly efficient star formation.
Jean L. Turner | Introduction
The prominent open cluster the Pleiades has been recognized as a group of stars since antiquity, while the Hyades forms part of Taurus, one of the oldest constellations. 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 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.
Open cluster | Historical observations
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.
Globular cluster | Observation history
Among his most important works was the triangulation of the Beehive Cluster in 1894 and the two star clusters h Persei and χ Persei, which together form the Double Cluster, between 1891 and 1896. Averaging the results of his Beehive Cluster measurements led to 123 equations with 74 unknowns, which he resolved according to Gauss' method of least squares. The calculations were a tremendous amount of work and thus required 10 weeks to complete. In addition to his own research, Schur took on a number of other projects, including administrative and historical tasks. Together with an assistant, he catalogued and organised over 11,000 books and brochures in the observatory's library over a period of a year and a half, finishing in 1899 and leaving the library "in an orderly condition for the first time in 44 years." In 1891 he published a star catalogue of 6000 astronomical objects, which his professor and fellow past director of the observatory Wilhelm Klinkerfues had observed from 1858 to 1869.
Wilhelm Schur | Career and later life
Nearly all globular clusters of the Milky Way's galactic halo formed at the same time (12-15 Gyr). Even the far situated NGC 2419 (~100 kpc from galactic center) has a similar age. This trend also applies to the age of globulars found in the Large Magellanic Cloud and Fornax Dwarf (~140 kpc from galactic center). However, a few globulars seem to be significantly younger than the rest; these include Palomar 1, Palomar 3, Palomar 4, Palomar 12, Palomar 14, Ruprecht 106, IC 4499, Arp 2, Eridanus, Fornax 4, and Terzan 7. In particular, the ones associated with the Sag DEG appear to have formed more recently.
Terzan 7 | Young globular
Gerry Gilmore worked as a research fellow at the Royal Observatory, Edinburgh, Scotland, between 1979 and 1984. He used the expertise gained during his PhD to measure brightnesses and numbers of stars from photographic sky surveys. Working with Neil Reid with data from the United Kingdom Schmidt Telescope, he found an excess of faint stars compared to standard models of the Galaxy that represented the distribution of stars as two simple components. Gilmore and Reid argued the observed numbers of stars implied the existence of an additional component they called the thick disc that exists alongside the main Galactic disc and the stellar halo.
Gerard F. Gilmore | Royal Observatory, Edinburgh
NGC 6352 (also known as Caldwell 81) is a globular cluster of stars in the southern constellation of Ara, located approximately from the Sun. It was discovered by Scottish astronomer James Dunlop on May 14, 1826. The cluster has a Shapley–Sawyer Concentration Class of XI:. A telescope with a aperture is required to resolve the stars within this loose cluster. This cluster is about 12.67 billion years old with two distinct stellar populations; the second generation is only around 10 million years younger than the first. It lies approximately from the galactic center and from the galactic plane. The orbital motion of this cluster through the Milky Way suggests it is a member of the bulge or disk population.
NGC 6352 | Introduction
The most of his work are connected with applying the methods of mathematical statistics and theory of random processes to stellar astronomy. In particular, studying the results of star and galaxy counts, he was able to separate the effects of real clustering and the effects due to a clumpy structure of the absorbing layer and to estimate the cluster parameters.
Tateos Agekian | Cluster parameters
Due to its enormous size, this cluster was never recognized as such before the 20th century. The British astronomer Philibert Jacques Melotte was the first to notice it, who described it in his 1915 catalogue of star clusters as a large group of stars scattered around the star 67 Ophiuchi. In 1931, it was re-observed by Swedish astronomer Per Collinder, who described it as a group of 15 stars devoid of appreciable concentration, providing measurements of its member stars.
Melotte 186 | History
He discovered that many of these features could be resolved into groupings of individual stars. Herschel conceived 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.
Open cluster | Historical observations
Galileo observed the Milky Way, previously believed to be nebulous, and found it to be a multitude of stars packed so densely that they appeared from Earth to be clouds. He located many other stars too distant to be visible with the naked eye. He observed the double star Mizar in Ursa Major in 1617. In the "Starry Messenger", Galileo reported that stars appeared as mere blazes of light, essentially unaltered in appearance by the telescope, and contrasted them to planets, which the telescope revealed to be discs. But shortly thereafter, in his "Letters on Sunspots", he reported that the telescope revealed the shapes of both stars and planets to be "quite round".
Galileo Galilei | Milky Way and stars
His earliest work had concerned the planet Mars and while at Harvard he used telescope observations from 1909 to 1958 to study the areographic coordinates of features on the surface of Mars. His later work focused on the study of galaxies and he co-authored the "Third Reference Catalogue of Bright Galaxies" with his wife Antoinette (1921-1987), a fellow UT Austin astronomer and lifelong collaborator. His specialty included reanalyzing Hubble and Sandage's galaxy atlas and recomputing the distance measurements utilizing a method of averaging many different kinds of metrics such as luminosity, the diameters of ring galaxies, brightest star clusters, etc., in a method he called "spreading the risks." During the 1950s he promoted the idea that galactic clusters are grouped into superclusters.
Gérard de Vaucouleurs | Life and career
He was one of the first to suspect that the galaxies are remote stellar systems at vast distances and not nearby objects belonging to our own galaxy, the Milky Way. In 1919 he measured the distance to M31 – the Andromeda Galaxy, to 650,000 light years (about a fourth of the present day value) using magnitudes of novae found in M31 and comparing them to nearby ones with known distances. Lundmark's work contributed to the later famous Great Debate over whether nebulae were galaxies or concentrations of glowing gas.
Knut Lundmark | Introduction
The small "h" followed with the catalogue entry number represented the item. In 1878, John Louis Emil Dreyer published a supplement to the "General Catalogue". In 1886, he suggested building a second supplement to the "General Catalogue", but the Royal Astronomical Society asked Dreyer to compile a new version instead. This led to the publication of the "New General Catalogue" (NGC) in 1888, and its two expansions, the "Index Catalogues" (IC), in 1895 and 1908.
Catalogue of Nebulae and Clusters of Stars | History
In his view, the Milky Way is celestial. This idea would be influential later in the Islamic world. The Persian astronomer Abū Rayhān al-Bīrūnī (973–1048) proposed that the Milky Way is "a collection of countless fragments of the nature of nebulous stars". The Andalusian astronomer Avempace ( 1138) proposed the Milky Way to be made up of many stars but appears to be a continuous image due to the effect of refraction in Earth's atmosphere, citing his observation of a conjunction of Jupiter and Mars in 1106 or 1107 as evidence. Ibn Qayyim Al-Jawziyya (1292–1350) proposed that the Milky Way is "a myriad of tiny stars packed together in the sphere of the fixed stars" and that these stars are larger than planets.
Milky Way | Astronomical history
During the long period of building the telescope, Mayall continued to use Lick's Crossley Reflector and focused his efforts on utilizing his slitless spectrograph, which was optimized for extended, low-surface-brightness clusters, galaxies, and nebulae. In 1946, he completed his pre-war effort to get integrated spectra of globular clusters and published the work. His paper was key in demonstrating that the system of Milky Way globular clusters shares only slightly the galactic rotation found in the flattened disc of interstellar matter and young stars in our galaxy. In 1948, Mayall serendipitously discovered a type II supernova while conducting other research. Other research Mayall performed included the 20 year collaboration (formulated in 1935 by Hubble) with Milton Humason, to gather redshift values for all northern galaxies brighter than +13 visual magnitude.
Nicholas Mayall | Postwar Lick research
Gilmore moved to the Institute of Astronomy of the University of Cambridge in 1984 on being awarded an advanced research fellowship by the Science and Engineering Research Council. He used observations of spectra of stars to study the structure of the Galaxy, particularly by measuring their radial velocities. Working with Konrad Kuijken, he used spectra of faint stars in the region around the South Galactic Pole obtained with the Anglo-Australian Telescope to measure the stars' radial velocities, and consequently to determine the surface mass density of the Galactic disc. They showed that there was little or no dark matter within the disc other than that part of the Galactic dark matter halo that extends through the disc.
Gerard F. Gilmore | University of Cambridge
A small number of galaxies or galaxy groups have been named after individual people. In most cases, the named individual was the person who discovered the object, who first brought attention to it, or who first studied it scientifically. Many of the brighter galaxies visible from the Northern Hemisphere have Messier numbers, named after Charles Messier. For instance, the Andromeda Galaxy is Messier 31 and the Whirlpool Galaxy is Messier 51. There are a few other comprehensive catalogs that assign the cataloguer's name to galaxies. For instance, Markarian galaxies, named after Benjamin Markarian, are galaxies with excess blue and ultraviolet emission; galaxies in the Atlas of Peculiar Galaxies are assigned an Arp number after Halton Arp who produced the catalog; etc.
List of galaxies named after people | Introduction
In 1921, Harlow Shapley became director of HCO, and soon afterward hired Ames as an assistant. Ames was Shapley's first graduate student and Ames went on to supervise her own graduate students. Her early work at Harvard focused on the identification of NGC/IC objects. In 1926, she and Shapley published several articles on the shapes, colors, and diameters of 103 NGC galaxies. In 1930, she published "A catalog of 2778 nebulae including the Coma-Virgo group", which identified 214 NGC and 342 IC objects in the area of the Virgo cluster.
Adelaide Ames | Research at Harvard
In 1959, while serving as a professor of astrophysics at the Case Institute of Technology (renamed Case Western Reserve University in 1967) in Cleveland, Ohio, Blanco discovered an open cluster. It was named Blanco 1 in his honor, and is centered on the blue star Zeta Sculptoris. According to "The Monthly Notices of the Royal Astronomical Society", Blanco 1 exhibits subsolar ratios that are not observed among nearby field stars. Together with his wife Betty, Blanco and Martin McCarthy conducted pioneering research into the stellar population in the central regions of our galaxy, and in the Magellanic Clouds. They discovered the change in the ratio of carbon stars to M-type stars from the nuclear bulge of our galaxy, in relation to both Large Magellanic Clouds and Small Magellanic Clouds.
Víctor Manuel Blanco | Accomplishments
GaBany's scientific collaboration with professional astronomers has resulted in his participation as co-author of 16 peer-reviewed papers including the discovery of stellar rings around NGC 4013, NGC 5907 and NGC 5055 (M63); the discovery of extended spiral arms around NGC 4736 (M94); the identification of Arps Loop as Milky Way cirrus; a summary of stellar stream discoveries around nearby galaxies; techniques that enable modest instruments to be used for science; a study of the Perseus galaxy cluster core in xrays; the formation of galactic shells from stars and dark matter clumps in a CDM Universe; an investigation into the first discovered stellar stream associated with a dwarf galaxy (NGC 4449).; an investigation of the stream surrounding NGC 4651, the Umbrella Galaxy, that also identified the progenitor's core; the discovery of a new star forming region where a stellar stream intersects with the disk of NGC 5387; the discovery of a stellar tidal stream around NGC 4631, the whale galaxy; the discovery of 11 satellite systems around nearby galaxies with modest telescopes and the discovery of a stellar stream around NGC 253.
R. Jay GaBany | Scientific collaborations
He discovered a relationship between the density of galaxies in a cluster and the size of the cluster: larger clusters have a lower density of galaxies. He provided the data that led to Willem Jacob Luyten's discovery of UV Ceti, the first known flare star.
Edwin Francis Carpenter | Introduction
Her research focuses on the evolution and dynamics of galaxies, in particular the Milky Way, using locations, velocities, ages, and chemical abundances of stars to understand the formation process of galaxies (stellar archaeology). She also studies the nature of dark matter. In her research, Helmi uses computer simulations as well as observational data from for example the Gaia space telescope. The Helmi stream was named after her.
Amina Helmi | Research
In 1997, the Hubble telescope discovered a large number of intergalactic stars in the Virgo cluster of galaxies. Later in the 1990s, scientists discovered another group of intergalactic stars in the Fornax cluster of galaxies. In 2005, at the Smithsonian Center for Astrophysics, Warren Brown and his team attempted to measure the speeds of hypervelocity stars by using the Doppler Technique, by which light is observed for the similar changes that occur in sound when an object is moving away or toward something. But the speeds found are only estimated minimums, as in reality their speeds may be larger than the speeds found by the researchers.
Intergalactic star | Observation history
Charles Messier publishes his catalog of star clusters and nebulas. Messier draws up the list to prevent these objects from being identified as comets. However, it soon becomes a standard reference for the study of star clusters and nebulas and is still in use today.
Timeline of astronomy | 1784
In "Meteorologica" (DK 59 A80), Aristotle (384–322 BC) wrote that the Greek philosophers Anaxagoras (–428 BC) and Democritus (460–370 BC) proposed that the Milky Way might consist of distant stars. However, Aristotle himself believed the Milky Way to be caused by "the ignition of the fiery exhalation of some stars which were large, numerous and close together" and that the "ignition takes place in the upper part of the atmosphere, in the region of the world which is continuous with the heavenly motions." The Neoplatonist philosopher Olympiodorus the Younger (–570 ) criticized this view, arguing that if the Milky Way were sublunary, it should appear different at different times and places on Earth, and that it should have parallax, which it does not.
Milky Way | Astronomical history
Kant also correctly deduced (though through usually false premises and fallacious reasoning, according to Bertrand Russell) that the Milky Way was a large disk of stars, which he theorized formed from a much larger spinning gas cloud. He further suggested that other distant "nebulae" might be other galaxies. These postulations opened new horizons for astronomy, for the first time extending it beyond the Solar System to galactic and intergalactic realms. According to Thomas Huxley (1867), Kant also made contributions to geology in his "Universal Natural History". From then on, Kant turned increasingly to philosophical issues, although he continued to write on the sciences throughout his life.
Immanuel Kant | Early work
The resulting disk of stars can be seen as a band on the sky from our perspective inside the disk. In a treatise in 1755, Immanuel Kant elaborated on Wright's idea about the structure of the Milky Way. The first project to describe the shape of the Milky Way and the position of the Sun was undertaken by William Herschel in 1785 by counting the number of stars in different regions of the sky. He produced a diagram of the shape of the galaxy with the Solar System close to the center. Using a refined approach, Kapteyn in 1920 arrived at the picture of a small (diameter about 15 kiloparsecs) ellipsoid galaxy with the Sun close to the center.
Galaxy | Milky Way
General research area is the nature of the oldest stars in the Galaxy and the early chemical evolution of the Universe. He has published more than 100 papers in the referred literature with ~7000 citations at the end of 2018. With Craig Hogan of the University of Chicago he published in 1995 an early prediction of what became known as "Dark Energy" as a dominant component of the Universe. With Kathryn Johnston (Columbia) and Lars Hernquist (Harvard) published one of the early papers on identifying "fossil" structures in the Galactic halo left by tidally shredded dwarf galaxies: "Fossil Signatures of Ancient Accretion Events in the Halo"
Michael Bolte | Research and notable publications
By the 1920s, a large fraction of the astronomical community had recognized that some of the diffuse, cloud-like objects, or nebulae, seen in the night sky were collections of stars located beyond our own, local collection of star clusters. These "galaxies" had diverse morphologies, ranging from ellipsoids to disks. The concentrated band of starlight that is the visible signature of the Milky Way was indicative of a disk structure for our galaxy; however, our location within our galaxy made structural determinations from observations difficult. Classical mechanics predicted that a collection of stars could be supported against gravitational collapse by either random velocities of the stars or their rotation about its center of mass.
Oort constants | Historical significance and background
ca. 14 billion years. Sandage performed photometric studies of globular clusters, and calculated their age to be at least 25 billion years. This led him to speculate that the universe did not merely expand, but actually expanded and contracted with a period of 80 billion years. The current cosmological estimates of the age of the universe, in contrast, are typically of the order of 14 billion years. As part of his studies concerning the formation of galaxies in the early universe, he co-wrote the paper now referred to as ELS after the authors Olin J. Eggen, Donald Lynden-Bell and Sandage, first describing the collapse of a proto-galactic gas cloud into our present Milky Way Galaxy.
Allan Sandage | Career
The Catalogue of Nebulae and Clusters of Stars (CN) is an astronomical catalogue of nebulae first published in 1786 by William Herschel, with the assistance of his sister Caroline Herschel. It was later expanded into the General Catalogue of Nebulae and Clusters of Stars (GC) by his son, John Herschel. The CN and GC are the precursors to John Louis Emil Dreyer's "New General Catalogue" (NGC) used by current astronomers.
Catalogue of Nebulae and Clusters of Stars | Introduction
In 1609, Galileo first telescopically observed the Beehive and was able to resolve it into 40 stars. Charles Messier added it to his famous catalog in 1769 after precisely measuring its position in the sky. Along with the Orion Nebula and the Pleiades cluster, Messier's inclusion of the Beehive has been noted as curious, as most of Messier's objects were much fainter and more easily confused with comets. Another possibility is that Messier simply wanted to have a larger catalog than his scientific rival Lacaille, whose 1755 catalog contained 42 objects, and so he added some well-known bright objects to boost his list.
Beehive Cluster | History
Kulkarni started off his career as a radio astronomer. He studied Milky Way Galaxy using HI absorption under the guidance of his advisor Carl Heiles, and observed its four arms. The review articles he wrote with Carl Heiles have been highly cited in the field of interstellar medium. He discovered the first millisecond pulsar called PSR B1937+21 with Donald Backer and colleagues, while he was a graduate student. In 1986, he found the first optical counterpart of binary pulsars, while he was a Millikan Fellow at California Institute of Technology. He was instrumental in discovery of the first globular cluster pulsar in 1987 using a supercomputer.
Shrinivas Kulkarni | Key discoveries
Gale Bruno van Albada (28 March 1911, Amsterdam – 18 December 1972, Amsterdam) was a Dutch astronomer, known for his orbital observations of binary stars and studies on the evolution of galaxy clusters.
Gale Bruno van Albada | Introduction
The cluster has a density of 0.21 solar masses per cubic parsec; roughly double the density of stars near the Sun. It is about 20 million years old, and may have been created from the same interstellar cloud that formed NGC 2516 and NGC 2547.
Collinder 140 | Introduction
Cohen's research in the structure and evolution of stars and galaxies has included developing instrumentation for the Keck observatory and leading the Caltech Faint Galaxy Redshift Survey, with more than 200 published papers. Her research has also been described in the popular press. Using laser-guided adaptive optics at the Keck observatory, she showed that several unlikely tight clusters of stars orbiting the Andromeda Galaxy were not actually clusters at all. Together with Evan Kirby, she has studied the mass of nearby dwarf galaxy Triangulum II, showing that this galaxy has a surprisingly large mass for its number of visible stars, making it a candidate dark matter galaxy.
Judith Gamora Cohen | Career and research
The stars in the cluster do not show a significant spread in ages, suggesting that the star formation was triggered all at once.
Messier 21 | Introduction
The cluster was discovered by Giovanni Batista Hodierna before 1654. Hodierna listed it as "Luminosae" in his catalogue of deep sky observations. This catalogue was included in his book "De Admirandis Coeli Characteribuse" published in 1654 at Palermo. It was independently observed by other astronomers after Hodierna, including Edmond Halley (1678), Jean-Philippe de Cheseaux (1745-46), and Abbe Lacaille (1751-52).
NGC 6231 | Discovery
In astronomy, Galactocentrism is the theory that the Milky Way Galaxy, home of Earths Solar System, is at or near the center of the Universe. Observations by William Herschel in 1785 suggested that the Milky Way was a disk-shaped galaxy with the sun in a central position. Although heliocentric, Herschel's observations were the first attempt at an observational cosmology. Herschel's heliocentric theory was overthrown by astronomer Harlow Shapley's work on globular clusters in 1918. Shapley's research marked the transition from heliocentrism to galactocentrism, placing the Galactic Center of the Milky Way Galaxy far away from the sun, towards Sagittarius. Heber Doust Curtis and Edwin Hubble further refuted the heliocentric view of the universe by showing that spirals are themselves far-flung galactic systems.
Galactocentrism | Introduction
Tateos Artemjevich Agekian (, , 12 May 1913, Batumi – 16 January 2006, Saint Petersburg) was a Soviet astrophysicist of Armenian descent, and one of the pioneers of Russian and world Stellar dynamics. Has found two evolutionary sequences of stellar systems: nearly spherical and strongly flattened. Suggested essentially new method to investigate the structure and kinematics of the Milky Way Galaxy. Found a new estimate for the dissipation rate in stellar clusters. Gave an exhaustive analysis of the photogravitational interaction between stars and gas clouds. A planet (3862, "Agekian") was named in honor of Tateos Artemjevich.
Tateos Agekian | Introduction
Prior to the 20th century, the notion that galaxies existed beyond our Milky Way was not well established. In fact, the idea was so controversial at the time that it led to what is now heralded as the "Shapley-Curtis Great Debate" aptly named after the astronomers Harlow Shapley and Heber Doust Curtis that debated the nature of "nebulae" and the size of the Milky Way at the National Academy of Sciences on April 26, 1920. Shapley argued that the Milky Way was the entire universe (spanning over 100,000 lightyears or 30 kiloparsec across) and that all of the observed "nebulae" (currently known as galaxies) resided within this region.
Satellite galaxy | Early 20th century
These stars are bluer, more luminous and more massive than they should be after 10 billion years of stellar evolution, and if they had truly formed that long ago, they should have already died. There are only 2 possible explanations for these stars to be the color that they are, and shine as bright as they do and those are; these stars have either stolen mass from companion stars, or are the result of mergers between two stars that collided. NGC 6362 is located about 25 000 light-years from Earth in the constellation of Ara (The Altar). British astronomer James Dunlop first observed this globular cluster on 30 June 1826.Believe it or not these clusters are showing up all over the our galaxy and in a lot of other surrounding galaxies.
NGC 6362 | Introduction
Their conclusion was that M73 was an old open cluster that was 9 arcmin wide. G. Carraro, however, published results in 2000 based on a similar analysis and concluded that the stars did not follow any color-luminosity relation. Carraro's conclusion was that M73 was an asterism. Adding to the controversy, E. Bica and collaborators concluded that the chance alignment of the four bright stars seen in the center of M73 as well as one other nearby star was highly unlikely, so M73 was probably a sparse open cluster. The controversy was solved in 2002, when M. Odenkirchen and C. Soubiran published an analysis of the high resolution spectra of the six brightest stars within 6 arcmin of the central position of M73.
Messier 73 | Scientific research: asterism or open cluster?
Edwin Hubble's arrival at Mount Wilson Observatory, California in 1919 coincided roughly with the completion of the Hooker Telescope, then the world's largest. At that time, the prevailing view of the cosmos was that the universe consisted entirely of the Milky Way Galaxy. Using the Hooker Telescope at Mt. Wilson, Hubble identified Cepheid variables (a kind of star that is used as a means to determine the distance from the galaxy – see also standard candle) in several spiral nebulae, including the Andromeda Nebula and Triangulum. His observations, made in 1924, proved conclusively that these nebulae were much too distant to be part of the Milky Way and were, in fact, entire galaxies outside our own, suspected by researchers at least as early as 1755 when Immanuel Kant's General History of Nature and Theory of the Heavens appeared.
Edwin Hubble | Universe goes beyond the Milky Way galaxy
The search for such very iron-poor stars has shown they are almost all anomalies in globular clusters and the Galactic Halo. This accords with a narrative that they are rare survivors of their generation. If so, the apparent visual data of the oldest of these enables us to longstop-date the reionization (first star formation) phase of the Universe independently of theories and evidence of the first few million years after the Big Bang. Most stars from Population II and Population III are no longer observable. Theories exist allowing for an older age of the universe than conventionally accepted, which can still accommodate the observed redshift of early objects and earlier radiation.
HD 140283 | Age and significance
To distinguish these groups, he called them "giant" and "dwarf" stars. The following year he began studying star clusters; large groupings of stars that are co-located at approximately the same distance. He published the first plots of color versus luminosity for these stars. These plots showed a prominent and continuous sequence of stars, which he named the Main Sequence. At Princeton University, Henry Norris Russell was following a similar course of research. He was studying the relationship between the spectral classification of stars and their actual brightness as corrected for distance—their absolute magnitude. For this purpose he used a set of stars that had reliable parallaxes and many of which had been categorized at Harvard.
Main sequence | History
Walter Scott Houston described its appearance as follows: "Photographs usually show a departure from circularity, a feature quite evident to visual observers. Older reports almost always mention a cross shape, which seems more pronounced with small instruments. A view with a 24-inch reflector on a fine Arizona night showed the cluster as irregular, and the host of stars made fruitless any effort to find a geometrical figure." At its distance of 1066 pc., its angular diameter of about 20 arc minutes corresponds to about 4.0 parsecs (13 light years), similar to that of its more distant neighbor M37. It is of intermediate age at about 290 million years.
Messier 38 | Introduction
Lindblad studied the theory of the rotation of galaxies. By making careful observations of the apparent motions of stars, he was able to study the rotation of the Milky Way. He deduced that the rate of rotation of the stars in the outer part of the galaxy, where the Sun is located, decreased with distance from the galactic core. This deduction was soon confirmed by Jan Oort in 1927. A certain class of resonances in rotating stellar or gaseous disks are named Lindblad resonances, after Bertil Lindblad.
Bertil Lindblad | Introduction