1.
Constellation
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A constellation is formally defined as a region of the celestial sphere, with boundaries laid down by the International Astronomical Union. The constellation areas mostly had their origins in Western-traditional patterns of stars from which the constellations take their names, in 1922, the International Astronomical Union officially recognized the 88 modern constellations, which cover the entire sky. They began as the 48 classical Greek constellations laid down by Ptolemy in the Almagest, Constellations in the far southern sky are late 16th- and mid 18th-century constructions. 12 of the 88 constellations compose the zodiac signs, though the positions of the constellations only loosely match the dates assigned to them in astrology. The term constellation can refer to the stars within the boundaries of that constellation. Notable groupings of stars that do not form a constellation are called asterisms, when astronomers say something is “in” a given constellation they mean it is within those official boundaries. Any given point in a coordinate system can unambiguously be assigned to a single constellation. Many astronomical naming systems give the constellation in which an object is found along with a designation in order to convey a rough idea in which part of the sky it is located. For example, the Flamsteed designation for bright stars consists of a number, the word constellation seems to come from the Late Latin term cōnstellātiō, which can be translated as set of stars, and came into use in English during the 14th century. It also denotes 88 named groups of stars in the shape of stellar-patterns, the Ancient Greek word for constellation was ἄστρον. Colloquial usage does not draw a distinction between constellation in the sense of an asterism and constellation in the sense of an area surrounding an asterism. The modern system of constellations used in astronomy employs the latter concept, the term circumpolar constellation is used for any constellation that, from a particular latitude on Earth, never sets below the horizon. From the North Pole or South Pole, all constellations south or north of the equator are circumpolar constellations. In the equatorial or temperate latitudes, the term equatorial constellation has sometimes been used for constellations that lie to the opposite the circumpolar constellations. They generally include all constellations that intersect the celestial equator or part of the zodiac, usually the only thing the stars in a constellation have in common is that they appear near each other in the sky when viewed from the Earth. In galactic space, the stars of a constellation usually lie at a variety of distances, since stars also travel on their own orbits through the Milky Way, the star patterns of the constellations change slowly over time. After tens to hundreds of thousands of years, their familiar outlines will become unrecognisable, the terms chosen for the constellation themselves, together with the appearance of a constellation, may reveal where and when its constellation makers lived. The earliest direct evidence for the constellations comes from inscribed stones and it seems that the bulk of the Mesopotamian constellations were created within a relatively short interval from around 1300 to 1000 BC
2.
Galaxy morphological classification
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Galaxy morphological classification is a system used by astronomers to divide galaxies into groups based on their visual appearance. The Hubble sequence is a classification scheme for galaxies invented by Edwin Hubble in 1926. It is often known colloquially as the “Hubble tuning-fork” because of the shape in which it is traditionally represented, Hubble’s scheme divides galaxies into three broad classes based on their visual appearance, Elliptical galaxies have smooth, featureless light distributions and appear as ellipses in images. They are denoted by the letter E, followed by an integer n representing their degree of ellipticity on the sky. Spiral galaxies consist of a disk, with stars forming a spiral structure, and a central concentration of stars known as the bulge. They are given the symbol S, roughly half of all spirals are also observed to have a bar-like structure, extending from the central bulge. These barred spirals are given the symbol SB and these broad classes can be extended to enable finer distinctions of appearance and to encompass other types of galaxies, such as irregular galaxies, which have no obvious regular structure. The Hubble sequence is represented in the form of a two-pronged fork, with the ellipticals on the left. Lenticular galaxies are placed between the ellipticals and the spirals, at the point where the two meet the “handle”. To this day, the Hubble sequence is the most commonly used system for classifying galaxies, the de Vaucouleurs system for classifying galaxies is a widely used extension to the Hubble sequence, first described by Gérard de Vaucouleurs in 1959. In particular, he argued that rings and lenses are important structural components of spiral galaxies, the de Vaucouleurs system retains Hubble’s basic division of galaxies into ellipticals, lenticulars, spirals and irregulars. For example, a barred spiral galaxy with loosely wound arms. Visually, the de Vaucouleurs system can be represented as a version of Hubble’s tuning fork, with stage on the x-axis, family on the y-axis. De Vaucouleurs also assigned numerical values to each class of galaxy in his scheme, values of the numerical Hubble stage T run from −6 to +10, with negative numbers corresponding to early-type galaxies and positive numbers to late types. Elliptical galaxies are divided into three stages, compact ellipticals, normal ellipticals and late types, lenticulars are similarly subdivided into early, intermediate and late types. Irregular galaxies can be of type magellanic irregulars or compact, the use of numerical stages allows for more quantitative studies of galaxy morphology. Created by American astronomer William Wilson Morgan, together with Philip Keenan, Morgan developed the MK system for the classification of stars through their spectra. The Yerkes scheme uses the spectra of stars in the galaxy, the shape, real and apparent, thus, for example, the Andromeda Galaxy is classified as kS5. H
3.
Cosmic distance ladder
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The cosmic distance ladder is the succession of methods by which astronomers determine the distances to celestial objects. A real direct distance measurement of an object is possible only for those objects that are close enough to Earth. The techniques for determining distances to more distant objects are all based on various measured correlations between methods that work at distances and methods that work at larger distances. Several methods rely on a candle, which is an astronomical object that has a known luminosity. The ladder analogy arises because no single technique can measure distances at all ranges encountered in astronomy, instead, one method can be used to measure nearby distances, a second can be used to measure nearby to intermediate distances, and so on. Each rung of the ladder provides information that can be used to determine the distances at the next higher rung, at the base of the ladder are fundamental distance measurements, in which distances are determined directly, with no physical assumptions about the nature of the object in question. The precise measurement of stellar positions is part of the discipline of astrometry, direct distance measurements are based upon the astronomical unit, which is the distance between the Earth and the Sun. Historically, observations of transits of Venus were crucial in determining the AU, in the first half of the 20th century, observations of asteroids were also important. Keplers laws provide precise ratios of the sizes of the orbits of objects orbiting the Sun, radar is used to measure the distance between the orbits of the Earth and of a second body. From that measurement and the ratio of the two sizes, the size of Earths orbit is calculated. The Earths orbit is known with a precision of a few meters, the most important fundamental distance measurements come from trigonometric parallax. As the Earth orbits the Sun, the position of stars will appear to shift slightly against the more distant background. These shifts are angles in a triangle, with 2 AU making the base leg of the triangle. The amount of shift is small, measuring 1 arcsecond for an object at the 1 parsec distance of the nearest stars. Astronomers usually express distances in units of parsecs, light-years are used in popular media, because parallax becomes smaller for a greater stellar distance, useful distances can be measured only for stars whose parallax is larger than a few times the precision of the measurement. Parallax measurements typically have an accuracy measured in milliarcseconds, the Hubble telescope WFC3 now has the potential to provide a precision of 20 to 40 microarcseconds, enabling reliable distance measurements up to 5,000 parsecs for small numbers of stars. By the early 2020s, the GAIA space mission will provide similarly accurate distances to all bright stars. Stars have a velocity relative to the Sun that causes proper motion, for a group of stars with the same spectral class and a similar magnitude range, a mean parallax can be derived from statistical analysis of the proper motions relative to their radial velocities
4.
Spiral galaxy
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A spiral galaxy is a type of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk containing stars, gas and dust, and these are surrounded by a much fainter halo of stars, many of which reside in globular clusters. Spiral galaxies are named for the structures that extend from the center into the galactic disc. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disc because of the young, hot OB stars that inhabit them. Roughly two-thirds of all spirals are observed to have a component in the form of a bar-like structure, extending from the central bulge. Our own Milky Way has recently confirmed to be a barred spiral. The most convincing evidence for its existence comes from a recent survey, performed by the Spitzer Space Telescope, together with irregular galaxies, spiral galaxies make up approximately 60% of galaxies in the local Universe. They are mostly found in low-density regions and are rare in the centers of galaxy clusters, Spiral arms are regions of stars that extend from the center of spiral and barred spiral galaxies. These long, thin regions resemble a spiral and thus give spiral galaxies their name, naturally, different classifications of spiral galaxies have distinct arm-structures. Sc and SBc galaxies, for instance, have very loose arms, whereas Sa, either way, spiral arms contain many young, blue stars, which make the arms so bright. A bulge is a huge, tightly packed group of stars, the term commonly refers to the central group of stars found in most spiral galaxies. Using the Hubble classification, the bulge of Sa galaxies is usually composed of Population II stars, further, the bulge of Sa and SBa galaxies tends to be large. In contrast, the bulges of Sc and SBc galaxies are much smaller and are composed of young, some bulges have similar properties to those of elliptical galaxies, others simply appear as higher density centers of disks, with properties similar to disk galaxies. Many bulges are thought to host a supermassive black hole at their centers, such black holes have never been directly observed, but many indirect proofs exist. In our own galaxy, for instance, the object called Sagittarius A* is believed to be a black hole. There is a correlation between the mass of the black hole and the velocity dispersion of the stars in the bulge. However, some stars inhabit a spheroidal halo or galactic spheroid, the orbital behaviour of these stars is disputed, but they may describe retrograde and/or highly inclined orbits, or not move in regular orbits at all. The galactic halo also contains many globular clusters, due to their irregular movement around the center of the galaxy—if they do so at all—these stars often display unusually high proper motion
5.
Canes Venatici
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Canes Venatici /ˈkeɪniːz vᵻˈnætᵻsaɪ/ is one of the 88 official modern constellations. It is a northern constellation that was created by Johannes Hevelius in the 17th century. Its name is Latin for hunting dogs, and the constellation is depicted in illustrations as representing the dogs of Boötes the Herdsman. The stars of Canes Venatici are not bright, in classical times, they were listed by Ptolemy as unfigured stars below the constellation Ursa Major in his star catalogue. In medieval times, the identification of these stars with the dogs of Boötes arose through a mistranslation, some of Boötess stars were traditionally described as representing the club of Boötes. When the Arabic text was translated into Latin, the translator Gerard of Cremona mistook the Arabic word كلاب for kilāb, meaning dogs. In 1533, the German astronomer Peter Apian depicted Boötes as having two dogs with him and these spurious dogs floated about the astronomical literature until Hevelius decided to specify their presence in the sky by making them a separate constellation in 1687. Hevelius chose the name Asterion for the dog and Chara for the southern dog, as Canes Venatici. In his star catalogue, the Czech astronomer Bečvář assigned the names Asterion to β CVn, Canes Venatici is bordered by Ursa Major to the north and west, Coma Berenices to the south, and Boötes to the east. The three-letter abbreviation for the constellation, as adopted by the International Astronomical Union in 1922, is CVn, the official constellation boundaries, as set by Eugène Delporte in 1930, are defined by a polygon of 14 sides. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 12h 06. 2m and 14h 07. 3m, while the coordinates are between +27. 84° and +52. 36°. Covering 465 square degrees, it ranks 38th of the 88 constellations in size, Canes Venatici contains no bright stars, α and β CVn being only of 3rd and 4th magnitude respectively. Flamsteed catalogued 25 stars in the constellation, labelling them 1 to 25 Canum Venaticorum, α CVn is the constellations brightest star, named by Sir Charles Scarborough in memory of King Charles I, the deposed king of Britain. Legend has it that α CVn was brighter than usual during the Restoration, cor Caroli is a wide double star, with a primary of magnitude 2.9 and a secondary of magnitude 5.6, the primary is 110 light-years from Earth. The primary also has an unusually strong variable magnetic field, β CVn is a yellow-hued main sequence star of magnitude 4.2,27 light-years from Earth. Its common name comes from the word for joy, Y CVn is a semiregular variable star that varies between magnitudes 5.0 and 6.5 over a period of around 158 days. It is a star and is being deep red in color. AM CVn, a blue star of magnitude 14, is the prototype of a special class of cataclysmic variable stars, in which the companion star is a white dwarf
6.
Charles Messier
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Charles Messier was a French astronomer most notable for publishing an astronomical catalogue consisting of nebulae and star clusters that came to be known as the 110 Messier objects. The purpose of the catalogue was to help astronomical observers, in particular comet hunters such as himself, Messier was born in Badonviller in the Lorraine region of France, being the tenth of twelve children of Françoise B. Grandblaise and Nicolas Messier, a Court usher, six of his brothers and sisters died while young and in 1741, his father died. Charles interest in astronomy was stimulated by the appearance of the spectacular, great six-tailed comet in 1744, in 1751 he entered the employ of Joseph Nicolas Delisle, the astronomer of the French Navy, who instructed him to keep careful records of his observations. Messiers first documented observation was that of the Mercury transit of 6 May 1753, according to Meyer, Messier is buried in Père Lachaise Cemetery, Paris, in Section 11. Messiers occupation as a comet hunter led him to come across fixed diffuse objects in the night sky which could be mistaken for comets. He compiled a list of them, in collaboration with his friend and assistant Pierre Méchain, the entries are now known to be galaxies, planetary nebulae, other types of nebulae, and star clusters. Messier did his observing with a 100 mm refracting telescope from Hôtel de Cluny, in downtown Paris, the list he compiled contains only objects found in the area of the sky he could observe, from the north celestial pole to a declination of about −35. 7°. They are not organized scientifically by object type, or even by location, the first version of Messiers catalogue contained 45 objects and was published in 1774 in the journal of the French Academy of Sciences in Paris. In addition to his own discoveries, this version included objects previously observed by other astronomers, by 1780 the catalog had increased to 80 objects. The final version of the catalogue was published in 1781, in the 1784 issue of Connaissance des Temps, the final list of Messier objects had grown to 103. These seven objects, M104 through M110, are accepted by astronomers as official Messier objects, the crater Messier on the Moon and the asteroid 7359 Messier were named in his honor. Deep sky object List of Messier objects Messier object Messier marathon Caldwell catalogue OMeara, deep Sky Companions, The Messier Objects. Charles Messier Biography at Students for the Exploration and Development of Space, retrieved July 2007 Short biography of Charles Messier and history of the Messier Object Catalog by Jon Zander at OurDarkSkies. com. Retrieved July 2007 Life of a Comet Hunter, Messier and Astrobiology Professor Mark Brake and Martin Griffiths, Astrobiology Magazine European Edition, retrieved July 2007 Interactive Messier Catalog Greenhawk Observatory Amateur Photos of Charles Messier Objects Biography - Messier website. Messier Marathon Attempts to find as many Messier objects as possible in one night, retrieved July 2007 Clickable table of Messier objects Charles Messier explains his catalog on YouTube
7.
Barred spiral galaxy
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A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in approximately two-thirds of all spiral galaxies, bars generally affect both the motions of stars and interstellar gas within spiral galaxies and can affect spiral arms as well. The Milky Way Galaxy, where our own Solar System is located, is classified as a spiral galaxy. Edwin Hubble classified spiral galaxies of this type as SB in his Hubble sequence, sBa types feature tightly bound arms, while SBc types are at the other extreme and have loosely bound arms. SB0 is a lenticular galaxy. Among other types in Hubbles classifications for the galaxies are, spiral galaxy, elliptical galaxy, Barred galaxies are apparently predominant, with surveys showing that up to two-thirds of all spiral galaxies contain a bar. The current hypothesis is that the bar structure acts as a type of stellar nursery, the bar is thought to act as a mechanism that channels gas inwards from the spiral arms through orbital resonance, in effect funneling the flow to create new stars. This process is thought to explain why many barred spiral galaxies have active galactic nuclei. The creation of the bar is thought to be the result of a density wave radiating from the center of the galaxy whose effects reshape the orbits of the inner stars. This effect builds over time to stars orbiting further out, which creates a bar structure. Bars are thought to be temporary phenomena in the lives of spiral galaxies, past a certain size the accumulated mass of the bar compromises the stability of the overall bar structure. Barred spiral galaxies with high mass accumulated in their center tend to have short, since so many spiral galaxies have bar structures, it is likely that they are recurring phenomena in spiral galaxy development. The oscillating evolutionary cycle from spiral galaxy to barred spiral galaxy is thought to take on the average about two billion years, recent studies have confirmed the idea that bars are a sign of galaxies reaching full maturity as the formative years end. The sub-categories are based on how open or tight the arms of the spiral are, sBa types feature tightly bound arms. SBc types are at the extreme and have loosely bound arms. SBm describes somewhat irregular barred spirals, sB0 is a barred lenticular galaxy. Galaxy morphological classification Galaxy formation and evolution Lenticular galaxy Spiral galaxy Firehose instability Britt, Milky Way’s Central Structure Seen with Fresh Clarity. An article about the Spitzer Space Telescopes Milky Way discovery Devitt, galactic survey reveals a new look for the Milky Way
8.
Star formation
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Star formation is the process by which dense regions within molecular clouds in interstellar space, sometimes referred to as stellar nurseries or star-forming regions, fuse to form stars. It is closely related to formation, another branch of astronomy. Star formation theory, as well as accounting for the formation of a star, must also account for the statistics of binary stars. In June 2015, astronomers reported evidence for Population III stars in the Cosmos Redshift 7 galaxy at z =6.60, a spiral galaxy like the Milky Way contains stars, stellar remnants, and a diffuse interstellar medium of gas and dust. The interstellar medium consists of 10−4 to 106 particles per cm3 and is composed of roughly 70% hydrogen by mass. This medium has been enriched by trace amounts of heavier elements that were ejected from stars as they passed beyond the end of their main sequence lifetime. Higher density regions of the interstellar medium form clouds, or diffuse nebulae, in the dense nebulae where stars are produced, much of the hydrogen is in the molecular form, so these nebulae are called molecular clouds. These giant molecular clouds have densities of 100 particles per cm3, diameters of 100 light-years, masses of up to 6 million solar masses. About half the mass of the galactic ISM is found in molecular clouds and in the Milky Way there are an estimated 6,000 molecular clouds. The nearest nebula to the Sun where massive stars are being formed is the Orion nebula,1,300 ly away, however, lower mass star formation is occurring about 400–450 light years distant in the ρ Ophiuchi cloud complex. A more compact site of formation is the opaque clouds of dense gas and dust known as Bok globules. These can form in association with collapsing molecular clouds or possibly independently, the Bok globules are typically up to a light year across and contain a few solar masses. They can be observed as dark clouds silhouetted against bright emission nebulae or background stars, over half the known Bok globules have been found to contain newly forming stars. An interstellar cloud of gas will remain in equilibrium as long as the kinetic energy of the gas pressure is in balance with the potential energy of the internal gravitational force. Mathematically this is expressed using the theorem, which states that, to maintain equilibrium. If a cloud is massive enough that the gas pressure is insufficient to support it, the mass above which a cloud will undergo such collapse is called the Jeans mass. The Jeans mass depends on the temperature and density of the cloud and this coincides with the typical mass of an open cluster of stars, which is the end product of a collapsing cloud. In triggered star formation, one of several events occur to compress a molecular cloud
9.
Bulge (astronomy)
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In astronomy, a bulge is a tightly packed group of stars within a larger formation. The term almost exclusively refers to the group of stars found in most spiral galaxies. It is now thought there are at least two types of bulges, bulges that are like ellipticals and bulges that are like spiral galaxies. Bulges that have similar to those of elliptical galaxies are often called classical bulges due to their similarity to the historic view of bulges. These bulges are composed primarily of stars that are older, Population II stars and these stars are also in orbits that are essentially random compared to the plane of the galaxy, giving the bulge a distinct spherical form. Due to the lack of dust and gasses, bulges tend to have almost no star formation, the distribution of light is described by de Vaucouleurs law. Classical bulges are thought to be the result of collisions of smaller structures and these disrupt the paths of stars, resulting in the randomness of bulge orbits. Following such a merger, gas clouds are more likely to be converted into stars, many bulges have properties more similar to those of the central regions of spiral galaxies than elliptical galaxies. They are often referred to as pseudobulges or disky-bulges and these bulges have stars that are not orbiting randomly, but rather orbit in an ordered fashion in the same plane as the stars in the outer disk. This contrasts greatly with elliptical galaxies, subsequent studies show that the bulges of many galaxies are not devoid of dust, but rather show a varied and complex structure. This structure often looks similar to a galaxy, but is much smaller. Giant spiral galaxies are typically 2–100 times the size of those spirals that exist in bulges, where they exist, these central spirals dominate the light of the bulge in which they reside. Typically the rate at which new stars are formed in pseudobulges is similar to the rate at which stars form in disk galaxies, sometimes bulges contain nuclear rings that are forming stars at much higher rate than is typically found in outer disks, as shown in NGC4314. Properties such as structure and young stars suggest that some bulges did not form through the same process that made elliptical galaxies. Yet the theories for the formation of pseudobulges are less certain than those for classical bulges, pseudobulges may be the result of extremely gas-rich mergers that happened more recently than those mergers that formed classical bulges. However, it is difficult for disks to survive the merging process, many astronomers suggest that bulges that appear similar to disks form outside of the disk, and are not the product of a merging process. When left alone, disk galaxies can rearrange their stars and gas, the products of this process are often observed in such galaxies, both spiral disks and galactic bars can result from secular evolution of galaxy disks. Secular evolution is expected to send gas and stars to the center of a galaxy
10.
Hubble Space Telescope
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The Hubble Space Telescope is a space telescope that was 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, with a 2. 4-meter mirror, Hubbles four main instruments observe in the near ultraviolet, visible, and near infrared spectra. Hubbles orbit outside the distortion of Earths atmosphere allows it to take extremely high-resolution images, Hubble has recorded some of the most detailed visible light images ever, 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, the Space Telescope Science Institute selects Hubbles 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, and the Challenger disaster. When finally launched in 1990, Hubbles main mirror was found to have been ground incorrectly, the optics were corrected to their intended quality by a servicing mission in 1993. Hubble is the telescope designed to be serviced in space by astronauts. After launch by Space Shuttle Discovery in 1990, five subsequent Space Shuttle missions repaired, upgraded, 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, the telescope is operating as of 2017, and could last until 2030–2040. Its scientific successor, the James Webb Space Telescope, is scheduled for launch in 2018, the history of the Hubble Space Telescope can be traced back as far as 1946, to the astronomer Lyman Spitzers paper Astronomical advantages of an extraterrestrial observatory. In it, he discussed the two advantages that a space-based observatory would have over ground-based telescopes. First, the resolution would be limited only by diffraction, rather than by the turbulence in the atmosphere. Second, a telescope could observe infrared and ultraviolet light. Spitzer devoted much of his career to pushing for the development of a space telescope, space-based astronomy had begun on a very small scale following World War II, as scientists made use of developments that had taken place in rocket technology. An orbiting solar telescope was launched in 1962 by the United Kingdom as part of the Ariel space program, oAO-1s battery failed after three days, terminating the mission. It was followed by OAO-2, which carried out observations of stars and galaxies from its launch in 1968 until 1972. The continuing success of the OAO program encouraged increasingly strong consensus within the community that the LST should be a major goal
11.
Milky Way
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The Milky Way is the galaxy that contains our Solar System. The descriptive milky is derived from the appearance from Earth of the galaxy – a band of light seen in the night sky formed from stars that cannot be distinguished by the naked eye. The term Milky Way is a translation of the Latin via lactea, 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. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies, the Milky Way is a barred spiral galaxy with a diameter between 100,000 light-years and 180,000 light-years. The Milky Way is estimated to contain 100–400 billion stars, there are probably at least 100 billion planets in the Milky Way. The Solar System is located within the disk, about 26,000 light-years from the Galactic Center, on the edge of one of the spiral-shaped concentrations of gas. The stars in the inner ≈10,000 light-years form a bulge, the very center is marked by an intense radio source, named Sagittarius A*, which is likely to be a supermassive black hole. Stars and gases at a range of distances from the Galactic Center orbit at approximately 220 kilometers per second. The constant rotation speed contradicts the laws of Keplerian dynamics and suggests much of the mass of the Milky Way does not emit or absorb electromagnetic radiation. This mass has been termed dark matter, the rotational period is about 240 million years at the position of the Sun. The Milky Way as a whole is moving at a velocity of approximately 600 km per second with respect to frames of reference. The oldest stars in the Milky Way are nearly as old as the Universe itself, the Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which is a component of the Virgo Supercluster, which is itself a component of the Laniakea Supercluster. The Milky Way can be seen as a band of white light some 30 degrees wide arcing across the sky. Dark regions within the band, such as the Great Rift, the area of the sky obscured by the Milky Way is called the Zone of Avoidance. The Milky Way has a low surface brightness. Its visibility can be reduced by background light such as light pollution or stray light from the Moon. The sky needs to be darker than about 20.2 magnitude per square arcsecond in order for the Milky Way to be seen and it should be visible when the limiting magnitude is approximately +5.1 or better and shows a great deal of detail at +6.1
. Bibcode:2001ApJ...546..681T. doi:10.1086/318301.
