The observable universe is a spherical region of the Universe comprising all matter that can be observed from Earth or its space-based telescopes and exploratory probes at the present time, because electromagnetic radiation from these objects has had time to reach the Solar System and Earth since the beginning of the cosmological expansion. There are at least 2 trillion galaxies in the observable universe. Assuming the Universe is isotropic, the distance to the edge of the observable universe is the same in every direction; that is, the observable universe has a spherical volume centered on the observer. Every location in the Universe has its own observable universe, which may or may not overlap with the one centered on Earth; the word observable in this sense does not refer to the capability of modern technology to detect light or other information from an object, or whether there is anything to be detected. It refers to the physical limit created by the speed of light itself; because no signals can travel faster than light, any object farther away from us than light could travel in the age of the Universe cannot be detected, as the signals could not have reached us yet.
Sometimes astrophysicists distinguish between the visible universe, which includes only signals emitted since recombination —and the observable universe, which includes signals since the beginning of the cosmological expansion. According to calculations, the current comoving distance—proper distance, which takes into account that the universe has expanded since the light was emitted—to particles from which the cosmic microwave background radiation was emitted, which represent the radius of the visible universe, is about 14.0 billion parsecs, while the comoving distance to the edge of the observable universe is about 14.3 billion parsecs, about 2% larger. The radius of the observable universe is therefore estimated to be about 46.5 billion light-years and its diameter about 28.5 gigaparsecs. The total mass of ordinary matter in the universe can be calculated using the critical density and the diameter of the observable universe to be about 1.5 × 1053 kg. In November 2018, astronomers reported that the extragalactic background light amounted to 4 × 1084 photons.
Since the expansion of the universe is known to accelerate and will become exponential in the future, the light emitted from all distant objects, past some time dependent on their current redshift, will never reach the Earth. In the future all observable objects will freeze in time while emitting progressively redder and fainter light. For instance, objects with the current redshift z from 5 to 10 will remain observable for no more than 4–6 billion years. In addition, light emitted by objects situated beyond a certain comoving distance will never reach Earth; some parts of the universe are too far away for the light emitted since the Big Bang to have had enough time to reach Earth or its scientific space-based instruments, so lie outside the observable universe. In the future, light from distant galaxies will have had more time to travel, so additional regions will become observable. However, due to Hubble's law, regions sufficiently distant from the Earth are expanding away from it faster than the speed of light and furthermore the expansion rate appears to be accelerating due to dark energy.
Assuming dark energy remains constant, so that the expansion rate of the universe continues to accelerate, there is a "future visibility limit" beyond which objects will never enter our observable universe at any time in the infinite future, because light emitted by objects outside that limit would never reach the Earth. This future visibility limit is calculated at a comoving distance of 19 billion parsecs, assuming the universe will keep expanding forever, which implies the number of galaxies that we can theoretically observe in the infinite future is only larger than the number observable by a factor of 2.36. Though in principle more galaxies will become observable in the future, in practice an increasing number of galaxies will become redshifted due to ongoing expansion, so much so that they will seem to disappear from view and become invisible. An additional subtlety is that a galaxy at a given comoving distance is defined to lie within the "observable universe" if we can receive signals emitted by the galaxy at any age in its past history, but because of the universe's expansion, there may be some age at which a signal sent from the same galaxy can never reach the Earth at any point in the infinite future (so, for example, we might never see what the galaxy looked like 10 billion years after the Bi
SIMBAD is an astronomical database of objects beyond the Solar System. It is maintained by the Centre de données astronomiques de France. SIMBAD was created by merging the Catalog of Stellar Identifications and the Bibliographic Star Index as they existed at the Meudon Computer Centre until 1979, expanded by additional source data from other catalogues and the academic literature; the first on-line interactive version, known as Version 2, was made available in 1981. Version 3, developed in the C language and running on UNIX stations at the Strasbourg Observatory, was released in 1990. Fall of 2006 saw the release of Version 4 of the database, now stored in PostgreSQL, the supporting software, now written in Java; as of 10 February 2017, SIMBAD contains information for 9,099,070 objects under 24,529,080 different names, with 327,634 bibliographical references and 15,511,733 bibliographic citations. The minor planet 4692 SIMBAD was named in its honour. Planetary Data System – NASA's database of information on SSSB, maintained by JPL and Caltech.
NASA/IPAC Extragalactic Database – a database of information on objects outside the Milky Way maintained by JPL. NASA Exoplanet Archive – an online astronomical exoplanet catalog and data service Bibcode SIMBAD, Strasbourg SIMBAD, Harvard
Hubble Space Telescope
The Hubble Space Telescope is a space telescope, launched into low Earth orbit in 1990 and remains in operation. Although not the first space telescope, Hubble is one of the largest and most versatile and is well known as both a vital research tool and a public relations boon for astronomy; the HST is named after the astronomer Edwin Hubble and is one of NASA's Great Observatories, along with the Compton Gamma Ray Observatory, the Chandra X-ray Observatory and the Spitzer Space Telescope. With a 2.4-meter mirror, Hubble's four main instruments observe in the ultraviolet and near infrared regions of the electromagnetic spectrum. Hubble's orbit outside the distortion of Earth's atmosphere allows it to take high-resolution images, with lower background light than ground-based telescopes. Hubble has recorded some of the most detailed visible light images allowing a deep view into space and time. Many Hubble observations have led to breakthroughs in astrophysics, such as determining the rate of expansion of the universe.
The HST was built by the United States space agency NASA, with contributions from the European Space Agency. The Space Telescope Science Institute selects Hubble's targets and processes the resulting data, while the Goddard Space Flight Center controls the spacecraft. Space telescopes were proposed as early as 1923. Hubble was funded in the 1970s, with a proposed launch in 1983, but the project was beset by technical delays, budget problems, the Challenger disaster; when launched in 1990, Hubble's main mirror was found to have been ground incorrectly, creating a spherical aberration, compromising the telescope's capabilities. The optics were corrected to their intended quality by a servicing mission in 1993. Hubble is the only telescope designed to be serviced in space by astronauts. After launch by Space Shuttle Discovery in 1990, five subsequent Space Shuttle missions repaired and replaced systems on the telescope, including all five of the main instruments; the fifth mission was canceled on safety grounds following the Columbia disaster.
However, after spirited public discussion, NASA administrator Mike Griffin approved the fifth servicing mission, completed in 2009. The telescope is operating as of 2019, could last until 2030–2040. After numerous delays, its successor, the James Webb Space Telescope, is scheduled to be launched in March 2021. In 1923, Hermann Oberth—considered a father of modern rocketry, along with Robert H. Goddard and Konstantin Tsiolkovsky—published Die Rakete zu den Planetenräumen, which mentioned how a telescope could be propelled into Earth orbit by a rocket; the history of the Hubble Space Telescope can be traced back as far as 1946, to the astronomer Lyman Spitzer's paper "Astronomical advantages of an extraterrestrial observatory". In it, he discussed the two main advantages that a space-based observatory would have over ground-based telescopes. First, the angular resolution would be limited only by diffraction, rather than by the turbulence in the atmosphere, which causes stars to twinkle, known to astronomers as seeing.
At that time ground-based telescopes were limited to resolutions of 0.5–1.0 arcseconds, compared to a theoretical diffraction-limited resolution of about 0.05 arcsec for a telescope with a mirror 2.5 m in diameter. Second, a space-based telescope could observe infrared and ultraviolet light, which are absorbed by the atmosphere. Spitzer devoted much of his career to pushing for the development of a space telescope. In 1962, a report by the US National Academy of Sciences recommended the development of a space telescope as part of the space program, in 1965 Spitzer was appointed as head of a committee given the task of defining scientific objectives for a large space telescope. Space-based astronomy had begun on a small scale following World War II, as scientists made use of developments that had taken place in rocket technology; the first ultraviolet spectrum of the Sun was obtained in 1946, the National Aeronautics and Space Administration launched the Orbiting Solar Observatory to obtain UV, X-ray, gamma-ray spectra in 1962.
An orbiting solar telescope was launched in 1962 by the United Kingdom as part of the Ariel space program, in 1966 NASA launched the first Orbiting Astronomical Observatory mission. OAO-1's battery failed after three days, it was followed by OAO-2, which carried out ultraviolet observations of stars and galaxies from its launch in 1968 until 1972, well beyond its original planned lifetime of one year. The OSO and OAO missions demonstrated the important role space-based observations could play in astronomy, in 1968, NASA developed firm plans for a space-based reflecting telescope with a mirror 3 m in diameter, known provisionally as the Large Orbiting Telescope or Large Space Telescope, with a launch slated for 1979; these plans emphasized the need for manned maintenance missions to the telescope to ensure such a costly program had a lengthy working life, the concurrent development of plans for the reusable Space Shuttle indicated that the technology to allow this was soon to become available.
The continuing success of the OAO program encouraged strong consensus within the astronomical community that the LST should be a major goal. In 1970, NASA established two committees, one to plan the engineering side of the space telescope project, the other to determine the scientific goals of the mission. Once these had been established, the next hurdle for NASA was to obtain funding for the instrument, which would be far more costly than any Earth-bas
Harlow Shapley was an American scientist, head of the Harvard College Observatory, political activist during the latter New Deal and Fair Deal. Shapley used RR Lyrae stars to estimate the size of the Milky Way Galaxy and the Sun's position within it by using parallax. In 1953 he proposed his "liquid water belt" theory, now known as the concept of a habitable zone. Shapley was born on a farm in Nashville, Missouri, to Willis and Sarah Shapley, dropped out of school with only the equivalent of a fifth-grade education. After studying at home and covering crime stories as a newspaper reporter, Shapley returned to complete a six-year high school program in only two years, graduating as class valedictorian. In 1907, Shapley went to study journalism at the University of Missouri; when he learned that the opening of the School of Journalism had been postponed for a year, he decided to study the first subject he came across in the course directory. Rejecting Archaeology, which Shapley claimed he could not pronounce, he chose the next subject, Astronomy.
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 to determine distances to globular clusters. He was instrumental in moving astronomy away from the idea that Cepheids were spectroscopic binaries, toward the concept that they were pulsators, he realized that the Milky Way Galaxy was far larger than believed, 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. Shapley participated in the "Great Debate" with Heber D. Curtis on the nature of nebulae and galaxies and the size of the Universe; the debate took place on April 26, 1920, in the hall of the United States National Academy of Sciences in Washington D. C. Shapley took the side that spiral nebulae are inside our Milky Way, while Curtis took the side that the spiral nebulae are'island universes' far outside our own Milky Way and comparable in size and nature to our own Milky Way.
This issue and debate are the start of extragalactic astronomy, while the detailed arguments and data with ambiguities, appeared together in 1921. Characteristic issues were whether Adriaan van Maanen had measured rotation in a spiral nebula, the nature and luminosity of the exploding novae and supernovae seen in spiral galaxies, the size of our own Milky Way. However, Shapley's actual talk and argument given during the Great Debate were different from the published paper. Historian Michael Hoskin says "His decision was to treat the National Academy of Sciences to an address so elementary that much of it was uncontroversial.", with Shapley's motivation being only to impress a delegation from Harvard who were interviewing him for a possible offer as the next Director of Harvard College Observatory. With the default by Shapley, Curtis won the debate; the astronomical issues were soon resolved in favor of Curtis' position when Edwin Hubble discovered Cepheid variable stars in the Andromeda Galaxy.
At the time of the debate, Shapley was working at the Mount Wilson Observatory, where he had been hired by George Ellery Hale. After the debate, however, he was hired to replace the deceased Edward Charles Pickering as director of the Harvard College Observatory, he is known to have incorrectly opposed Edwin Hubble's observations that there are additional galaxies in the universe other than the Milky Way. Shapley fiercely regarded his work as junk science. However, after he received a letter from Hubble showing Hubble's observed light curve of V1, he withdrew his criticism, he told a colleague, "Here is the letter that destroyed my universe." He encouraged Hubble to write a paper for a joint meeting of the American Astronomical Society and American Association for the Advancement of Science. Hubble's findings went on to reshape fundamentally the scientific view of the universe, he served as director of the HCO from 1921–52. During this time, he hired Cecilia Payne, who, in 1925, became the first person to earn a doctorate at Radcliffe College in the field of astronomy, for work done at Harvard College Observatory.
From 1941 he was on the original standing committee of the Foundation for the Study of Cycles. He served on the board of trustees of Science Service, now known as Society for Science & the Public, from 1935-71. In the 1940s, Shapley helped found government funded scientific associations, including the National Science Foundation, he is responsible for the addition of the "S" in UNESCO. On November 14, 1946, Shapley appeared under subpoena by the House Un-American Activities Committee in his role as member of the Independent Committee of the Arts and Professions, "major political arm of the Russophile left", for opposing U. S. Representative Joseph William Martin Jr. during mid-term elections that year. In 1947, he became President of the American Association for the Advancement of Science. In his inaugural address he referred to the danger of the "genius maniac" and proposed the elimination of "all primates that show any evidence of signs of genius or talent". Other global threats he listed were: drugs.
In 1950, Shapley was instrumental in organizing a campaign in academia against the controversial US bestseller book Worlds in
The Galactic Center, or Galactic Centre, is the rotational center of the Milky Way. It is 8,122 ± 31 parsecs away from Earth in the direction of the constellations Sagittarius and Scorpius where the Milky Way appears brightest, it coincides with the compact radio source Sagittarius A*. There are around 10 million stars within one parsec of the Galactic Center, dominated by red giants, with a significant population of massive supergiants and Wolf-Rayet stars from a star formation event around one million years ago, one supermassive black hole of 4.100 ± 0.034 million solar masses at the Galactic Center, which powers the Sagittarius A* radio source. Because of interstellar dust along the line of sight, the Galactic Center cannot be studied at visible, ultraviolet, or soft X-ray wavelengths; the available information about the Galactic Center comes from observations at gamma ray, hard X-ray, infrared and radio wavelengths. Immanuel Kant stated in General Natural History and Theory of the Heavens that a large star was at the center of the Milky Way Galaxy, 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. In the early 1940s Walter Baade at Mount Wilson Observatory took advantage of wartime blackout conditions in nearby Los Angeles to conduct a search for the center with the 100-inch Hooker Telescope, he found that near the star Alnasl there is a one-degree-wide void in the interstellar dust lanes, which provides a clear view of the swarms of stars around the nucleus of our Milky Way Galaxy. This gap has been known as Baade's Window since. At Dover Heights in Sydney, Australia, a team of radio astronomers from the Division of Radiophysics at the CSIRO, led by Joseph Lade Pawsey, used'sea interferometry' to discover some of the first interstellar and intergalactic radio sources, including Taurus A, Virgo A and Centaurus A. By 1954 they had built an 80-foot fixed dish antenna and used it to make a detailed study of an extended powerful belt of radio emission, detected in Sagittarius.
They named an intense point-source near the center of this belt Sagittarius A, realised that it was located at the center of our Galaxy, despite being some 32 degrees south-west of the conjectured galactic center of the time. In 1958 the International Astronomical Union decided to adopt the position of Sagittarius A as the true zero co-ordinate point for the system of galactic latitude and longitude. In the equatorial coordinate system the location is: RA 17h 45m 40.04s, Dec −29° 00′ 28.1″. The exact distance between the Solar System and the Galactic Center is not certain, although estimates since 2000 have remained within the range 24–28.4 kilolight-years. The latest estimates from geometric-based methods and standard candles yield the following distances to the Galactic Center: 7.4±0.2 ± 0.2 or 7.4±0.3 kpc 7.62±0.32 kpc 7.7±0.7 kpc 7.94 or 8.0±0.5 kpc 7.98±0.15 ± 0.20 or 8.0±0.25 kpc 8.33±0.35 kpc 8.7±0.5 kpc An accurate determination of the distance to the Galactic Center as established from variable stars or standard candles is hindered by countless effects, which include: an ambiguous reddening law.
The nature of the Milky Way's bar, which extends across the Galactic Center, is actively debated, with estimates for its half-length and orientation spanning between 1–5 kpc and 10–50°. Certain authors advocate that the Milky Way features two distinct bars, one nestled within the other; the bar is delineated by red-clump stars. The bar may be surrounded by a ring called the 5-kpc ring that contains a large fraction of the molecular hydrogen present in the Milky Way, most of the Milky Way's star formation activity. Viewed from the Andromeda Galaxy, it would be the brightest feature of the Milky Way; the complex astronomical radio source Sagittarius A appears to be located exactly at the Galactic Center, contains an intense compact radio source, Sagittarius A*, which coincides with a supermassive black hole at the center of the Milky Way. Accretion of gas onto the black hole involving an accretion disk around it, would release energy to power the radio source, itself much larger than the black hole.
The latter is too small to see with present instruments. A study in 2008 which linked radio telescopes in Hawaii and California measured the diameter of Sagittarius A* to be 44 million kilometers. For comparison, the radius of Earth's orbit around the Sun is about 150 million kilometers, whereas the distance of Mercury from the Sun at closest approach is 46 million kilometers. Thus, the diameter of the radio source is less than the distance from Mercury to the Sun. Scientists at the Max Planck Institute for Extraterrestrial Physics in Germany using Chilean telescopes have confirmed the existence of a superm
The Hertzsprung–Russell diagram, abbreviated as H–R diagram, HR diagram or HRD, is a scatter plot of stars showing the relationship between the stars' absolute magnitudes or luminosities versus their stellar classifications or effective temperatures. More it plots each star on a graph plotting the star's brightness against its temperature; the diagram was created circa 1910 by Ejnar Hertzsprung and Henry Norris Russell and represents a major step towards an understanding of stellar evolution. The related color–magnitude diagram plots the apparent magnitudes of stars against their color for a cluster so that the stars are all at the same distance. In the nineteenth-century large-scale photographic spectroscopic surveys of stars were performed at Harvard College Observatory, producing spectral classifications for tens of thousands of stars, culminating in the Henry Draper Catalogue. In one segment of this work Antonia Maury included divisions of the stars by the width of their spectral lines.
Hertzsprung noted that stars described with narrow lines tended to have smaller proper motions than the others of the same spectral classification. He took this as an indication of greater luminosity for the narrow-line stars, computed secular parallaxes for several groups of these, allowing him to estimate their absolute magnitude. In 1910 Hans Rosenberg published a diagram plotting the apparent magnitude of stars in the Pleiades cluster against the strengths of the calcium K line and two hydrogen Balmer lines; these spectral lines serve as a proxy for the temperature of the star, an early form of spectral classification. The apparent magnitude of stars in the same cluster is equivalent to their absolute magnitude and so this early diagram was a plot of luminosity against temperature; the same type of diagram is still used today as a means of showing the stars in clusters without having to know their distance and luminosity. Hertzsprung had been working with this type of diagram, but his first publications showing it were not until 1911.
This was the form of the diagram using apparent magnitudes of a cluster of stars all at the same distance. Russell's early versions of the diagram included Maury's giant stars identified by Hertzsprung, those nearby stars with parallaxes measured at the time, stars from the Hyades, several moving groups, for which the moving cluster method could be used to derive distances and thereby obtain absolute magnitudes for those stars. There are several forms of the Hertzsprung–Russell diagram, the nomenclature is not well defined. All forms share the same general layout: stars of greater luminosity are toward the top of the diagram, stars with higher surface temperature are toward the left side of the diagram; the original diagram displayed the spectral type of stars on the horizontal axis and the absolute visual magnitude on the vertical axis. The spectral type is not a numerical quantity, but the sequence of spectral types is a monotonic series that reflects the stellar surface temperature. Modern observational versions of the chart replace spectral type by a color index of the stars.
This type of diagram is what is called an observational Hertzsprung–Russell diagram, or a color–magnitude diagram, it is used by observers. In cases where the stars are known to be at identical distances such as within a star cluster, a color–magnitude diagram is used to describe the stars of the cluster with a plot in which the vertical axis is the apparent magnitude of the stars. For cluster members, by assumption there is a single additive constant difference between their apparent and absolute magnitudes, called the distance modulus, for all of that cluster of stars. Early studies of nearby open clusters by Hertzsprung and Rosenberg produced the first CMDs, antedating by a few years Russell's influential synthesis of the diagram collecting data for all stars for which absolute magnitudes could be determined. Another form of the diagram plots the effective surface temperature of the star on one axis and the luminosity of the star on the other invariably in a log-log plot. Theoretical calculations of stellar structure and the evolution of stars produce plots that match those from observations.
This type of diagram could be called temperature-luminosity diagram, but this term is hardly used. A peculiar characteristic of this form of the H–R diagram is that the temperatures are plotted from high temperature to low temperature, which aids in comparing this form of the H–R diagram with the observational form. Although the two types of diagrams are similar, astronomers make a sharp distinction between the two; the reason for this distinction is that the exact transformation from one to the other is not trivial. To go between effective temperature and color requires a color–temperature relation, constructing, difficult; when converting luminosity or absolute bolometric magnitude to apparent or absolute visual magnitude, one requires a bolometric correction, which may or may not come from the same source as the color–temperature relation. One needs to know the distance to the observed objects and the effects of interstellar obscuration, both in the color and in the apparent magnitude. Color distortion and extinction are apparent in stars having
The Local Group is the galaxy group that includes the Milky Way. Its has a total diameter of 3 Mpc, a total mass of the order of 2×1012 solar masses, it consists of two clusters of galaxies in a "dumbbell" shape, the Milky Way and its satellites on one hand, the Andromeda Galaxy and its satellites on the other. The two clusters are separated by about 0.8 Mpc and move towards one another with a velocity of 123 km/h. The group itself is a part of the larger Virgo Supercluster, which may be a part of the Laniakea Supercluster; the total number of galaxies in the Local Group is unknown but known to exceed 54, most of them being dwarf galaxies. The two largest members, the Andromeda Galaxy and the Milky Way, are both spiral galaxies with masses of about 1012 solar masses each, each have their own system of satellite galaxies: The Andromeda Galaxy's satellite system consists of Messier 32, Messier 110, NGC 147, NGC 185, Andromeda I, And II, And III, And V, And VI, And VII, And VIII, And IX, And X, And XI, And XIX, And XXI and And XXII, plus several additional ultra-faint dwarf spheroidal galaxies.
The Milky Way's satellite galaxies system comprises Sagittarius Dwarf Galaxy, Large Magellanic Cloud, Small Magellanic Cloud, Canis Major Dwarf Galaxy, Ursa Minor Dwarf Galaxy, Draco Dwarf Galaxy, Carina Dwarf Galaxy, Sextans Dwarf Galaxy, Sculptor Dwarf Galaxy, Fornax Dwarf Galaxy, Leo I, Leo II, Ursa Major I Dwarf Galaxy and Ursa Major II Dwarf Galaxy, plus several additional ultra-faint dwarf spheroidal galaxies. The Triangulum Galaxy is the third largest member of the Local Group, at about 5×1010 M☉, the third spiral galaxy, it may not be a companion to the Andromeda Galaxy. Pisces Dwarf Galaxy is equidistant from the Andromeda Galaxy and the Triangulum Galaxy, so it may be a satellite of either; the membership of NGC 3109, with its companions Sextans A and the Antlia Dwarf Galaxy, is uncertain due to extreme distances from the center of the Local Group. The other members of the group are gravitationally secluded from these large subgroups: IC 10, IC 1613, Phoenix Dwarf Galaxy, Leo A, Tucana Dwarf Galaxy, Cetus Dwarf Galaxy, Pegasus Dwarf Irregular Galaxy, Wolf–Lundmark–Melotte, Aquarius Dwarf Galaxy, Sagittarius Dwarf Irregular Galaxy.
The term "The Local Group" was introduced by Edwin Hubble in Chapter VI of his 1936 book The Realm of the Nebulae. There, he described it as "a typical small group of nebulae, isolated in the general field" and delineated, by decreasing luminosity, its members to be M31, Milky Way, M33, Large Magellanic Cloud, Small Magellanic Cloud, M32, NGC 205, NGC 6822, NGC 185, IC 1613 and NGC 147, he identified IC 10 as a possible part of Local Group. By 2003, the number of known Local Group members had increased from his initial 12 to 36. Smith's Cloud, a high-velocity cloud, between 32,000 and 49,000 light years from Earth and 8,000 light years from the disk of the Milky Way galaxy HVC 127-41-330, a high-velocity cloud, 2.3 million light-years from Earth Monoceros Ring, a ring of stars around the Milky Way, proposed to consist of a stellar stream torn from the Canis Major Dwarf Galaxy Galaxy cluster List of nearest galaxies List of galaxy clusters IC 342/Maffei Group, the group of galaxies nearest to the Local Group Local Supercluster List of Andromeda's satellite galaxies List of Milky Way's satellite galaxies The Local Group of Galaxies, SEDS Messier pages A Survey of the Resolved Stellar Content of Nearby Galaxies Currently Forming Stars, Lowell Observatory van den Bergh, Sidney.
"Updated Information on the Local Group". The Publications of the Astronomical Society of the Pacific. 112: 529–536. ArXiv:astro-ph/0001040. Bibcode:2000PASP..112..529V. Doi:10.1086/316548