Supermassive black hole
A supermassive black hole is the largest type of black hole, containing a mass of the order of hundreds of thousands, to billions of times, the mass of the Sun. Black holes are a class of astronomical object that have undergone gravitational collapse, leaving behind spheroidal regions of space from which nothing can escape, not light. Observational evidence indicates that nearly all large galaxies contain a supermassive black hole, located at the galaxy's center. In the case of the Milky Way, the supermassive black hole corresponds to the location of Sagittarius A* at the Galactic Core. Accretion of interstellar gas onto supermassive black holes is the process responsible for powering quasars and other types of active galactic nuclei. Supermassive black holes have properties. First, the average density of a SMBH can be less than the density of water in the case of some SMBHs; this is. Since the volume of a spherical object is directly proportional to the cube of the radius, the density of a black hole is inversely proportional to the square of the mass, thus higher mass black holes have lower average density.
In addition, the tidal forces in the vicinity of the event horizon are weaker for supermassive black holes. The tidal force on a body at the event horizon is inversely proportional to the square of the mass: a person on the surface of the Earth and one at the event horizon of a 10 million M☉ black hole experience about the same tidal force between their head and feet. Unlike with stellar mass black holes, one would not experience significant tidal force until deep into the black hole; some astronomers have begun labeling black holes of at least 10 billion M☉ as ultramassive black holes. Most of these are associated with exceptionally energetic quasars; the story of how supermassive black holes were found began with the investigation by Maarten Schmidt of the radio source 3C 273 in 1963. This was thought to be a star, but the spectrum proved puzzling, it was determined to be hydrogen emission lines, red shifted, indicating the object was moving away from the Earth. Hubble's law showed that the object was located several billion light-years away, thus must be emitting the energy equivalent of hundreds of galaxies.
The rate of light variations of the source, dubbed a quasi-stellar object, or quasar, suggested the emitting region had a diameter of one parsec or less. Four such sources had been identified by 1964. In 1963, Fred Hoyle and W. A. Fowler proposed the existence of hydrogen burning supermassive stars as an explanation for the compact dimensions and high energy output of quasars; these would have a mass of about 105 – 109 M☉. However, Richard Feynman noted stars above a certain critical mass are dynamically unstable and would collapse into a black hole, at least if they were non-rotating. Fowler proposed that these supermassive stars would undergo a series of collapse and explosion oscillations, thereby explaining the energy output pattern. Appenzeller and Fricke built models of this behavior, but found that the resulting star would still undergo collapse, concluding that a non-rotating 0.75×106 M☉ SMS "cannot escape collapse to a black hole by burning its hydrogen through the CNO cycle". Edwin E. Salpeter and Yakov B.
Zel'dovich made the proposal in 1964 that matter falling onto a massive compact object would explain the properties of quasars. It would require a mass of around 108 M☉ to match the output of these objects. Donald Lynden-Bell noted in 1969 that the infalling gas would form a flat disk that spirals into the central "Schwarzschild throat", he noted that the low output of nearby galactic cores implied these were old, inactive quasars. Meanwhile, in 1967, Martin Ryle and Malcolm Longair suggested that nearly all sources of extra-galactic radio emission could be explained by a model in which particles are ejected from galaxies at relativistic velocities. Martin Ryle, Malcolm Longair, Peter Scheuer proposed in 1973 that the compact central nucleus could be the original energy source for these relativistic jets. Arthur M. Wolfe and Geoffrey Burbidge noted in 1970 that the large velocity dispersion of the stars in the nuclear region of elliptical galaxies could only be explained by a large mass concentration at the nucleus.
They showed that the behavior could be explained by a massive black hole with up to 1010 M☉, or a large number of smaller black holes with masses below 103 M☉. Dynamical evidence for a massive dark object was found at the core of the active elliptical galaxy Messier 87 in 1978 estimated at 5×109 M☉. Discovery of similar behavior in other galaxies soon followed, including the Andromeda Galaxy in 1984 and the Sombrero Galaxy in 1988. Donald Lynden-Bell and Martin Rees hypothesized in 1971 that the center of the Milky Way galaxy would contain a massive black hole. Sagittarius A* was discovered and named on February 13 and 15, 1974, by astronomers Bruce Balick and Robert Brown using the Green Bank Interferometer of the National Radio Astronomy Observatory, they discovered a radio source. This was, the first indication that a supermassive black hole exists in the center of the Milky Way; the Hubble Space Telescope, launched in 1990, provided the resolution needed to perform more refined observations of galactic nuclei.
In 1994 th
Sloan Digital Sky Survey
The Sloan Digital Sky Survey or SDSS is a major multi-spectral imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project was named after the Alfred P. Sloan Foundation. Data collection began in 2000; the main galaxy sample has a median redshift of z = 0.1. Data release 8, released in January 2011, includes all photometric observations taken with the SDSS imaging camera, covering 14,555 square degrees on the sky. Data release 9, released to the public on 31 July 2012, includes the first results from the Baryon Oscillation Spectroscopic Survey spectrograph, including over 800,000 new spectra. Over 500,000 of the new spectra are of objects in the Universe 7 billion years ago. Data release 10, released to the public on 31 July 2013, includes all data from previous releases, plus the first results from the APO Galactic Evolution Experiment spectrograph, including over 57,000 high-resolution infrared spectra of stars in the Milky Way.
DR10 includes over 670,000 new BOSS spectra of galaxies and quasars in the distant universe. The publicly available images from the survey were made between 1998 and 2009. SDSS uses a dedicated 2.5 m wide-angle optical telescope. The imaging camera was retired in late 2009, since the telescope has observed in spectroscopic mode. Images were taken using a photometric system of five filters; these images are processed to produce lists of objects observed and various parameters, such as whether they seem pointlike or extended and how the brightness on the CCDs relates to various kinds of astronomical magnitude. For imaging observations, the SDSS telescope used the drift scanning technique, which tracks the telescope along a great circle on the sky and continuously records small strips of the sky; the image of the stars in the focal plane drifts along the CCD chip, the charge is electronically shifted along the detectors at the same rate, instead of staying fixed as in tracked telescopes.. This method allows consistent astrometry over the widest possible field, minimises overheads from reading out the detectors.
The disadvantage is minor distortion effects. The telescope's imaging camera is made up of 30 CCD chips, each with a resolution of 2048×2048 pixels, totaling 120 megapixels; the chips are arranged in 5 rows of 6 chips. Each row has a different optical filter with average wavelengths of 355.1, 468.6, 616.5, 748.1 and 893.1 nm, with 95% completeness in typical seeing to magnitudes of 22.0, 22.2, 22.2, 21.3, 20.5, for u, g, r, i, z respectively. The filters are placed on the camera in the order r, i, u, z, g. To reduce noise, the camera is cooled to 190 kelvins by liquid nitrogen. Using these photometric data, stars and quasars are selected for spectroscopy; the spectrograph operates by feeding an individual optical fibre for each target through a hole drilled in an aluminum plate. Each hole is positioned for a selected target, so every field in which spectra are to be acquired requires a unique plate; the original spectrograph attached to the telescope was capable of recording 640 spectra while the updated spectrograph for SDSS III can record 1000 spectra at once.
Over the course of each night, between six and nine plates are used for recording spectra. In spectroscopic mode, the telescope tracks the sky in the standard way, keeping the objects focused on their corresponding fibre tips; every night the telescope produces about 200 GB of data. During its first phase of operations, 2000–2005, the SDSS imaged more than 8,000 square degrees of the sky in five optical bandpasses, it obtained spectra of galaxies and quasars selected from 5,700 square degrees of that imaging, it obtained repeated imaging of a 300 square degree stripe in the southern Galactic cap. In 2005 the survey entered a new phase, the SDSS-II, by extending the observations to explore the structure and stellar makeup of the Milky Way, the SEGUE and the Sloan Supernova Survey, which watches after supernova Ia events to measure the distances to far objects; the survey covers over 7,500 square degrees of the Northern Galactic Cap with data from nearly 2 million objects and spectra from over 800,000 galaxies and 100,000 quasars.
The information on the position and distance of the objects has allowed the large-scale structure of the Universe, with its voids and filaments, to be investigated for the first time. All of these data were obtained in SDSS-I, but a small part of the footprint was finished in SDSS-II; the Sloan Extension for Galactic Understanding and Exploration obtained spectra of 240,000 stars in order to create a detailed three-dimensional map of the Milky Way. SEGUE data provide evidence for the age and phase space distribution of stars within the various Galactic components, providing crucial clues for understanding the structure, formation a
Pierre François André Méchain was a French astronomer and surveyor who, with Charles Messier, was a major contributor to the early study of deep sky objects and comets. Pierre Méchain was born in Laon, the son of the ceiling designer and plasterer Pierre François Méchain and Marie–Marguerite Roze, he displayed mental gifts in mathematics and physics but had to give up his studies for lack of money. However, his talents in astronomy were noticed by Joseph Jérôme Lalande, for whom he became a friend and proof-reader of the second edition of his book "L'Astronomie". Lalande secured a position for him as assistant hydrographer with the Naval Depot of Maps and Charts at Versailles, where he worked through the 1770s engaged in hydrographic work and coastline surveying, it was during this time—approximately 1774—that he met Charles Messier, they became friends. In the same year, he produced his first astronomical work, a paper on an occultation of Aldebaran by the Moon and presented it as a memoir to the Academy of Sciences.
In 1777, he married Barbe-Thérèse Marjou. They had two sons: Jérôme, born 1780, Augustin, born 1784, one daughter, he was admitted to the French Académie des sciences in 1782, was the editor of Connaissance des Temps from 1785 to 1792. In 1789 he was elected a Fellow of the Royal Society, he participated in the Anglo-French Survey to measure by trigonometry the precise distance between the Paris Observatory and the Royal Greenwich Observatory. This project was initiated by Dominique, comte de Cassini, in 1787 Méchain visited Dover and London with Cassini and Adrien-Marie Legendre to facilitate its progress; the three men visited the astronomer William Herschel at Slough. With his surveying skills, he worked on maps of Northern Italy and Germany after this, but his most important mapping work was geodetic: the determination of the southern part of the meridian arc of the Earth's surface between Dunkirk and Barcelona beginning in 1791; this measurement would become the basis of the metric system's unit of the meter.
He encountered numerous difficulties on this project stemming from the effects of the French Revolution. He was arrested after it was suspected his instruments were weapons, he was interned in Barcelona after war broke out between France and Spain, his property in Paris was confiscated during The Terror, he was released from Spain to live in Italy returned home in 1795. A intriguing fact about this project was that Méchain was uncertain of the precision of his measurements owing to anomalous results in verifying his latitude by astronomical observation; the distance from the pole to the equator, which Méchain and his associate Jean Baptiste Joseph Delambre had intended to be ten million meters, was determined in the late 20th century by space satellites to be 10,002,290 meters. This small error of 2,290 meters equals 1.423 statute miles. It represents in each meter an error of 0.23 millimetres – more than the width of a single strand of human hair. This discrepancy is sometimes mentioned as "Méchain's error", with the suggestion that the tiny variation in the length of the meridian can be attributed to Méchain's calculations.
But analysis of Méchain's figures reveals that Méchain kept the discrepancy tiny forcing his individual reported measurements to appear more precise and consistent than would be reasonably expected of a survey involving more than a hundred measurements of rough country using 18th century equipment. From 1799, he was the director of the Paris Observatory. Continuing doubts about his measurements of the Dunkirk-Barcelona arc led him to return to that work; this took him back to Spain in 1804, where he died in Castellón de la Plana. Méchain discovered either 25 or 26 deep-sky objects, depending on how one counts M102. However, Méchain disavowed the observation from 1783 onwards as a mistaken re-observation of M101. Since that time, others have proposed that he did in fact observe another object, suggested what they might be. See the discussion The M102 Controversy for more details. Open cluster Globular cluster Nebula Planetary nebula Supernova remnant Galaxy Other He independently discovered four others discovered by someone else but unknown to him at the time and included in the Messier catalogue: M71, discovered by Jean-Philippe de Chéseaux in the 1740s.
Six other discoveries are "honorary Messier objects" added to the list in the 20th century: He discovered NGC 5195, the companion galaxy that makes M51 so distinctive. Méchain never set out to observe deep-sky objects. Like Messier, he was interested in cataloguing objects that might be mistaken for comets. All together, he discovered eight comets, co-discovered three, his sole discoveries are: C/1781 M1, 1781 I C/1781 T1, 1781 II C/1785 E1, 1785 II 2P/Encke, discovered in 1786 C/1787 G1, 1787 I 8P/Tuttle, discovered in 1790 C/1799 P1 (Méchai
Sky & Telescope
Sky & Telescope is a monthly American magazine covering all aspects of amateur astronomy, including the following: current events in astronomy and space exploration. The articles are intended for the informed lay reader and include detailed discussions of current discoveries by participating scientists; the magazine is illustrated in full color, with both amateur and professional photography of celestial sights, as well as tables and charts of upcoming celestial events. Sky & Telescope was founded by Charles A Federer and his wife Helen Spence Federer and began publication at Harvard College Observatory in November 1941, as a result of the merger of the separate magazines, The Sky and The Telescope. In 2005, Sky Publishing Corporation was acquired by New Track Media, a portfolio company of the private equity firm Boston Ventures. In 2014, New Track was sold to F+W Media; the magazine played an important role in the dissemination of knowledge about telescope making, through the column "Gleanings for ATMs" that ran from 1933 to 1990.
Its main competitor is Astronomy. Amateur astronomy Amateur telescope making Official website
Astronomy & Astrophysics
Astronomy & Astrophysics is a peer-reviewed scientific journal covering theoretical and instrumental astronomy and astrophysics. It is one of the premier journals for astronomy in the world; the journal is published by EDP Sciences in 16 issues per year. The editor-in-chief is Thierry Forveille. Previous editors in chief include Claude Bertout, James Lequeux, Michael Grewing, Catherine Cesarsky and George Contopoulos. Astronomy & Astrophysics was formed in 1969 by the merging of several national journals of individual European countries into one comprehensive publication; these journals, with their ISSN and date of first publication are as follows: Annales d'Astrophysique ISSN 0365-0499, established in 1938 Arkiv för Astronomi ISSN 0004-2048, established in 1948 Bulletin of the Astronomical Institutes of the Netherlands ISSN 0365-8910, established in 1921 Bulletin Astronomique ISSN 0245-9787, established in 1884 Journal des Observateurs ISSN 0368-3389, established in 1915 Zeitschrift für Astrophysik ISSN 0372-8331, established in 1930The publishing of Astronomy & Astrophysics was further extended in 1992 by the incorporation of Bulletin of the Astronomical Institutes of Czechoslovakia, established in 1947.
Astronomy & Astrophysics published articles in either English, French, or German, but articles in French and German were always few. They were discontinued, in part due to difficulties in finding adequately specialized independent referees who were fluent in those languages; the original sponsoring countries were the four countries whose journals merged to form Astronomy & Astrophysics, together with Belgium, Denmark and Norway. The European Southern Observatory participated as a "member country". Norway withdrew, but Austria, Italy and Switzerland all joined; the Czech Republic, Hungary and Slovakia all joined as new members in the 1990s. In 2001 the words "A European Journal" were removed from the front cover in recognition of the fact that the journal was becoming global in scope, in 2002 Argentina was admitted as an "observer". In 2004 the Board of Directors decided that the journal "will henceforth consider applications for sponsoring membership from any country in the world with well-documented active and excellent astronomical research".
Argentina became the first non-European country to gain full membership in 2005. Brazil and Portugal all gained "observer" status at this time and have since progressed to full membership; this journal is listed in the following databases: All letters to the editor and all articles published in the online sections of the journal are open access upon publication. Articles in the other sections of the journal are made available 12 months after publication, through the publisher's site and via the Astrophysics Data System. Authors have the option to pay for immediate open access; the Astrophysical Journal The Astronomical Journal Monthly Notices of the Royal Astronomical Society History and purpose of Astronomy & Astrophysics journal. S. R. Pottasch. EDP Sciences. 2012
Messier 2 or M2 is a globular cluster in the constellation Aquarius, five degrees north of the star Beta Aquarii. It was discovered by Jean-Dominique Maraldi in 1746, is one of the largest known globular clusters. M2 was discovered by the French astronomer Jean-Dominique Maraldi in 1746 while observing a comet with Jacques Cassini. Charles Messier rediscovered it in 1760, but thought it a nebula without any stars associated with it. William Herschel, in 1783, was the first to resolve individual stars in the cluster. M2 is, under good conditions, just visible to the naked eye. Binoculars or a small telescope will identify this cluster as non-stellar, while larger telescopes will resolve individual stars, of which the brightest are of apparent magnitude 13.1. M2 is about 55,000 light-years distant from Earth. At 175 light-years in diameter, it is one of the larger globular clusters known; the cluster is rich and elliptical. It is 13 billion years old and one of the older globulars associated with the Milky Way galaxy.
M2 contains about 150,000 stars, including 21 known variable stars. Its brightest stars are yellow giant stars; the overall spectral type is F4. M2 is part of the Gaia Sausage, the hypothesised remains of a merged dwarf galaxy. M2,SEDS Messier pages M2, Galactic Globular Clusters Database page Historic observations of M2 Messier 2 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Sky Map and images