The Message Digest 5 (MD5) hash is commonly used as for integrity verification in the forensic imaging process. The ability to force MD5 hash collisions has been a reality for more than a decade, although there is a general consensus that hash collisions are of minimal impact to the practice of computer forensics. This paper describes an experiment to determine the results of imaging two disks that are identical except for one file, the two versions of which have different content but otherwise occupy the same byte positions on the disk, are the same size, and have the same hash value.
P2001) - Disk 3.zip
The Milky Way[c] is the galaxy that includes the Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. The term Milky Way is a translation of the Latin via lactea, from the Greek γαλακτικός κύκλος (galaktikos kýklos), meaning "milky circle".[24][25] From Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610. Until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe.[26] Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Doust Curtis,[27] observations by Edwin Hubble showed that the Milky Way is just one of many galaxies.
Proof of the Milky Way consisting of many stars came in 1610 when Galileo Galilei used a telescope to study the Milky Way and discovered that it is composed of a huge number of faint stars. Galileo also concluded that the appearance of the Milky Way was due to refraction of the Earth's atmosphere .[74][75][67] In a treatise in 1755, Immanuel Kant, drawing on earlier work by Thomas Wright,[76] speculated (correctly) that the Milky Way might be a rotating body of a huge number of stars, held together by gravitational forces akin to the Solar System but on much larger scales.[77] The resulting disk of stars would be seen as a band on the sky from our perspective inside the disk. Wright and Kant also conjectured that some of the nebulae visible in the night sky might be separate "galaxies" themselves, similar to our own. Kant referred to both the Milky Way and the "extragalactic nebulae" as "island universes", a term still current up to the 1930s.[78][79][80]
The Milky Way is one of the two largest galaxies in the Local Group (the other being the Andromeda Galaxy), although the size for its galactic disc and how much it defines the isophotal diameter is not well understood.[113] It is estimated that the significant bulk of stars in the galaxy lies within the 26 kiloparsecs (80,000 light-years) diameter, and that the number of stars beyond the outermost disc dramatically reduces to a very low number, with respect to an extrapolation of the exponential disk with the scale length of the inner disc.[114][113]
A 2020 study predicted the edge of the Milky Way's dark matter halo being around 292 61 kpc (952,000 199,000 ly), which translates to a diameter of 584 122 kpc (1.905 0.3979 Mly).[28][29] The Milky Way's stellar disk is also estimated to be approximately up to 1.35 kpc (4,000 ly) thick.[125][126]
The Milky Way contains between 100 and 400 billion stars[10][11] and at least that many planets.[144] An exact figure would depend on counting the number of very-low-mass stars, which are difficult to detect, especially at distances of more than 300 ly (90 pc) from the Sun. As a comparison, the neighboring Andromeda Galaxy contains an estimated one trillion (1012) stars.[145] The Milky Way may contain ten billion white dwarfs, a billion neutron stars, and a hundred million stellar black holes.[f][148][149] Filling the space between the stars is a disk of gas and dust called the interstellar medium. This disk has at least a comparable extent in radius to the stars,[150] whereas the thickness of the gas layer ranges from hundreds of light-years for the colder gas to thousands of light-years for warmer gas.[151][152]
The Milky Way consists of a bar-shaped core region surrounded by a warped disk of gas, dust and stars.[168][169] The mass distribution within the Milky Way closely resembles the type Sbc in the Hubble classification, which represents spiral galaxies with relatively loosely wound arms.[5] Astronomers first began to conjecture that the Milky Way is a barred spiral galaxy, rather than an ordinary spiral galaxy, in the 1960s.[170][171][172] These conjectures were confirmed by the Spitzer Space Telescope observations in 2005 that showed the Milky Way's central bar to be larger than previously thought.[173]
Outside the gravitational influence of the Galactic bar, the structure of the interstellar medium and stars in the disk of the Milky Way is organized into four spiral arms.[198] Spiral arms typically contain a higher density of interstellar gas and dust than the Galactic average as well as a greater concentration of star formation, as traced by H II regions[199][200] and molecular clouds.[201]
Outside of the major spiral arms is the Monoceros Ring (or Outer Ring), a ring of gas and stars torn from other galaxies billions of years ago. However, several members of the scientific community recently restated their position affirming the Monoceros structure is nothing more than an over-density produced by the flared and warped thick disk of the Milky Way.[218] The structure of the Milky Way's disk is warped along an "S" curve.[219]
The Galactic disk is surrounded by a spheroidal halo of old stars and globular clusters, of which 90% lie within 100,000 light-years (30 kpc) of the Galactic Center.[220] However, a few globular clusters have been found farther, such as PAL 4 and AM 1 at more than 200,000 light-years from the Galactic Center. About 40% of the Milky Way's clusters are on retrograde orbits, which means they move in the opposite direction from the Milky Way rotation.[221] The globular clusters can follow rosette orbits about the Milky Way, in contrast to the elliptical orbit of a planet around a star.[222]
Although the disk contains dust that obscures the view in some wavelengths, the halo component does not. Active star formation takes place in the disk (especially in the spiral arms, which represent areas of high density), but does not take place in the halo, as there is little cool gas to collapse into stars.[105] Open clusters are also located primarily in the disk.[223]
The Milky Way began as one or several small overdensities in the mass distribution in the Universe shortly after the Big Bang 13.61 billion years ago.[237][238][239] Some of these overdensities were the seeds of globular clusters in which the oldest remaining stars in what is now the Milky Way formed. Nearly half the matter in the Milky Way may have come from other distant galaxies.[237] Nonetheless, these stars and clusters now comprise the stellar halo of the Milky Way. Within a few billion years of the birth of the first stars, the mass of the Milky Way was large enough so that it was spinning relatively quickly. Due to conservation of angular momentum, this led the gaseous interstellar medium to collapse from a roughly spheroidal shape to a disk. Therefore, later generations of stars formed in this spiral disk. Most younger stars, including the Sun, are observed to be in the disk.[240][241]
The age of stars in the galactic thin disk has also been estimated using nucleocosmochronology. Measurements of thin disk stars yield an estimate that the thin disk formed 8.8 1.7 billion years ago. These measurements suggest there was a hiatus of almost 5 billion years between the formation of the galactic halo and the thin disk.[260] Recent analysis of the chemical signatures of thousands of stars suggests that stellar formation might have dropped by an order of magnitude at the time of disk formation, 10 to 8 billion years ago, when interstellar gas was too hot to form new stars at the same rate as before.[261]
In 2014 researchers reported that most satellite galaxies of the Milky Way lie in a very large disk and orbit in the same direction.[271] This came as a surprise: according to standard cosmology, the satellite galaxies should form in dark matter halos, and they should be widely distributed and moving in random directions. This discrepancy is still not fully explained.[272]
In January 2006, researchers reported that the heretofore unexplained warp in the disk of the Milky Way has now been mapped and found to be a ripple or vibration set up by the Large and Small Magellanic Clouds as they orbit the Milky Way, causing vibrations when they pass through its edges. Previously, these two galaxies, at around 2% of the mass of the Milky Way, were considered too small to influence the Milky Way. However, in a computer model, the movement of these two galaxies creates a dark matter wake that amplifies their influence on the larger Milky Way.[273]
From long ago, numerous studies have been undertaken on the flow induced by a rotating object in a vessel, as has been seen in fluid machinery such as centrifugal pumps and water wheels, and where energy is transferred to and from the fluid by the rotation of an impeller; such studies were performed by Bödewadt [1] and others. If there is a large gap between both disks, the flow has a separating boundary layer, and this case was studied by Batchelor [2]. Other studies investigated the flow around the eccentric rotating porous disks [3] or the nonlinear vortex structures in obliquely rotating fluid [4].
However, in many previous studies, the rotating disk has only been used to give a circumferential velocity component, with little consideration given to the shape of the disk. However, it is known that the radial gap greatly affects the flow between the fixed and rotating disks at the bottom of the vessel [5]. The effect of this radial gap on the flow between the bottom of the vessel and the rotating disk [6] is called the end-face effect. 2ff7e9595c
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