Galactocentrism

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In astronomy, galactocentrism is the theory that the Milky Way Galaxy, home of Earth 's Solar System, is at or near the center of the Universe. [1] [2]

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Thomas Wright and Immanuel Kant first speculated that fuzzy patches of light called nebulae were actually distant "island universes" consisting of many stellar systems. [3] The shape of the solar system's own parent 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. [4]

William Herschel's heliocentric model of the universe, which was regarded as the standard model of cosmology in the 19th century, was overthrown by astronomer Harlow Shapley's work on globular clusters in 1918. Shapley's research marked the transition from heliocentrism to galactocentrism, [5] [6] [7] 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. By 1925, the galactocentric model was established. [7]

The theory of galactocentrism was an important step in the development of cosmological models as speculation on the existence of other galaxies, comparable in size and structure to the Milky Way, placed the Earth in its proper perspective with respect to the rest of the universe. Shifts from heliocentrism to galactocentrism and later acentrism have been compared in significance to the Copernican Revolution. [7]

Harlow Shapley and 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. [6] 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. [8] The Galactic Center was determined to be in the direction of the Sagittarius constellation, approximately 50,000 light-years from us. [8]

Sextans A, a member of the local group of galaxies, which includes the Andromeda and Milky Way spirals, Lowell Observatory, 2013 Local.group.arp.600pix.jpg
Sextans A, a member of the local group of galaxies, which includes the Andromeda and Milky Way spirals, Lowell Observatory, 2013

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. [7] 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. [9] [10] [11]

"The realization, only a few decades ago, that our Galaxy is not unique and central in the universe ranks with the acceptance of the Copernican system as one of the great advances in cosmological thought." George O. Abell [7]

When astronomers realized that starlight can be absorbed by clouds of gas and dust, infrared radiation was used to penetrate the dust clouds. [12] Estimates dating after 2000 locate the Solar System within the range 24–28.4 kilolight-years (7.4–8.7 kiloparsecs ) from the Galactic Center of the Milky Way galaxy. [13]

Shift from galactocentrism to acentrism

Hubble's observations of redshift in light from distant galaxies indicated that the universe was expanding and acentric. [7] As a result, galactocentrism was abandoned in favor of the Big Bang model of the acentric expanding universe. Further assumptions, such as the Copernican principle, the cosmological principle, dark energy, and dark matter, eventually lead to the current model of cosmology, Lambda-CDM.

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<span class="mw-page-title-main">Copernican principle</span> Principle that humans are not privileged observers of the universe

In physical cosmology, the Copernican principle states that humans, on the Earth or in the Solar System, are not privileged observers of the universe, that observations from the Earth are representative of observations from the average position in the universe. Named for Copernican heliocentrism, it is a working assumption that arises from a modified cosmological extension of Copernicus' argument of a moving Earth.

<span class="mw-page-title-main">Edwin Hubble</span> American astronomer (1889–1953)

Edwin Powell Hubble was an American astronomer. He played a crucial role in establishing the fields of extragalactic astronomy and observational cosmology.

<span class="mw-page-title-main">Galaxy</span> Large gravitationally bound system of stars and interstellar matter

A galaxy is a system of stars, stellar remnants, interstellar gas, dust, and dark matter bound together by gravity. The word is derived from the Greek galaxias (γαλαξίας), literally 'milky', a reference to the Milky Way galaxy that contains the Solar System. Galaxies, averaging an estimated 100 million stars, range in size from dwarfs with less than a thousand stars, to the largest galaxies known – supergiants with one hundred trillion stars, each orbiting its galaxy's center of mass. Most of the mass in a typical galaxy is in the form of dark matter, with only a few percent of that mass visible in the form of stars and nebulae. Supermassive black holes are a common feature at the centres of galaxies.

<span class="mw-page-title-main">Globular cluster</span> Spherical collection of stars

A globular cluster is a spheroidal conglomeration of stars that is bound together by gravity, with a higher concentration of stars towards its center. It can contain anywhere from tens of thousands to many millions of member stars, all orbiting in a stable, compact formation. Globular clusters are similar in form to dwarf spheroidal galaxies, and the distinction between the two is not always clear. Their name is derived from Latin globulus. Globular clusters are occasionally known simply as "globulars".

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

The following is a timeline of galaxies, clusters of galaxies, and large-scale structure of the universe.

<span class="mw-page-title-main">Timeline of cosmological theories</span>

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<span class="mw-page-title-main">Andromeda Galaxy</span> Barred spiral galaxy in the Local Group

The Andromeda Galaxy is a barred spiral galaxy and is the nearest major galaxy to the Milky Way. It was originally named the Andromeda Nebula and is cataloged as Messier 31, M31, and NGC 224. Andromeda has a D25 isophotal diameter of about 46.56 kiloparsecs (152,000 light-years) and is approximately 765 kpc (2.5 million light-years) from Earth. The galaxy's name stems from the area of Earth's sky in which it appears, the constellation of Andromeda, which itself is named after the princess who was the wife of Perseus in Greek mythology.

<span class="mw-page-title-main">Spiral galaxy</span> Class of galaxy that has spiral structures extending from their cores.

Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, form part of the Hubble sequence. Most spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and a central concentration of stars known as the bulge. These are often surrounded by a much fainter halo of stars, many of which reside in globular clusters.

<span class="mw-page-title-main">Heliocentrism</span> Sun-centered astronomical model

Heliocentrism is a superseded astronomical model in which the Earth and planets revolve around the Sun at the center of the universe. Historically, heliocentrism was opposed to geocentrism, which placed the Earth at the center. The notion that the Earth revolves around the Sun had been proposed as early as the third century BC by Aristarchus of Samos, who had been influenced by a concept presented by Philolaus of Croton. In the 5th century BC the Greek Philosophers Philolaus and Hicetas had the thought on different occasions that the Earth was spherical and revolving around a "mystical" central fire, and that this fire regulated the universe. In medieval Europe, however, Aristarchus' heliocentrism attracted little attention—possibly because of the loss of scientific works of the Hellenistic period.

<span class="mw-page-title-main">Harlow Shapley</span> American scientist and political activist (1885–1972)

Harlow Shapley was an American scientist, head of the Harvard College Observatory (1921–1952), and political activist during the latter New Deal and Fair Deal.

<span class="mw-page-title-main">Thomas Wright (astronomer)</span> English polymath

Thomas Wright was an English astronomer, mathematician, instrument maker, architect and garden designer. He was the first to describe the shape of the Milky Way and to speculate that faint nebulæ were distant galaxies.

<span class="mw-page-title-main">Heber Doust Curtis</span> American astronomer

Heber Doust Curtis was an American astronomer. He participated in 11 expeditions for the study of solar eclipses, and, as an advocate and theorist that additional galaxies existed outside of the Milky Way, was involved in the 1920 Shapley–Curtis Debate concerning the size and galactic structure of the universe.

<span class="mw-page-title-main">Omega Centauri</span> Globular cluster in the constellation Centaurus

Omega Centauri is a globular cluster in the constellation of Centaurus that was first identified as a non-stellar object by Edmond Halley in 1677. Located at a distance of 17,090 light-years, it is the largest known globular cluster in the Milky Way at a diameter of roughly 150 light-years. It is estimated to contain approximately 10 million stars, with a total mass of 4 million solar masses, making it the most massive known globular cluster in the Milky Way.

<span class="mw-page-title-main">NGC 6397</span> Globular cluster of stars in the Milky Way

NGC 6397 is a globular cluster in the constellation Ara that was discovered by French astronomer Nicolas-Louis de Lacaille in 1752. It is located about 7,800 light-years from Earth, making it one of the two nearest globular clusters to Earth. The cluster contains around 400,000 stars, and can be seen with the naked eye under good observing conditions.

<span class="mw-page-title-main">Messier 80</span> Globular cluster in the constellation Scorpius

Messier 80 is a globular cluster in the constellation Scorpius. It was discovered by Charles Messier in 1781, being one of his first discoveries.

<span class="mw-page-title-main">Messier 68</span> Globular cluster in the constellation Hydra

Messier 68 is a globular cluster found in the east south-east of Hydra, away from its precisely equatorial part. It was discovered by Charles Messier in 1780. William Herschel described it as "a beautiful cluster of stars, extremely rich, and so compressed that most of the stars are blended together". His son John noted that it was "all clearly resolved into stars of 12th magnitude, very loose and ragged at the borders".

<span class="mw-page-title-main">Great Debate (astronomy)</span> 1920 debate between Harlow Shapley and Heber Curtis on whether there were other galaxies

The Great Debate, also called the Shapley–Curtis Debate, was held on 26 April 1920 at the Smithsonian Museum of Natural History, between the astronomers Harlow Shapley and Heber Curtis. It concerned the nature of so-called spiral nebulae and the size of the universe. Shapley believed that these nebulae were relatively small and lay within the outskirts of the Milky Way galaxy, while Curtis held that they were in fact independent galaxies, implying that they were exceedingly large and distant.

<span class="mw-page-title-main">Satellite galaxy</span> Galaxy that orbits a larger galaxy due to gravitational attraction

A satellite galaxy is a smaller companion galaxy that travels on bound orbits within the gravitational potential of a more massive and luminous host galaxy. Satellite galaxies and their constituents are bound to their host galaxy, in the same way that planets within the Solar System are gravitationally bound to the Sun. While most satellite galaxies are dwarf galaxies, satellite galaxies of large galaxy clusters can be much more massive. The Milky Way is orbited by about fifty satellite galaxies, the largest of which is the Large Magellanic Cloud.

<span class="mw-page-title-main">History of the center of the Universe</span> Historical concept in cosmology

The center of the Universe is a concept that lacks a coherent definition in modern astronomy; according to standard cosmological theories on the shape of the universe, it has no distinct spatial center.

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

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

References

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