Nuclear fusion
In nuclear chemistry, nuclear fusion is a reaction in which two or more atomic nuclei are combined to form one or more different atomic nuclei and subatomic particles. The difference in mass between the reactants and products is manifested as either the release or absorption of energy; this difference in mass arises due to the difference in atomic "binding energy" between the atomic nuclei before and after the reaction. Fusion is other high magnitude stars. A fusion process that produces a nucleus lighter than iron-56 or nickel-62 will yield a net energy release; these elements have the smallest mass per nucleon and the largest binding energy per nucleon, respectively. Fusion of light elements toward these releases energy, while a fusion producing nuclei heavier than these elements will result in energy retained by the resulting nucleons, the resulting reaction is endothermic; the opposite is true for nuclear fission. This means that the lighter elements, such as helium, are in general more fusible.
The extreme astrophysical event of a supernova can produce enough energy to fuse nuclei into elements heavier than iron. In 1920, Arthur Eddington suggested hydrogen-helium fusion could be the primary source of stellar energy. Quantum tunneling was discovered by Friedrich Hund in 1929, shortly afterwards Robert Atkinson and Fritz Houtermans used the measured masses of light elements to show that large amounts of energy could be released by fusing small nuclei. Building on the early experiments in nuclear transmutation by Ernest Rutherford, laboratory fusion of hydrogen isotopes was accomplished by Mark Oliphant in 1932. In the remainder of that decade, the theory of the main cycle of nuclear fusion in stars were worked out by Hans Bethe. Research into fusion for military purposes began in the early 1940s as part of the Manhattan Project. Fusion was accomplished in 1951 with the Greenhouse Item nuclear test. Nuclear fusion on a large scale in an explosion was first carried out on 1 November 1952, in the Ivy Mike hydrogen bomb test.
Research into developing controlled thermonuclear fusion for civil purposes began in earnest in the 1940s, it continues to this day. The release of energy with the fusion of light elements is due to the interplay of two opposing forces: the nuclear force, which combines together protons and neutrons, the Coulomb force, which causes protons to repel each other. Protons are positively charged and repel each other by the Coulomb force, but they can nonetheless stick together, demonstrating the existence of another, short-range, force referred to as nuclear attraction. Light nuclei are sufficiently small and proton-poor allowing the nuclear force to overcome repulsion; this is because the nucleus is sufficiently small that all nucleons feel the short-range attractive force at least as as they feel the infinite-range Coulomb repulsion. Building up nuclei from lighter nuclei by fusion releases the extra energy from the net attraction of particles. For larger nuclei, however, no energy is released, since the nuclear force is short-range and cannot continue to act across longer atomic length scales.
Thus, energy is not released with the fusion of such nuclei. Fusion powers stars and produces all elements in a process called nucleosynthesis; the Sun is a main-sequence star, and, as such, generates its energy by nuclear fusion of hydrogen nuclei into helium. In its core, the Sun fuses 620 million metric tons of hydrogen and makes 606 million metric tons of helium each second; the fusion of lighter elements in stars releases the mass that always accompanies it. For example, in the fusion of two hydrogen nuclei to form helium, 0.7% of the mass is carried away in the form of kinetic energy of an alpha particle or other forms of energy, such as electromagnetic radiation. It takes considerable energy to force nuclei to fuse those of the lightest element, hydrogen; when accelerated to high enough speeds, nuclei can overcome this electrostatic repulsion and brought close enough such that the attractive nuclear force is greater than the repulsive Coulomb force. The strong force grows once the nuclei are close enough, the fusing nucleons can "fall" into each other and the result is fusion and net energy produced.
The fusion of lighter nuclei, which creates a heavier nucleus and a free neutron or proton releases more energy than it takes to force the nuclei together. Energy released in most nuclear reactions is much larger than in chemical reactions, because the binding energy that holds a nucleus together is greater than the energy that holds electrons to a nucleus. For example, the ionization energy gained by adding an electron to a hydrogen nucleus is 13.6 eV—less than one-millionth of the 17.6 MeV released in the deuterium–tritium reaction shown in the adjacent diagram. The complete conversion of one gram of matter would release 9×1013 joules of energy. Fusion reactions have an energy density many times greater than nuclear fission. Only direct conversion of mass into energy, such as that caused by the annihilatory collision of matter and antimatter, is more energetic per unit of mass than nuclear fusion. Research into using fusion for the p
Luminosity
In astronomy, luminosity is the total amount of energy emitted per unit of time by a star, galaxy, or other astronomical object. As a term for energy emitted per unit time, luminosity is synonymous with power. In SI units luminosity is measured in joules per second or watts. Values for luminosity are given in the terms of the luminosity of the Sun, L⊙. Luminosity can be given in terms of the astronomical magnitude system: the absolute bolometric magnitude of an object is a logarithmic measure of its total energy emission rate, while absolute magnitude is a logarithmic measure of the luminosity within some specific wavelength range or filter band. In contrast, the term brightness in astronomy is used to refer to an object's apparent brightness: that is, how bright an object appears to an observer. Apparent brightness depends on both the luminosity of the object and the distance between the object and observer, on any absorption of light along the path from object to observer. Apparent magnitude is a logarithmic measure of apparent brightness.
The distance determined by luminosity measures can be somewhat ambiguous, is thus sometimes called the luminosity distance. In astronomy, luminosity is the amount of electromagnetic energy; when not qualified, the term "luminosity" means bolometric luminosity, measured either in the SI units, watts, or in terms of solar luminosities. A bolometer is the instrument used to measure radiant energy over a wide band by absorption and measurement of heating. A star radiates neutrinos, which carry off some energy, contributing to the star's total luminosity; the IAU has defined a nominal solar luminosity of 3.828×1026 W to promote publication of consistent and comparable values in units of the solar luminosity. While bolometers do exist, they cannot be used to measure the apparent brightness of a star because they are insufficiently sensitive across the electromagnetic spectrum and because most wavelengths do not reach the surface of the Earth. In practice bolometric magnitudes are measured by taking measurements at certain wavelengths and constructing a model of the total spectrum, most to match those measurements.
In some cases, the process of estimation is extreme, with luminosities being calculated when less than 1% of the energy output is observed, for example with a hot Wolf-Rayet star observed only in the infra-red. Bolometric luminosities can be calculated using a bolometric correction to a luminosity in a particular passband; the term luminosity is used in relation to particular passbands such as a visual luminosity of K-band luminosity. These are not luminosities in the strict sense of an absolute measure of radiated power, but absolute magnitudes defined for a given filter in a photometric system. Several different photometric systems exist; some such as the UBV or Johnson system are defined against photometric standard stars, while others such as the AB system are defined in terms of a spectral flux density. A star's luminosity can be determined from two stellar characteristics: size and effective temperature; the former is represented in terms of solar radii, R⊙, while the latter is represented in kelvins, but in most cases neither can be measured directly.
To determine a star's radius, two other metrics are needed: the star's angular diameter and its distance from Earth. Both can be measured with great accuracy in certain cases, with cool supergiants having large angular diameters, some cool evolved stars having masers in their atmospheres that can be used to measure the parallax using VLBI. However, for most stars the angular diameter or parallax, or both, are far below our ability to measure with any certainty. Since the effective temperature is a number that represents the temperature of a black body that would reproduce the luminosity, it cannot be measured directly, but it can be estimated from the spectrum. An alternative way to measure stellar luminosity is to measure the star's apparent brightness and distance. A third component needed to derive the luminosity is the degree of interstellar extinction, present, a condition that arises because of gas and dust present in the interstellar medium, the Earth's atmosphere, circumstellar matter.
One of astronomy's central challenges in determining a star's luminosity is to derive accurate measurements for each of these components, without which an accurate luminosity figure remains elusive. Extinction can only be measured directly if the actual and observed luminosities are both known, but it can be estimated from the observed colour of a star, using models of the expected level of reddening from the interstellar medium. In the current system of stellar classification, stars are grouped according to temperature, with the massive young and energetic Class O stars boasting temperatures in excess of 30,000 K while the less massive older Class M stars exhibit temperatures less than 3,500 K; because luminosity is proportional to temperature to the fourth power, the large variation in stellar temperatures produces an vaster variation in stellar luminosity. Because the luminosity depends on a high power of the stellar mass, high mass luminous stars have much shorter lifetimes; the most luminous stars are always young stars, no more than a few million years for the most extreme.
In the Hertzsprung–Russell diagram, the x-axis represents temperature or spectral type while the y-axis represents luminosity or magnitude. The vast majority of stars are found along the main sequence with blue Class O stars found at the top left of the chart while red Class M stars fall to the bottom right. Certain stars like Deneb and Betelgeuse are
Andromeda Galaxy
The Andromeda Galaxy known as Messier 31, M31, or NGC 224, is a spiral galaxy 780 kiloparsecs from Earth, the nearest major galaxy to the Milky Way. Its name stems from the area of the Earth's sky; the virial mass of the Andromeda Galaxy is of the same order of magnitude as that of the Milky Way, at a trillion solar masses. The mass of either galaxy is difficult to estimate with any accuracy, but it was long thought that the Andromeda Galaxy is more massive than the Milky Way by a margin of some 25% to 50%; this has been called into question by a 2018 study which cited a lower estimate on the mass of the Andromeda Galaxy, combined with preliminary reports on a 2019 study estimating a higher mass of the Milky Way. The Andromeda Galaxy has a diameter of about 220,000 light-years, making it the largest member of the Local Group at least in terms of extension, if not mass; the number of stars contained in the Andromeda Galaxy is estimated at one trillion, or twice the number estimated for the Milky Way.
The Milky Way and Andromeda galaxies are expected to collide in ~4.5 billion years, merging to form a giant elliptical galaxy or a large disc galaxy. With an apparent magnitude of 3.4, the Andromeda Galaxy is among the brightest of the Messier objects making it visible to the naked eye from Earth on moonless nights when viewed from areas with moderate light pollution. Around the year 964, the Persian astronomer Abd al-Rahman al-Sufi described the Andromeda Galaxy, in his Book of Fixed Stars as a "nebulous smear". Star charts of that period labeled it as the Little Cloud. In 1612, the German astronomer Simon Marius gave an early description of the Andromeda Galaxy based on telescopic observations; the German philosopher Immanuel Kant in 1755 in his work Universal Natural History and Theory of the Heavens conjectured that the blurry spot was an island universe. In 1764, Charles Messier cataloged Andromeda as object M31 and incorrectly credited Marius as the discoverer despite it being visible to the naked eye.
In 1785, the astronomer William Herschel noted a faint reddish hue in the core region of Andromeda. He believed Andromeda to be the nearest of all the "great nebulae", based on the color and magnitude of the nebula, he incorrectly guessed that it is no more than 2,000 times the distance of Sirius. In 1850, William Parsons, 3rd Earl of Rosse and made the first drawing of Andromeda's spiral structure. In 1864, William Huggins noted; the spectra of Andromeda displays a continuum of frequencies, superimposed with dark absorption lines that help identify the chemical composition of an object. Andromeda's spectrum is similar to the spectra of individual stars, from this, it was deduced that Andromeda has a stellar nature. In 1885, a supernova was seen in the first and so far only one observed in that galaxy. At the time Andromeda was considered to be a nearby object, so the cause was thought to be a much less luminous and unrelated event called a nova, was named accordingly. In 1887, Isaac Roberts took the first photographs of Andromeda, still thought to be a nebula within our galaxy.
Roberts mistook Andromeda and similar spiral nebulae as solar systems being formed. In 1912, Vesto Slipher used spectroscopy to measure the radial velocity of Andromeda with respect to our Solar System—the largest velocity yet measured, at 300 kilometres per second. In 1917, Heber Curtis observed a nova within Andromeda. Searching the photographic record, 11 more novae were discovered. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred elsewhere in the sky; as a result, he was able to come up with a distance estimate of 500,000 light-years. He became a proponent of the so-called "island universes" hypothesis, which held that spiral nebulae were independent galaxies. In 1920, the Great Debate between Harlow Shapley and Curtis took place concerning the nature of the Milky Way, spiral nebulae, the dimensions of the Universe. To support his claim of the Great Andromeda Nebula being, in fact, an external galaxy, Curtis noted the appearance of dark lanes within Andromeda which resembled the dust clouds in our own galaxy, as well as historical observations of Andromeda Galaxy's significant Doppler shift.
In 1922 Ernst Öpik presented a method to estimate the distance of Andromeda using the measured velocities of its stars. His result placed the Andromeda Nebula far outside our galaxy at a distance of about 450,000 parsecs. Edwin Hubble settled the debate in 1925 when he identified extragalactic Cepheid variable stars for the first time on astronomical photos of Andromeda; these were made using the 2.5-metre Hooker telescope, they enabled the distance of Great Andromeda Nebula to be determined. His measurement demonstrated conclusively that this feature was not a cluster of stars and gas within our own galaxy, but an separate galaxy located a significant distance from the Milky Way. In 1943, Walter Baade was the first person to resolve stars in the central region of the Andromeda Galaxy. Baade identified two distinct populations of stars based on their metallicity, naming the young, high-velocity stars in the disk Type I and the older, red stars in the bulge Type II; this nomenclature was subsequently adopted for stars within the Milky Way, elsewhere.
Baade discovered that there were two types of Cepheid variables, which resulted in a doubling of the distance estimate to Andromeda, as well as the remainder o
Cygnus (constellation)
Cygnus is a northern constellation lying on the plane of the Milky Way, deriving its name from the Latinized Greek word for swan. Cygnus is one of the most recognizable constellations of the northern summer and autumn, it features a prominent asterism known as the Northern Cross. Cygnus was among the 48 constellations listed by the 2nd century astronomer Ptolemy, it remains one of the 88 modern constellations. Cygnus contains Deneb -which is one of the brightest stars in the night sky and is the most distant first-magnitude star- as its "tail star" and one corner of the Summer Triangle, it has some notable X-ray sources and the giant stellar association of Cygnus OB2. Cygnus is known as the Northern Cross. One of the stars of this association, NML Cygni, is one of the largest stars known; the constellation is home to Cygnus X-1, a distant X-ray binary containing a supergiant and unseen massive companion, the first object held to be a black hole. Many star systems in Cygnus have known planets as a result of the Kepler Mission observing one patch of the sky, an area around Cygnus.
In addition, most of the eastern part of Cygnus is dominated by the Hercules–Corona Borealis Great Wall, a giant galaxy filament, the largest known structure in the observable universe, covering most of the northern sky. In Greek mythology, Cygnus has been identified with several different legendary swans. Zeus disguised himself as a swan to seduce Leda, Spartan king Tyndareus's wife, who gave birth to the Gemini, Helen of Troy, Clytemnestra; the Greeks associated this constellation with the tragic story of Phaethon, the son of Helios the sun god, who demanded to ride his father's sun chariot for a day. Phaethon, was unable to control the reins, forcing Zeus to destroy the chariot with a thunderbolt, causing it to plummet to the earth into the river Eridanus. According to the myth, Phaethon's brother, grieved bitterly and spent many days diving into the river to collect Phaethon's bones to give him a proper burial; the gods were so touched by Cygnus's devotion to his brother that they turned him into a swan and placed him among the stars.
In Ovid's Metamorphoses, there are three people named Cygnus, all of whom are transformed into swans. Alongside Cygnus, noted above, he mentions a boy from Tempe who commits suicide when Phyllius refuses to give him a tamed bull that he demands, but is transformed into a swan and flies away, he mentions a son of Neptune, an invulnerable warrior in the Trojan War, defeated by Achilles, but Neptune saves him by transforming him into a swan. Together with other avian constellations near the summer solstice, Vultur cadens and Aquila, Cygnus may be a significant part of the origin of the myth of the Stymphalian Birds, one of The Twelve Labours of Hercules. In Hinduism, the period of time or the Muhurta which lasts from 4:24 AM to 5:12 AM is called the "Brahma Muhurta" translating to "The moment of the Universe" and the Star system in correlation is the Cygnus constellation; this is a auspicious time to do any task or start the day. In Polynesia, Cygnus was recognized as a separate constellation. In Tonga it was called Tuula-lupe, in the Tuamotus it was called Fanui-tai.
Deneb was often given a name. The name Deneb comes from the Arabic name dhaneb, meaning "tail", from the phrase Dhanab ad-Dajājah, which means “the tail of the hen”. In New Zealand it was called Mara-tea, in the Society Islands it was called Pirae-tea or Taurua-i-te-haapa-raa-manu, in the Tuamotus it was called Fanui-raro. Beta Cygni was named in New Zealand. Gamma Cygni was called Fanui-runga in the Tuamotus. A large constellation, Cygnus is bordered by Cepheus to the north and east, Draco to the north and west, Lyra to the west, Vulpecula to the south, Pegasus to the southeast and Lacerta to the east; the three-letter abbreviation for the constellation, as adopted by the IAU in 1922, is'Cyg'. The official constellation boundaries, as set by Eugène Delporte in 1930, are defined as a polygon of 28 segments. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 19h 07.3m and 22h 02.3m, while the declination coordinates are between 27.73° and 61.36°. Covering 804 square degrees and around 1.9% of the night sky, Cygnus ranks 16th of the 88 constellations in size.
Cygnus culminates at midnight on 29 June, is most visible in the evening from the early summer to mid-autumn in the Northern Hemisphere. Cygnus is depicted with Delta and Epsilon Cygni as its wings. Deneb, the brightest in the constellation is at its tail, Albireo as the tip of its beak. There are several asterisms in Cygnus. In the 17th-century German celestial cartographer Johann Bayer's star atlas the Uranometria, Alpha and Gamma Cygni form the pole of a cross, while Delta and Epsilon form the cross beam; the nova P Cygni was considered to be the body of Christ. Bayer catalogued many stars in the constellation, giving them the Bayer designations from Alpha to Omega and using lowercase Roman letters to g. John Flamsteed were dropped by Francis Baily. There are several bright stars in Cygnus. Alpha Cygni, called Deneb, is the brightest star in Cygnus, it is a white supergiant star of spectral type A2Iae that varies between magnitudes 1.21 and 1.29, one of the largest and most luminous A-class stars known.
It is located about 3200
Binary star
A binary star is a star system consisting of two stars orbiting around their common barycenter. Systems of two or more stars are called multiple star systems; these systems when more distant appear to the unaided eye as a single point of light, are revealed as multiple by other means. Research over the last two centuries suggests that half or more of visible stars are part of multiple star systems; the term double star is used synonymously with binary star. Optical doubles are so called because the two stars appear close together in the sky as seen from the Earth, their "doubleness" depends only on this optical effect. A double star can be revealed as optical by means of differences in their parallax measurements, proper motions, or radial velocities. Most known double stars have not been studied adequately to determine whether they are optical doubles or doubles physically bound through gravitation into a multiple star system. Binary star systems are important in astrophysics because calculations of their orbits allow the masses of their component stars to be directly determined, which in turn allows other stellar parameters, such as radius and density, to be indirectly estimated.
This determines an empirical mass-luminosity relationship from which the masses of single stars can be estimated. Binary stars are detected optically, in which case they are called visual binaries. Many visual binaries have long orbital periods of several centuries or millennia and therefore have orbits which are uncertain or poorly known, they may be detected by indirect techniques, such as spectroscopy or astrometry. If a binary star happens to orbit in a plane along our line of sight, its components will eclipse and transit each other. If components in binary star systems are close enough they can gravitationally distort their mutual outer stellar atmospheres. In some cases, these close binary systems can exchange mass, which may bring their evolution to stages that single stars cannot attain. Examples of binaries are Sirius, Cygnus X-1. Binary stars are common as the nuclei of many planetary nebulae, are the progenitors of both novae and type Ia supernovae; the term binary was first used in this context by Sir William Herschel in 1802, when he wrote: If, on the contrary, two stars should be situated near each other, at the same time so far insulated as not to be materially affected by the attractions of neighbouring stars, they will compose a separate system, remain united by the bond of their own mutual gravitation towards each other.
This should be called a real double star. By the modern definition, the term binary star is restricted to pairs of stars which revolve around a common center of mass. Binary stars which can be resolved with a telescope or interferometric methods are known as visual binaries. For most of the known visual binary stars one whole revolution has not been observed yet, they are observed to have travelled along a curved path or a partial arc; the more general term double star is used for pairs of stars which are seen to be close together in the sky. This distinction is made in languages other than English. Double stars may be binary systems or may be two stars that appear to be close together in the sky but have vastly different true distances from the Sun; the latter are termed optical optical pairs. Since the invention of the telescope, many pairs of double stars have been found. Early examples include Acrux. Mizar, in the Big Dipper, was observed to be double by Giovanni Battista Riccioli in 1650; the bright southern star Acrux, in the Southern Cross, was discovered to be double by Father Fontenay in 1685.
John Michell was the first to suggest that double stars might be physically attached to each other when he argued in 1767 that the probability that a double star was due to a chance alignment was small. William Herschel began observing double stars in 1779 and soon thereafter published catalogs of about 700 double stars. By 1803, he had observed changes in the relative positions in a number of double stars over the course of 25 years, concluded that they must be binary systems. Since this time, many more double stars have been measured; the Washington Double Star Catalog, a database of visual double stars compiled by the United States Naval Observatory, contains over 100,000 pairs of double stars, including optical doubles as well as binary stars. Orbits are known for only a few thousand of these double stars, most have not been ascertained to be either true binaries or optical double stars; this can be determined by observing the relative motion of the pairs. If the motion is part of an orbit, or if the stars have similar radial velocities and the difference in their proper motions is small compared to their common proper motion, the pair is physical.
One of the tasks that remains for visual observers of double stars is to obtain sufficient observations to prove or disprove gravitational connection. Binary stars are classified into four types accordi
Nebula
A nebula is an interstellar cloud of dust, hydrogen and other ionized gases. The term was used to describe any diffuse astronomical object, including galaxies beyond the Milky Way; the Andromeda Galaxy, for instance, was once referred to as the Andromeda Nebula before the true nature of galaxies was confirmed in the early 20th century by Vesto Slipher, Edwin Hubble and others. Most nebulae are of vast size. A nebula, visible to the human eye from Earth would appear larger, but no brighter, from close by; the Orion Nebula, the brightest nebula in the sky and occupying an area twice the diameter of the full Moon, can be viewed with the naked eye but was missed by early astronomers. Although denser than the space surrounding them, most nebulae are far less dense than any vacuum created on Earth – a nebular cloud the size of the Earth would have a total mass of only a few kilograms. Many nebulae are visible due to fluorescence caused by embedded hot stars, while others are so diffuse they can only be detected with long exposures and special filters.
Some nebulae are variably illuminated by T Tauri variable stars. Nebulae are star-forming regions, such as in the "Pillars of Creation" in the Eagle Nebula. In these regions the formations of gas and other materials "clump" together to form denser regions, which attract further matter, will become dense enough to form stars; the remaining material is believed to form planets and other planetary system objects. Around 150 AD, Claudius Ptolemaeus recorded, in books VII–VIII of his Almagest, five stars that appeared nebulous, he noted a region of nebulosity between the constellations Ursa Major and Leo, not associated with any star. The first true nebula, as distinct from a star cluster, was mentioned by the Persian astronomer Abd al-Rahman al-Sufi, in his Book of Fixed Stars, he noted "a little cloud". He cataloged the Omicron Velorum star cluster as a "nebulous star" and other nebulous objects, such as Brocchi's Cluster; the supernova that created the Crab Nebula, the SN 1054, was observed by Arabic and Chinese astronomers in 1054.
In 1610, Nicolas-Claude Fabri de Peiresc discovered the Orion Nebula using a telescope. This nebula was observed by Johann Baptist Cysat in 1618. However, the first detailed study of the Orion Nebula was not performed until 1659, by Christiaan Huygens, who believed he was the first person to discover this nebulosity. In 1715, Edmund Halley published a list of six nebulae; this number increased during the century, with Jean-Philippe de Cheseaux compiling a list of 20 in 1746. From 1751 to 1753, Nicolas Louis de Lacaille cataloged 42 nebulae from the Cape of Good Hope, most of which were unknown. Charles Messier compiled a catalog of 103 "nebulae" by 1781; the number of nebulae was greatly increased by the efforts of William Herschel and his sister Caroline Herschel. Their Catalogue of One Thousand New Nebulae and Clusters of Stars was published in 1786. A second catalog of a thousand was published in 1789 and the third and final catalog of 510 appeared in 1802. During much of their work, William Herschel believed that these nebulae were unresolved clusters of stars.
In 1790, however, he discovered a star surrounded by nebulosity and concluded that this was a true nebulosity, rather than a more distant cluster. Beginning in 1864, William Huggins examined the spectra of about 70 nebulae, he found that a third of them had the emission spectrum of a gas. The rest thus were thought to consist of a mass of stars. A third category was added in 1912 when Vesto Slipher showed that the spectrum of the nebula that surrounded the star Merope matched the spectra of the Pleiades open cluster, thus the nebula radiates by reflected star light. About 1923, following the Great Debate, it had become clear that many "nebulae" were in fact galaxies far from our own. Slipher and Edwin Hubble continued to collect the spectra from many different nebulae, finding 29 that showed emission spectra and 33 that had the continuous spectra of star light. In 1932, Hubble announced that nearly all nebula are associated with stars, their illumination comes from star light, he discovered that the emission spectrum nebulae are nearly always associated with stars having spectral classifications of B or hotter, while nebulae with continuous spectra appear with cooler stars.
Both Hubble and Henry Norris Russell concluded that the nebulae surrounding the hotter stars are transfomed in some manner. There are a variety of formation mechanisms for the different types of nebulae; some nebulae form from gas, in the interstellar medium while others are produced by stars. Examples of the former case are giant molecular clouds, the coldest, densest phase of interstellar gas, which can form by the cooling and condensation of more diffuse gas. Examples of the latter case are planetary nebulae formed from material shed by a star in late stages of its stellar evolution. Star-forming regions are a class of emission nebula associated with giant molecular clouds; these form as a molecular cloud collapses under its own weight. Massive stars may form in the center, their ultraviolet radiation ionizes the surrounding gas, making it visible at optical wavelengths; the region of ionized hydrogen surrounding th
Cassiopeia (constellation)
Cassiopeia is a constellation in the northern sky, named after the vain queen Cassiopeia in Greek mythology, who boasted about her unrivaled beauty. Cassiopeia was one of the 48 constellations listed by the 2nd-century Greek astronomer Ptolemy, it remains one of the 88 modern constellations today, it is recognizable due to its distinctive'W' shape, formed by five bright stars. It is opposite Ursa Major. In northern locations above latitude 34ºN it is visible year-round and in the tropics it can be seen at its clearest from September to early November. In low southern latitudes below 25ºS it can be seen low in the North. At magnitude 2.2, Alpha Cassiopeiae, or Schedar, is the brightest star in Cassiopeia, though is shaded by Gamma Cassiopeiae, which has brightened to magnitude 1.6 on occasion. The constellation hosts some of the most luminous stars known, including the yellow hypergiants Rho Cassiopeiae and V509 Cassiopeiae and white hypergiant 6 Cassiopeiae; the semiregular variable. In 1572, Tycho Brahe's supernova flared brightly in Cassiopeia.
Cassiopeia A is a supernova remnant and the brightest extrasolar radio source in the sky at frequencies above 1 GHz. Fourteen star systems have been found to have exoplanets, one of which—HR 8832—is thought to host seven planets. A rich section of the Milky Way runs through Cassiopeia, containing a number of open clusters, young luminous galactic disc stars, nebulae. IC 10 is an irregular galaxy, the closest known starburst galaxy and the only one in the Local Group of galaxies; the constellation is named after the queen of Aethiopia. Cassiopeia was the wife of mother of Princess Andromeda. Cepheus and Cassiopeia were placed next to each other among the stars, along with Andromeda, she was placed in the sky as a punishment after enraging Poseidon with the boast that her daughter Andromeda was more beautiful than the Nereids or, that she herself was more beautiful than the sea nymphs. She was forced to wheel around the North Celestial Pole on her throne, spending half of her time clinging to it so she does not fall off, Poseidon decreed that Andromeda should be bound to a rock as prey for the monster Cetus.
Andromeda was rescued by the hero Perseus, whom she married. Cassiopeia has been variously portrayed throughout her history as a constellation. In Persia, she was drawn by al-Sufi as a queen holding a staff with a crescent moon in her right hand, wearing a crown, as well as a two-humped camel. In France, she was portrayed as having a marble throne and a palm leaf in her left hand, holding her robe in her right hand; this depiction is from Augustin Royer's 1679 atlas. In Chinese astronomy, the stars forming the constellation Cassiopeia are found among three areas: the Purple Forbidden enclosure, the Black Tortoise of the North, the White Tiger of the West; the Chinese astronomers saw several figures in. Kappa, Mu Cassiopeiae formed a constellation called the Bridge of the Kings; the charioteer's whip was represented by Gamma Cassiopeiae, sometimes called "Tsih", the Chinese word for "whip". In the 1600s, various Biblical figures were depicted in the stars of Cassiopeia; these included Solomon's mother.
A figure called the "Tinted Hand" appeared in the stars of Cassiopeia in some Arab atlases. This is variously said to represent a woman's hand dyed red with henna, as well as the bloodied hand of Muhammad's daughter Fatima; the hand is made up of the stars α Cas, β Cas, γ Cas, δ Cas, ε Cas, η Cas. The arm is made up of the stars α Per, γ Per, δ Per, ε Per, η Per, ν Per. Another Arab constellation that incorporated the stars of Cassiopeia was the Camel, its head was composed of Lambda, Kappa and Phi Andromedae. Other cultures see a moose antlers in the pattern; these include the Lapps. The Chukchi of Siberia saw the five main stars as five reindeer stags; the people of the Marshall Islands saw Cassiopeia as part of a great porpoise constellation. The main stars of Cassiopeia make its tail and Triangulum form its body, Aries makes its head. In Hawaii, Alpha and Gamma Cassiopeiae were named. Alpha Cassiopeiae was called Poloahilani, Beta Cassiopeiae was called Polula, Gamma Cassiopeiae was called Mulehu.
The people of Pukapuka saw the figure of Cassiopeia as a distinct constellation called Na Taki-tolu-a-Mataliki. In Modern Indian Astronomy Cassiopeia is known as Sharmishtha. In Hindu mythology, Sharmistha known as Sharmista or Sharmishtha, was the daughter of the great Devil King Vrishparva, she was a friend of Devayani for whom she becomes a servant. Covering 598.4 square degrees and hence 1.451% of the sky, Cassiopeia ranks 25th of the 88 constellations in area. It is bordered by Cepheus to the north and west, Andromeda to the south and west, Perseus to the southeast and Camelopardalis to the east, shares a short border with Lacerta to the west; the three-letter abbreviation for the constellation, as adopted by the International Astronomical Union in 1922, is'Cas'. The official constellation boundaries, as set by Eugène Delporte in 1930, are defined by a polygon of 30 segments. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 00h 27m 03s and 23h 41m 06s, while the decl
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