A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas and dark matter. The word galaxy is derived from the Greek galaxias "milky", a reference to the Milky Way. Galaxies range in size from dwarfs with just a few hundred million stars to giants with one hundred trillion stars, each orbiting its galaxy's center of mass. Galaxies are categorized according to their visual morphology as spiral, or irregular. Many galaxies are thought to have supermassive black holes at their centers; the Milky Way's central black hole, known as Sagittarius A*, has a mass four million times greater than the Sun. As of March 2016, GN-z11 is the oldest and most distant observed galaxy with a comoving distance of 32 billion light-years from Earth, observed as it existed just 400 million years after the Big Bang. Research released in 2016 revised the number of galaxies in the observable universe from a previous estimate of 200 billion to a suggested 2 trillion or more, containing more stars than all the grains of sand on planet Earth.
Most of the galaxies are 1,000 to 100,000 parsecs in diameter and separated by distances on the order of millions of parsecs. For comparison, the Milky Way has a diameter of at least 30,000 parsecs and is separated from the Andromeda Galaxy, its nearest large neighbor, by 780,000 parsecs; the space between galaxies is filled with a tenuous gas having an average density of less than one atom per cubic meter. The majority of galaxies are gravitationally organized into groups and superclusters; the Milky Way is part of the Local Group, dominated by it and the Andromeda Galaxy and is part of the Virgo Supercluster. At the largest scale, these associations are arranged into sheets and filaments surrounded by immense voids; the largest structure of galaxies yet recognised is a cluster of superclusters, named Laniakea, which contains the Virgo supercluster. The origin of the word galaxy derives from the Greek term for the Milky Way, galaxias, or kyklos galaktikos due to its appearance as a "milky" band of light in the sky.
In Greek mythology, Zeus places his son born by a mortal woman, the infant Heracles, on Hera's breast while she is asleep so that the baby will drink her divine milk and will thus become immortal. Hera wakes up while breastfeeding and realizes she is nursing an unknown baby: she pushes the baby away, some of her milk spills, it produces the faint band of light known as the Milky Way. In the astronomical literature, the capitalized word "Galaxy" is used to refer to our galaxy, the Milky Way, to distinguish it from the other galaxies in our universe; the English term Milky Way can be traced back to a story by Chaucer c. 1380: "See yonder, lo, the Galaxyë Which men clepeth the Milky Wey, For hit is whyt." Galaxies were discovered telescopically and were known as spiral nebulae. Most 18th to 19th Century astronomers considered them as either unresolved star clusters or anagalactic nebulae, were just thought as a part of the Milky Way, but their true composition and natures remained a mystery. Observations using larger telescopes of a few nearby bright galaxies, like the Andromeda Galaxy, began resolving them into huge conglomerations of stars, but based on the apparent faintness and sheer population of stars, the true distances of these objects placed them well beyond the Milky Way.
For this reason they were popularly called island universes, but this term fell into disuse, as the word universe implied the entirety of existence. Instead, they became known as galaxies. Tens of thousands of galaxies have been catalogued, but only a few have well-established names, such as the Andromeda Galaxy, the Magellanic Clouds, the Whirlpool Galaxy, the Sombrero Galaxy. Astronomers work with numbers from certain catalogues, such as the Messier catalogue, the NGC, the IC, the CGCG, the MCG and UGC. All of the well-known galaxies appear in one or more of these catalogues but each time under a different number. For example, Messier 109 is a spiral galaxy having the number 109 in the catalogue of Messier, having the designations NGC 3992, UGC 6937, CGCG 269-023, MCG +09-20-044, PGC 37617; the realization that we live in a galaxy, one among many galaxies, parallels major discoveries that were made about the Milky Way and other nebulae. The Greek philosopher Democritus proposed that the bright band on the night sky known as the Milky Way might consist of distant stars.
Aristotle, believed the Milky Way to be caused by "the ignition of the fiery exhalation of some stars that were large and close together" and that the "ignition takes place in the upper part of the atmosphere, in the region of the World, continuous with the heavenly motions." The Neoplatonist philosopher Olympiodorus the Younger was critical of this view, arguing that if the Milky Way is sublunary it should appear different at different times and places on Earth, that it should have parallax, which it does not. In his view, the Milky Way is celestial. According to Mohani Mohamed, the Arabian astronomer Alhazen made the first attempt at observing and measuring the Milky Way's parallax, he thus "determined that because the Milky Way had no parallax, it must be remote from the Earth, not belonging to the atmosphere." The Persian astronomer al-Bīrūnī
NGC 7002 is a large elliptical galaxy around 320 million Light-years away from Earth in the constellation of Indus. The galaxy was discovered by English astronomer John Herschel on September 30, 1834. NGC 7002 is part of a group of galaxies that contains the nearby galaxy NGC 7004. IC 1101 Massive elliptical galaxy, one of the largest known galaxies M87 Famous large elliptical galaxy about 50 mly in the constellation Virgo. List of NGC objects Elliptical galaxy NGC 7002 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Sky Map and images
Aquarius is a constellation of the zodiac, situated between Capricornus and Pisces. Its name is Latin for "water-carrier" or "cup-carrier", its symbol is, a representation of water. Aquarius is one of the oldest of the recognized constellations along the zodiac, it was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, it remains one of the 88 modern constellations. It is found in a region called the Sea due to its profusion of constellations with watery associations such as Cetus the whale, Pisces the fish, Eridanus the river. At apparent magnitude 2.9, Beta Aquarii is the brightest star in the constellation. Aquarius is identified as GU. LA "The Great One" in the Babylonian star catalogues and represents the god Ea himself, depicted holding an overflowing vase; the Babylonian star-figure appears on entitlement stones and cylinder seals from the second millennium. It contained the winter solstice in the Early Bronze Age. In Old Babylonian astronomy, Ea was the ruler of the southernmost quarter of the Sun's path, the "Way of Ea", corresponding to the period of 45 days on either side of winter solstice.
Aquarius was associated with the destructive floods that the Babylonians experienced, thus was negatively connoted. In Ancient Egypt astronomy, Aquarius was associated with the annual flood of the Nile. In the Greek tradition, the constellation came to be represented as a single vase from which a stream poured down to Piscis Austrinus; the name in the Hindu zodiac is kumbha "water-pitcher". In Greek mythology, Aquarius is sometimes associated with Deucalion, the son of Prometheus who built a ship with his wife Pyrrha to survive an imminent flood, they sailed for nine days before washing ashore on Mount Parnassus. Aquarius is sometimes identified with beautiful Ganymede, a youth in Greek mythology and the son of Trojan king Tros, taken to Mount Olympus by Zeus to act as cup-carrier to the gods. Neighboring Aquila represents the eagle, under Zeus' command. An alternative version of the tale recounts Ganymede's kidnapping by the goddess of the dawn, motivated by her affection for young men, yet another figure associated with the water bearer is Cecrops I, a king of Athens who sacrificed water instead of wine to the gods.
In the first century, Ptolemy's Almagest established the common Western depiction of Aquarius. His water jar, an asterism itself, consists of Gamma, Pi, Zeta Aquarii; the water bearer's head is represented by 5th magnitude 25 Aquarii while his left shoulder is Beta Aquarii. In Chinese astronomy, the stream of water flowing from the Water Jar was depicted as the "Army of Yu-Lin"; the name "Yu-lin" means "feathers and forests", referring to the numerous light-footed soldiers from the northern reaches of the empire represented by these faint stars. The constellation's stars were the most numerous of any Chinese constellation, numbering 45, the majority of which were located in modern Aquarius; the celestial army was protected by the wall Leibizhen, which counted Iota, Lambda and Sigma Aquarii among its 12 stars. 88, 89, 98 Aquarii represent Fou-youe, the axes used as weapons and for hostage executions. In Aquarius is Loui-pi-tchin, the ramparts that stretch from 29 and 27 Piscium and 33 and 30 Aquarii through Phi, Lambda and Iota Aquarii to Delta, Gamma and Epsilon Capricorni.
Near the border with Cetus, the axe Fuyue was represented by three stars. Tienliecheng has a disputed position; the Water Jar asterism was seen to the ancient Chinese as Fenmu. Nearby, the emperors' mausoleum Xiuliang stood, demarcated by Kappa Aquarii and three other collinear stars. Ku and Qi, each composed of two stars, were located in the same region. Three of the Chinese lunar mansions shared their name with constellations. Nu the name for the 10th lunar mansion, was a handmaiden represented by Epsilon, Mu, 3, 4 Aquarii; the 11th lunar mansion shared its name with the constellation Xu, formed by Beta Aquarii and Alpha Equulei. Wei, the rooftop and 12th lunar mansion, was a V-shaped constellation formed by Alpha Aquarii, Theta Pegasi, Epsilon Pegasi. Despite both its prominent position on the zodiac and its large size, Aquarius has no bright stars, its four brightest stars being less than magnitude 2. However, recent research has shown that there are several stars lying within its borders that possess planetary systems.
The two brightest stars and Beta Aquarii, are luminous yellow supergiants, of spectral types G0Ib and G2Ib that were once hot blue-white B-class main sequence stars 5 to 9 times as massive as the Sun. The two are moving through space perpendicular to the plane of the Milky Way. Just shading Alpha, Beta Aquarii is the brightest star in Aquarius with an apparent magnitude of 2.91. It has the proper name of Sadalsuud. Having cooled and swollen to around 50 times the Sun
NGC 7006 is a globular cluster in the constellation Delphinus. NGC 7006 resides in the outskirts of the Milky Way, it is about 135,000 light-years away, five times the distance between the Sun and the centre of the galaxy, it is part of the galactic halo. This spherical region of the Milky Way is made up of dark matter and sparsely distributed stellar clusters. NGC 7006 appears in the science fiction novel Beyond the Farthest Star by Edgar Rice Burroughs, where it is used as a point of reference by the inhabitants of the planet Poloda to determine the approximate location of Earth. NGC 7006 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Sky Map and images
NGC 7004 is a spiral galaxy around 330 million light-years away from Earth in the constellation Indus. NGC 7004 has an estimated diameter of 166,980 light-years. NGC 7004 was discovered by astronomer John Herschel on October 2, 1834. NGC 7004 is part of a group of galaxies that contains the nearby galaxy NGC 7002. NGC 6861 NGC 2787 List of NGC objects NGC 7004 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Sky Map and images
Barred spiral galaxy
A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in two thirds of all spiral galaxies. Bars 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 barred spiral galaxy. Edwin Hubble classified spiral galaxies of this type as "SB" in his Hubble sequence and arranged them into sub-categories based on how open the arms of the spiral are. SBa types feature bound arms, while SBc types are at the other extreme and have loosely bound arms. SBb-type galaxies lie in between the two. SB0 is a barred lenticular galaxy. A new type, SBm, was subsequently created to describe somewhat irregular barred spirals, such as the Magellanic Clouds, which were once classified as irregular galaxies, but have since been found to contain barred spiral structures. Among other types in Hubble's classifications for the galaxies are the spiral galaxy, elliptical galaxy and irregular galaxy.
Barred galaxies are 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, fueling star birth at their centers; 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, such as that seen in the Southern Pinwheel Galaxy; 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 self-perpetuating 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 stubby bars.
Since so many spiral galaxies have bar structures, it is 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. A 2008 investigation found that only 20 percent of the spiral galaxies in the distant past possessed bars, compared with about 65 percent of their local counterparts; the general classification is "SB". The sub-categories are based on how tight the arms of the spiral are. SBa types feature bound arms. SBc types are at the other extreme. SBb galaxies lie in between. SBm describes somewhat irregular barred spirals. SB0 is a barred lenticular galaxy. Galaxy morphological classification Galaxy formation and evolution Lenticular galaxy Firehose instability Britt, Robert Roy. "Milky Way’s Central Structure Seen with Fresh Clarity."
SPACE.com 16 August 2005. An article about the Spitzer Space Telescope's Milky Way discovery Devitt, Terry. "Galactic survey reveals a new look for the Milky Way." 16 August 2005. The original press release regarding the article above, from the Univ. of Wisconsin'Barred' Spiral Galaxy Pic Highlights Stellar Birth." SPACE.com 2 March 2001. Hastings and Jane Hastings. Classifying Galaxies: Barred Spirals, 1995. "Astronomers Find Multiple Generations of Star Formation in Central Starburst Ring of a Barred Spiral Galaxy." January 15, 2000. A press release concerning NGC 1326 Barred spirals come and go Sky & Telescope April 2002. "ESO Provides An Infrared Portrait of the Barred Spiral Galaxy Messier 83." November 29, 2001. A press release from the European Southern Observatory. Horton, Adam. "Spitzer NGC 1291 barred spiral galaxy seen in infrared." 22 October 2014
The apparent magnitude of an astronomical object is a number, a measure of its brightness as seen by an observer on Earth. The magnitude scale is logarithmic. A difference of 1 in magnitude corresponds to a change in brightness by a factor of 5√100, or about 2.512. The brighter an object appears, the lower its magnitude value, with the brightest astronomical objects having negative apparent magnitudes: for example Sirius at −1.46. The measurement of apparent magnitudes or brightnesses of celestial objects is known as photometry. Apparent magnitudes are used to quantify the brightness of sources at ultraviolet and infrared wavelengths. An apparent magnitude is measured in a specific passband corresponding to some photometric system such as the UBV system. In standard astronomical notation, an apparent magnitude in the V filter band would be denoted either as mV or simply as V, as in "mV = 15" or "V = 15" to describe a 15th-magnitude object; the scale used to indicate magnitude originates in the Hellenistic practice of dividing stars visible to the naked eye into six magnitudes.
The brightest stars in the night sky were said to be of first magnitude, whereas the faintest were of sixth magnitude, the limit of human visual perception. Each grade of magnitude was considered twice the brightness of the following grade, although that ratio was subjective as no photodetectors existed; this rather crude scale for the brightness of stars was popularized by Ptolemy in his Almagest and is believed to have originated with Hipparchus. In 1856, Norman Robert Pogson formalized the system by defining a first magnitude star as a star, 100 times as bright as a sixth-magnitude star, thereby establishing the logarithmic scale still in use today; this implies that a star of magnitude m is about 2.512 times as bright as a star of magnitude m + 1. This figure, the fifth root of 100, became known as Pogson's Ratio; the zero point of Pogson's scale was defined by assigning Polaris a magnitude of 2. Astronomers discovered that Polaris is variable, so they switched to Vega as the standard reference star, assigning the brightness of Vega as the definition of zero magnitude at any specified wavelength.
Apart from small corrections, the brightness of Vega still serves as the definition of zero magnitude for visible and near infrared wavelengths, where its spectral energy distribution approximates that of a black body for a temperature of 11000 K. However, with the advent of infrared astronomy it was revealed that Vega's radiation includes an Infrared excess due to a circumstellar disk consisting of dust at warm temperatures. At shorter wavelengths, there is negligible emission from dust at these temperatures. However, in order to properly extend the magnitude scale further into the infrared, this peculiarity of Vega should not affect the definition of the magnitude scale. Therefore, the magnitude scale was extrapolated to all wavelengths on the basis of the black-body radiation curve for an ideal stellar surface at 11000 K uncontaminated by circumstellar radiation. On this basis the spectral irradiance for the zero magnitude point, as a function of wavelength, can be computed. Small deviations are specified between systems using measurement apparatuses developed independently so that data obtained by different astronomers can be properly compared, but of greater practical importance is the definition of magnitude not at a single wavelength but applying to the response of standard spectral filters used in photometry over various wavelength bands.
With the modern magnitude systems, brightness over a wide range is specified according to the logarithmic definition detailed below, using this zero reference. In practice such apparent magnitudes do not exceed 30; the brightness of Vega is exceeded by four stars in the night sky at visible wavelengths as well as the bright planets Venus and Jupiter, these must be described by negative magnitudes. For example, the brightest star of the celestial sphere, has an apparent magnitude of −1.4 in the visible. Negative magnitudes for other bright astronomical objects can be found in the table below. Astronomers have developed other photometric zeropoint systems as alternatives to the Vega system; the most used is the AB magnitude system, in which photometric zeropoints are based on a hypothetical reference spectrum having constant flux per unit frequency interval, rather than using a stellar spectrum or blackbody curve as the reference. The AB magnitude zeropoint is defined such that an object's AB and Vega-based magnitudes will be equal in the V filter band.
As the amount of light received by a telescope is reduced by transmission through the Earth's atmosphere, any measurement of apparent magnitude is corrected for what it would have been as seen from above the atmosphere. The dimmer an object appears, the higher the numerical value given to its apparent magnitude, with a difference of 5 magnitudes corresponding to a brightness factor of 100. Therefore, the apparent magnitude m, in the spectral band x, would be given by m x = − 5 log 100 , more expressed in terms of common logarithms as m x