Messier 108 is a barred spiral galaxy in the constellation Ursa Major. It was discovered by Pierre Méchain in 1781 or 1782. From the perspective of the Earth, this galaxy is seen edge-on; this galaxy is an isolated member of the Ursa Major Cluster of galaxies in the Virgo supercluster. It has a morphological classification of type SBbc in the de Vaucouleurs system, which means it is a barred spiral galaxy with somewhat loosely wound arms; the maximum angular size of the galaxy in the optical band is 11′.1 × 4′.6, it is inclined 75° to the line of sight. This galaxy has an estimated mass of 125 billion times the mass of the Sun and includes about 290 ± 80 globular clusters. Examination of the distribution of neutral hydrogen in this galaxy shows shells of expanding gas extending for several kiloparsecs, known as H1 supershells; these may be driven by bursts of star formation activity. Alternatively they may result from an infall of gas by radio jets. Observations with the Chandra X-ray Observatory have identified 83 X-ray sources, including a source located at the nucleus.
The brightest of these sources may be an intermediate-mass black hole, accreting matter. The galaxy is emitting a diffuse soft X-ray radiation within 10 kpc of the optical galaxy; the spectrum of the X-ray source at the core is consistent with an active galactic nucleus, but an examination with the Spitzer Space Telescope shows no indication of activity. The supermassive black hole at the core has an estimated mass equal to 24 million times the mass of the Sun. NGC 2403 - a similar spiral galaxy NGC 4631 - a similar spiral galaxy NGC 7793 - a similar spiral galaxy SEDS: Spiral Galaxy M108 Messier 108 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Sky Map and images
ArXiv is a repository of electronic preprints approved for posting after moderation, but not full peer review. It consists of scientific papers in the fields of mathematics, astronomy, electrical engineering, computer science, quantitative biology, mathematical finance and economics, which can be accessed online. In many fields of mathematics and physics all scientific papers are self-archived on the arXiv repository. Begun on August 14, 1991, arXiv.org passed the half-million-article milestone on October 3, 2008, had hit a million by the end of 2014. By October 2016 the submission rate had grown to more than 10,000 per month. ArXiv was made possible by the compact TeX file format, which allowed scientific papers to be transmitted over the Internet and rendered client-side. Around 1990, Joanne Cohn began emailing physics preprints to colleagues as TeX files, but the number of papers being sent soon filled mailboxes to capacity. Paul Ginsparg recognized the need for central storage, in August 1991 he created a central repository mailbox stored at the Los Alamos National Laboratory which could be accessed from any computer.
Additional modes of access were soon added: FTP in 1991, Gopher in 1992, the World Wide Web in 1993. The term e-print was adopted to describe the articles, it began as a physics archive, called the LANL preprint archive, but soon expanded to include astronomy, computer science, quantitative biology and, most statistics. Its original domain name was xxx.lanl.gov. Due to LANL's lack of interest in the expanding technology, in 2001 Ginsparg changed institutions to Cornell University and changed the name of the repository to arXiv.org. It is now hosted principally with eight mirrors around the world, its existence was one of the precipitating factors that led to the current movement in scientific publishing known as open access. Mathematicians and scientists upload their papers to arXiv.org for worldwide access and sometimes for reviews before they are published in peer-reviewed journals. Ginsparg was awarded a MacArthur Fellowship in 2002 for his establishment of arXiv; the annual budget for arXiv is $826,000 for 2013 to 2017, funded jointly by Cornell University Library, the Simons Foundation and annual fee income from member institutions.
This model arose in 2010, when Cornell sought to broaden the financial funding of the project by asking institutions to make annual voluntary contributions based on the amount of download usage by each institution. Each member institution pledges a five-year funding commitment to support arXiv. Based on institutional usage ranking, the annual fees are set in four tiers from $1,000 to $4,400. Cornell's goal is to raise at least $504,000 per year through membership fees generated by 220 institutions. In September 2011, Cornell University Library took overall administrative and financial responsibility for arXiv's operation and development. Ginsparg was quoted in the Chronicle of Higher Education as saying it "was supposed to be a three-hour tour, not a life sentence". However, Ginsparg remains on the arXiv Scientific Advisory Board and on the arXiv Physics Advisory Committee. Although arXiv is not peer reviewed, a collection of moderators for each area review the submissions; the lists of moderators for many sections of arXiv are publicly available, but moderators for most of the physics sections remain unlisted.
Additionally, an "endorsement" system was introduced in 2004 as part of an effort to ensure content is relevant and of interest to current research in the specified disciplines. Under the system, for categories that use it, an author must be endorsed by an established arXiv author before being allowed to submit papers to those categories. Endorsers are not asked to review the paper for errors, but to check whether the paper is appropriate for the intended subject area. New authors from recognized academic institutions receive automatic endorsement, which in practice means that they do not need to deal with the endorsement system at all. However, the endorsement system has attracted criticism for restricting scientific inquiry. A majority of the e-prints are submitted to journals for publication, but some work, including some influential papers, remain purely as e-prints and are never published in a peer-reviewed journal. A well-known example of the latter is an outline of a proof of Thurston's geometrization conjecture, including the Poincaré conjecture as a particular case, uploaded by Grigori Perelman in November 2002.
Perelman appears content to forgo the traditional peer-reviewed journal process, stating: "If anybody is interested in my way of solving the problem, it's all there – let them go and read about it". Despite this non-traditional method of publication, other mathematicians recognized this work by offering the Fields Medal and Clay Mathematics Millennium Prizes to Perelman, both of which he refused. Papers can be submitted in any of several formats, including LaTeX, PDF printed from a word processor other than TeX or LaTeX; the submission is rejected by the arXiv software if generating the final PDF file fails, if any image file is too large, or if the total size of the submission is too large. ArXiv now allows one to store and modify an incomplete submission, only finalize the submission when ready; the time stamp on the article is set. The standard access route is through one of several mirrors. Sev
NGC 3501 is a spiral galaxy 80 million light years away. It is located in the constellation Leo; the galaxy was imaged by the Hubble Space Telescope in 2014.
NGC 3576 is a minor nebula in the Sagittarius arm of the galaxy a few thousand light-years away from the Eta Carinae nebula. This nebula received six different classification numbers. Astronomers call the entire nebula NGC 3576. A popular nickname is "The Statue of Liberty Nebula" because of the distinctive shape in the middle of the nebula; the name was first suggested in 2009 by Dr. Steve Mazlin, a member of Star Shadows Remote Observatory. Photo of the "Statue of Liberty"
A supernova is an event that occurs upon the death of certain types of stars. Supernovae are more energetic than novae. In Latin, nova means "new", referring astronomically to what appears to be a temporary new bright star. Adding the prefix "super-" distinguishes supernovae from ordinary novae, which are far less luminous; the word supernova was coined by Walter Baade and Fritz Zwicky in 1931. Only three Milky Way, naked-eye supernova events have been observed during the last thousand years, though many have been seen in other galaxies; the most recent directly observed supernova in the Milky Way was Kepler's Supernova in 1604, but two more recent supernova remnants have been found. Statistical observations of supernovae in other galaxies suggest they occur on average about three times every century in the Milky Way, that any galactic supernova would certainly be observable with modern astronomical telescopes. Supernovae may expel much, if not all, of the material away from a star at velocities up to 30,000 km/s or 10% of the speed of light.
This drives an expanding and fast-moving shock wave into the surrounding interstellar medium, in turn, sweeping up an expanding shell of gas and dust, observed as a supernova remnant. Supernovae create and eject the bulk of the chemical elements produced by nucleosynthesis. Supernovae play a significant role in enriching the interstellar medium with the heavier atomic mass chemical elements. Furthermore, the expanding shock waves from supernovae can trigger the formation of new stars. Supernova remnants are expected to accelerate a large fraction of galactic primary cosmic rays, but direct evidence for cosmic ray production was found only in a few of them so far, they are potentially strong galactic sources of gravitational waves. Theoretical studies indicate that most supernovae are triggered by one of two basic mechanisms: the sudden re-ignition of nuclear fusion in a degenerate star or the sudden gravitational collapse of a massive star's core. In the first instance, a degenerate white dwarf may accumulate sufficient material from a binary companion, either through accretion or via a merger, to raise its core temperature enough to trigger runaway nuclear fusion disrupting the star.
In the second case, the core of a massive star may undergo sudden gravitational collapse, releasing gravitational potential energy as a supernova. While some observed supernovae are more complex than these two simplified theories, the astrophysical collapse mechanics have been established and accepted by most astronomers for some time. Owing to the wide range of astrophysical consequences of these events, astronomers now deem supernova research, across the fields of stellar and galactic evolution, as an important area for investigation; the earliest recorded supernova HB9 was viewed by Indians 5,000-years ago and recorded in the oldest Star chart. The SN 185, was viewed by Chinese astronomers in 185 AD; the brightest recorded supernova was SN 1006, which occurred in 1006 AD and was described by observers across China, Iraq and Europe. The observed supernova SN 1054 produced the Crab Nebula. Supernovae SN 1572 and SN 1604, the latest to be observed with the naked eye in the Milky Way galaxy, had notable effects on the development of astronomy in Europe because they were used to argue against the Aristotelian idea that the universe beyond the Moon and planets was static and unchanging.
Johannes Kepler began observing SN 1604 at its peak on October 17, 1604, continued to make estimates of its brightness until it faded from naked eye view a year later. It was the second supernova to be observed in a generation. There is some evidence that the youngest galactic supernova, G1.9+0.3, occurred in the late 19th century more than Cassiopeia A from around 1680. Neither supernova was noted at the time. In the case of G1.9+0.3, high extinction along the plane of the galaxy could have dimmed the event sufficiently to go unnoticed. The situation for Cassiopeia A is less clear. Infrared light echos have been detected showing that it was a type IIb supernova and was not in a region of high extinction. Before the development of the telescope, only five supernovae were seen in the last millennium. Compared to a star's entire history, the visual appearance of a galactic supernova is brief spanning several months, so that the chances of observing one is once in a lifetime. Only a tiny fraction of the 100 billion stars in a typical galaxy have the capacity to become a supernova, restricted to either those having large mass or extraordinarily rare kinds of binary stars containing white dwarfs.
However and discovery of extragalactic supernovae are now far more common. The first such observation was of SN 1885A in the Andromeda galaxy. Today and professional astronomers are finding several hundred every year, some when near maximum brightness, others on old astronomical photographs or plates. American astronomers Rudolph Minkowski and Fritz Zwicky developed the modern supernova classification scheme beginning in 1941. During the 1960s, astronomers found that the maximum intensities of supernovae could be used as standard candles, hence indicators of astronomical distances; some of the most distant supernovae observed in 2003, appeared dimmer than expected. This supports the view. Techniques were developed for reconstructing supernovae events that have no written records of being observed; the date of the Cassiopeia A supernova event was determined from light echoes off nebulae, while the age of supernova remnant RX J0852.0-4622 was estimated from temperature
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
A constellation is a group of stars that forms an imaginary outline or pattern on the celestial sphere representing an animal, mythological person or creature, a god, or an inanimate object. The origins of the earliest constellations go back to prehistory. People used them to relate stories of their beliefs, creation, or mythology. Different cultures and countries adopted their own constellations, some of which lasted into the early 20th century before today's constellations were internationally recognized. Adoption of constellations has changed over time. Many have changed in shape; some became popular. Others were limited to single nations; the 48 traditional Western constellations are Greek. They are given in Aratus' work Phenomena and Ptolemy's Almagest, though their origin predates these works by several centuries. Constellations in the far southern sky were added from the 15th century until the mid-18th century when European explorers began traveling to the Southern Hemisphere. Twelve ancient constellations belong to the zodiac.
The origins of the zodiac remain uncertain. In 1928, the International Astronomical Union formally accepted 88 modern constellations, with contiguous boundaries that together cover the entire celestial sphere. Any given point in a celestial coordinate system lies in one of the modern constellations; some astronomical naming systems include the constellation where a given celestial object is found to convey its approximate location in the sky. The Flamsteed designation of a star, for example, consists of a number and the genitive form of the constellation name. Other star patterns or groups called asterisms are not constellations per se but are used by observers to navigate the night sky. Examples of bright asterisms include the Pleiades and Hyades within the constellation Taurus or Venus' Mirror in the constellation of Orion.. Some asterisms, like the False Cross, are split between two constellations; the word "constellation" comes from the Late Latin term cōnstellātiō, which can be translated as "set of stars".
The Ancient Greek word for constellation is ἄστρον. A more modern astronomical sense of the term "constellation" is as a recognisable pattern of stars whose appearance is associated with mythological characters or creatures, or earthbound animals, or objects, it can specifically denote the recognized 88 named constellations used today. Colloquial usage does not draw a sharp distinction between "constellations" and smaller "asterisms", yet the modern accepted astronomical constellations employ such a distinction. E.g. the Pleiades and the Hyades are both asterisms, each lies within the boundaries of the constellation of Taurus. Another example is the northern asterism known as the Big Dipper or the Plough, composed of the seven brightest stars within the area of the IAU-defined constellation of Ursa Major; the southern False Cross asterism includes portions of the constellations Carina and Vela and the Summer Triangle.. A constellation, viewed from a particular latitude on Earth, that never sets below the horizon is termed circumpolar.
From the North Pole or South Pole, all constellations south or north of the celestial equator are circumpolar. Depending on the definition, equatorial constellations may include those that lie between declinations 45° north and 45° south, or those that pass through the declination range of the ecliptic or zodiac ranging between 23½° north, the celestial equator, 23½° south. Although stars in constellations appear near each other in the sky, they lie at a variety of distances away from the Earth. Since stars have their own independent motions, all constellations will change over time. After tens to hundreds of thousands of years, familiar outlines will become unrecognizable. Astronomers can predict the past or future constellation outlines by measuring individual stars' common proper motions or cpm by accurate astrometry and their radial velocities by astronomical spectroscopy; the earliest evidence for the humankind's identification of constellations comes from Mesopotamian inscribed stones and clay writing tablets that date back to 3000 BC.
It seems that the bulk of the Mesopotamian constellations were created within a short interval from around 1300 to 1000 BC. Mesopotamian constellations appeared in many of the classical Greek constellations; the oldest Babylonian star catalogues of stars and constellations date back to the beginning in the Middle Bronze Age, most notably the Three Stars Each texts and the MUL. APIN, an expanded and revised version based on more accurate observation from around 1000 BC. However, the numerous Sumerian names in these catalogues suggest that they built on older, but otherwise unattested, Sumerian traditions of the Early Bronze Age; the classical Zodiac is a revision of Neo-Babylonian constellations from the 6th century BC. The Greeks adopted the Babylonian constellations in the 4th century BC. Twenty Ptolemaic constellations are from the Ancient Near East. Another ten have the same stars but different names. Biblical scholar, E. W. Bullinger interpreted some of the creatures mentioned in the books of Ezekiel and Revelation as the middle signs of the four quarters of the Zodiac, with the Lion as Leo, the Bull as Taurus, the Man representing Aquarius and the Eagle standing in for Scorpio.
The biblical Book of Job also