The galactic plane is the plane on which the majority of a disk-shaped galaxy's mass lies. The directions perpendicular to the galactic plane point to the galactic poles. In actual usage, the terms galactic plane and galactic poles refer to the plane and poles of the Milky Way, in which Planet Earth is located; some galaxies do not have any well-defined disk. In the case of a barred spiral galaxy like the Milky Way, defining the galactic plane is imprecise and arbitrary since the stars are not coplanar. In 1959, the IAU defined the position of the Milky Way's north galactic pole as RA = 12h 49m, Dec = 27° 24′ in the then-used B1950 epoch; this position is near the bright star Arcturus. The "zero of longitude" of galactic coordinates was defined in 1959 to be at position angle 123° from the north celestial pole, thus the zero longitude point on the galactic equator was at 17h 42m 26.603s, −28° 55′ 00.445″ or 17h 45m 37.224s, −28° 56′ 10.23″, its J2000 position angle is 122.932°. The galactic center is located at position angle 31.72 31.40 ° east of north.
Galactic coordinate system Reid, M. J.. "The Proper Motion of Sagittarius A*. II; the Mass of Sagittarius A*", The Astrophysical Journal, 616: 872–884, arXiv:astro-ph/0408107, Bibcode:2004ApJ...616..872R, doi:10.1086/424960. See appendix for the numbers listed above
In astronomy, a bulge is a packed group of stars within a larger formation. The term exclusively refers to the central group of stars found in most spiral galaxies. Bulges were thought to be elliptical galaxies that happened to have a disk of stars around them, but high-resolution images using the Hubble Space Telescope have revealed that many bulges lie at the heart of a spiral galaxy, it is now thought that there are at least two types of bulges: bulges that are like ellipticals and bulges that are like spiral galaxies. Bulges that have properties similar to those of elliptical galaxies are called "classical bulges" due to their similarity to the historic view of bulges; these bulges are composed of stars that are older, Population II stars, hence have a reddish hue. These stars are in orbits that are random compared to the plane of the galaxy, giving the bulge a distinct spherical form. Due to the lack of dust and gases, bulges tend to have no star formation; the distribution of light is described by a Sersic profile.
Classical bulges are thought to be the result of collisions of smaller structures. Convulsing gravitational forces and torques disrupt the orbital paths of stars, resulting in the randomised bulge orbits. If either progenitor galaxy was gas-rich, the tidal forces can cause inflows to the newly merged galaxy nucleus. Following a major merger, gas clouds are more to convert into stars, due to shocks. One study has suggested that about 80% of galaxies in the field lack a classical bulge, indicating that they have never experienced a major merger; the bulgeless galaxy fraction of the Universe has remained constant for at least the last 8 billion years. In contrast, about two thirds of galaxies in dense galaxy clusters do possess a classical bulge, demonstrating the disruptive effect of their crowding. Many bulges have properties more similar to those of the central regions of spiral galaxies than elliptical galaxies, they are referred to as pseudobulges or disky-bulges. 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 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 looks similar to a spiral galaxy, but is much smaller. Giant spiral galaxies are 2–100 times the size of those spirals that exist in bulges. Where they exist, these central spirals dominate the light of the bulge; 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 found in outer disks, as shown in NGC 4314. Properties such as spiral structure and young stars suggest that some bulges did not form through the same process that made elliptical galaxies and classical bulges, yet the theories for the formation of pseudobulges are less certain than those for classical bulges. Pseudobulges may be the result of gas-rich mergers that happened more than those mergers that formed classical bulges.
However, it is difficult for disks to survive casting doubt on this scenario. Many astronomers suggest that bulges that appear similar to disks form outside of the disk, 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 observed in such galaxies. Secular evolution is expected to send gas and stars to the center of a galaxy. If this happens that would increase the density at the center of the galaxy, thus make a bulge that has properties similar to those of disk galaxies. If secular evolution, or the slow, steady evolution of a galaxy, is responsible for the formation of a significant number of bulges that many galaxies have not experienced a merger since the formation of their disk; this would mean that current theories of galaxy formation and evolution over-predict the number of mergers in the past few billion years. Most bulges and pseudo-bulges are thought to host a central relativistic compact mass, traditionally assumed to be a supermassive black hole.
Such black holes by definition can not be observed directly, but various pieces of evidence suggest their existence, both in the bulges of spiral galaxies and in the centers of ellipticals. The masses of the black holes correlate with bulge properties; the M–sigma relation relates black hole mass to the velocity dispersion of bulge stars, while other correlations involve the total stellar mass or luminosity of the bulge, the central concentration of stars in the bulge, the richness of the globular cluster system orbiting in the galaxy's far outskirts, the winding angle of the spiral arms. Until it was thought that one could not have a supermassive black hole without a surrounding bulge. Galaxies hosting supermassive black holes without accompanying bulges have now been observed; the implication is that the bulge environment is not essential to the initial seeding and growth of massive black holes. Disc galaxy – A galaxy characterized by a flattened circular volume of stars, that may include a central bulge Spiral galaxy Galactic coordinate system – A celestial coordinat
A disc galaxy is a galaxy characterized by a disc, a flattened circular volume of stars. These galaxies may not include a central non-disc-like region. Junko Ueda observed that galaxy collisions result in disc galaxies, within 40 million light years from the Earth. While the galaxies are interacting, they change shape in cosmic time. Disc galaxy types include: spiral galaxies unbarred spiral galaxies barred spiral galaxies intermediate spiral galaxies lenticular galaxies Galaxies that are not disc types include: elliptical galaxies irregular galaxies
A constellation is a group of stars that forms an imaginary outline or pattern on the celestial sphere representing an animal, mythological person or creature, a god, or an inanimate object. The origins of the earliest constellations go back to prehistory. People used them to relate stories of their beliefs, creation, or mythology. Different cultures and countries adopted their own constellations, some of which lasted into the early 20th century before today's constellations were internationally recognized. Adoption of constellations has changed over time. Many have changed in shape; some became popular. Others were limited to single nations; the 48 traditional Western constellations are Greek. They are given in Aratus' work Phenomena and Ptolemy's Almagest, though their origin predates these works by several centuries. Constellations in the far southern sky were added from the 15th century until the mid-18th century when European explorers began traveling to the Southern Hemisphere. Twelve ancient constellations belong to the zodiac.
The origins of the zodiac remain uncertain. In 1928, the International Astronomical Union formally accepted 88 modern constellations, with contiguous boundaries that together cover the entire celestial sphere. Any given point in a celestial coordinate system lies in one of the modern constellations; some astronomical naming systems include the constellation where a given celestial object is found to convey its approximate location in the sky. The Flamsteed designation of a star, for example, consists of a number and the genitive form of the constellation name. Other star patterns or groups called asterisms are not constellations per se but are used by observers to navigate the night sky. Examples of bright asterisms include the Pleiades and Hyades within the constellation Taurus or Venus' Mirror in the constellation of Orion.. Some asterisms, like the False Cross, are split between two constellations; the word "constellation" comes from the Late Latin term cōnstellātiō, which can be translated as "set of stars".
The Ancient Greek word for constellation is ἄστρον. A more modern astronomical sense of the term "constellation" is as a recognisable pattern of stars whose appearance is associated with mythological characters or creatures, or earthbound animals, or objects, it can specifically denote the recognized 88 named constellations used today. Colloquial usage does not draw a sharp distinction between "constellations" and smaller "asterisms", yet the modern accepted astronomical constellations employ such a distinction. E.g. the Pleiades and the Hyades are both asterisms, each lies within the boundaries of the constellation of Taurus. Another example is the northern asterism known as the Big Dipper or the Plough, composed of the seven brightest stars within the area of the IAU-defined constellation of Ursa Major; the southern False Cross asterism includes portions of the constellations Carina and Vela and the Summer Triangle.. A constellation, viewed from a particular latitude on Earth, that never sets below the horizon is termed circumpolar.
From the North Pole or South Pole, all constellations south or north of the celestial equator are circumpolar. Depending on the definition, equatorial constellations may include those that lie between declinations 45° north and 45° south, or those that pass through the declination range of the ecliptic or zodiac ranging between 23½° north, the celestial equator, 23½° south. Although stars in constellations appear near each other in the sky, they lie at a variety of distances away from the Earth. Since stars have their own independent motions, all constellations will change over time. After tens to hundreds of thousands of years, familiar outlines will become unrecognizable. Astronomers can predict the past or future constellation outlines by measuring individual stars' common proper motions or cpm by accurate astrometry and their radial velocities by astronomical spectroscopy; the earliest evidence for the humankind's identification of constellations comes from Mesopotamian inscribed stones and clay writing tablets that date back to 3000 BC.
It seems that the bulk of the Mesopotamian constellations were created within a short interval from around 1300 to 1000 BC. Mesopotamian constellations appeared in many of the classical Greek constellations; the oldest Babylonian star catalogues of stars and constellations date back to the beginning in the Middle Bronze Age, most notably the Three Stars Each texts and the MUL. APIN, an expanded and revised version based on more accurate observation from around 1000 BC. However, the numerous Sumerian names in these catalogues suggest that they built on older, but otherwise unattested, Sumerian traditions of the Early Bronze Age; the classical Zodiac is a revision of Neo-Babylonian constellations from the 6th century BC. The Greeks adopted the Babylonian constellations in the 4th century BC. Twenty Ptolemaic constellations are from the Ancient Near East. Another ten have the same stars but different names. Biblical scholar, E. W. Bullinger interpreted some of the creatures mentioned in the books of Ezekiel and Revelation as the middle signs of the four quarters of the Zodiac, with the Lion as Leo, the Bull as Taurus, the Man representing Aquarius and the Eagle standing in for Scorpio.
The biblical Book of Job also
An elliptical galaxy is a type of galaxy with an ellipsoidal shape and a smooth, nearly featureless image. They are one of the three main classes of galaxy described by Edwin Hubble in his Hubble sequence and 1936 work The Realm of the Nebulae, along with spiral and lenticular galaxies. Elliptical galaxies are, together with lenticular galaxies with their large-scale disks, ES galaxies with their intermediate scale disks, a subset of the "early-type" galaxy population. Most elliptical galaxies are composed of older, low-mass stars, with a sparse interstellar medium and minimal star formation activity, they tend to be surrounded by large numbers of globular clusters. Elliptical galaxies are believed to make up 10%–15% of galaxies in the Virgo Supercluster, they are not the dominant type of galaxy in the universe overall, they are preferentially found close to the centers of galaxy clusters. Elliptical galaxies range in size from tens of millions to over one hundred trillion stars. Edwin Hubble hypothesized that elliptical galaxies evolved into spiral galaxies, discovered to be false, although the accretion of gas and smaller galaxies may build a disk around a pre-existing ellipsoidal structure.
Stars found inside of elliptical galaxies are on average much older than stars found in spiral galaxies. M49 M59 M60 M87 M89 M105 IC one of the largest galaxies in the observable universe. Maffei 1, the closest giant elliptical galaxy. CGCG 049-033, known for having the longest galactic jet discovered. Centaurus A, a radio galaxy, elliptical/lenticular disputed. Elliptical galaxies are characterized by several properties that make them distinct from other classes of galaxy, they are ovoid masses of stars, starved of star-making gases. The smallest known elliptical galaxy is about one-tenth the size of the Milky Way; the motion of stars in elliptical galaxies is predominantly radial, unlike the disks of spiral galaxies, which are dominated by rotation. Furthermore, there is little interstellar matter, which results in low rates of star formation, few open star clusters, few young stars. Large elliptical galaxies have an extensive system of globular clusters; the dynamical properties of elliptical galaxies and the bulges of disk galaxies are similar, suggesting that they may be formed by the same physical processes, although this remains controversial.
The luminosity profiles of both elliptical galaxies and bulges are well fit by Sersic's law, a range of scaling relations between the elliptical galaxies' structural parameters unify the population. Every massive elliptical galaxy contains a supermassive black hole at its center. Observations of 46 elliptical galaxies, 20 classical bulges, 22 pseudobulges show that each contain a black hole at the center; the mass of the black hole is correlated with the mass of the galaxy, evidenced through correlations such as the M–sigma relation which relates the velocity dispersion of the surrounding stars to the mass of the black hole at the center. Elliptical galaxies are preferentially found in compact groups of galaxies. Unlike flat spiral galaxies with organization and structure, elliptical galaxies are more three-dimensional, without much structure, their stars are in somewhat random orbits around the center. Elliptical galaxies vary in both size and mass with diameters ranging from 3000 lightyears to more than 700,000 lightyears, masses from 105 to nearly 1013 solar masses.
This range is much broader for this galaxy type than for any other. The smallest, the dwarf elliptical galaxies, may be no larger than a typical globular cluster, but contain a considerable amount of dark matter not present in clusters. Most of these small galaxies may not be related to other ellipticals; the Hubble classification of elliptical galaxies contains an integer that describes how elongated the galaxy image is. The classification is determined by the ratio of the major to the minor axes of the galaxy's isophotes: 10 × Thus for a spherical galaxy with a equal to b, the number is 0, the Hubble type is E0. While the limit in the literature is about E7, it has been known since 1966 that the E4 to E7 galaxies are misclassified lenticular galaxies with disks inclined at different angles to our line-of-sight; this has been confirmed through spectral observations revealing the rotation of their stellar disks. Hubble recognized that his shape classification depends both on the intrinsic shape of the galaxy, as well as the angle with which the galaxy is observed.
Hence, some galaxies with Hubble type E0 are elongated. It is sometimes said that there are two physical types of ellipticals: the giant ellipticals with "boxy"-shaped isophotes, whose shapes result from random motion, greater in some directions than in others; this is, however, an abuse of the nomenclature, as there two types of early-type galaxy, those with disks and those without. Given the existence of ES galaxies with intermediate-scale disks, it is reasonable to expect that there is a continuity from E to ES, onto the S0 galaxies with their large-scale stellar disks that dominate the light at large radii. Dwarf spheroidal galaxies appear to be a distinct class: their properties are more similar to those of irregulars and late spiral-type galaxies. At the large end of the elliptical spectrum, there
A star is type of astronomical object consisting of a luminous spheroid of plasma held together by its own gravity. The nearest star to Earth is the Sun. Many other stars are visible to the naked eye from Earth during the night, appearing as a multitude of fixed luminous points in the sky due to their immense distance from Earth; the most prominent stars were grouped into constellations and asterisms, the brightest of which gained proper names. Astronomers have assembled star catalogues that identify the known stars and provide standardized stellar designations. However, most of the estimated 300 sextillion stars in the Universe are invisible to the naked eye from Earth, including all stars outside our galaxy, the Milky Way. For at least a portion of its life, a star shines due to thermonuclear fusion of hydrogen into helium in its core, releasing energy that traverses the star's interior and radiates into outer space. All occurring elements heavier than helium are created by stellar nucleosynthesis during the star's lifetime, for some stars by supernova nucleosynthesis when it explodes.
Near the end of its life, a star can contain degenerate matter. Astronomers can determine the mass, age and many other properties of a star by observing its motion through space, its luminosity, spectrum respectively; the total mass of a star is the main factor. Other characteristics of a star, including diameter and temperature, change over its life, while the star's environment affects its rotation and movement. A plot of the temperature of many stars against their luminosities produces a plot known as a Hertzsprung–Russell diagram. Plotting a particular star on that diagram allows the age and evolutionary state of that star to be determined. A star's life begins with the gravitational collapse of a gaseous nebula of material composed of hydrogen, along with helium and trace amounts of heavier elements; when the stellar core is sufficiently dense, hydrogen becomes converted into helium through nuclear fusion, releasing energy in the process. The remainder of the star's interior carries energy away from the core through a combination of radiative and convective heat transfer processes.
The star's internal pressure prevents it from collapsing further under its own gravity. A star with mass greater than 0.4 times the Sun's will expand to become a red giant when the hydrogen fuel in its core is exhausted. In some cases, it will fuse heavier elements in shells around the core; as the star expands it throws a part of its mass, enriched with those heavier elements, into the interstellar environment, to be recycled as new stars. Meanwhile, the core becomes a stellar remnant: a white dwarf, a neutron star, or if it is sufficiently massive a black hole. Binary and multi-star systems consist of two or more stars that are gravitationally bound and move around each other in stable orbits; when two such stars have a close orbit, their gravitational interaction can have a significant impact on their evolution. Stars can form part of a much larger gravitationally bound structure, such as a star cluster or a galaxy. Stars have been important to civilizations throughout the world, they have used for celestial navigation and orientation.
Many ancient astronomers believed that stars were permanently affixed to a heavenly sphere and that they were immutable. By convention, astronomers grouped stars into constellations and used them to track the motions of the planets and the inferred position of the Sun; the motion of the Sun against the background stars was used to create calendars, which could be used to regulate agricultural practices. The Gregorian calendar used nearly everywhere in the world, is a solar calendar based on the angle of the Earth's rotational axis relative to its local star, the Sun; the oldest dated star chart was the result of ancient Egyptian astronomy in 1534 BC. The earliest known star catalogues were compiled by the ancient Babylonian astronomers of Mesopotamia in the late 2nd millennium BC, during the Kassite Period; the first star catalogue in Greek astronomy was created by Aristillus in 300 BC, with the help of Timocharis. The star catalog of Hipparchus included 1020 stars, was used to assemble Ptolemy's star catalogue.
Hipparchus is known for the discovery of the first recorded nova. Many of the constellations and star names in use today derive from Greek astronomy. In spite of the apparent immutability of the heavens, Chinese astronomers were aware that new stars could appear. In 185 AD, they were the first to observe and write about a supernova, now known as the SN 185; the brightest stellar event in recorded history was the SN 1006 supernova, observed in 1006 and written about by the Egyptian astronomer Ali ibn Ridwan and several Chinese astronomers. The SN 1054 supernova, which gave birth to the Crab Nebula, was observed by Chinese and Islamic astronomers. Medieval Islamic astronomers gave Arabic names to many stars that are still used today and they invented numerous astronomical instruments that could compute the positions of the stars, they built the first large observatory research institutes for the purpose of producing Zij star catalogues. Among these, the Book of Fixed Stars was written by the Persian astronomer Abd al-Rahman al-Sufi, who observed a number of stars, star clusters and galaxies.
According to A. Zahoor, in the 11th century, the Persian polymath scholar Abu Rayhan Biruni described the Milky
A dwarf galaxy is a small galaxy composed of about 100 million up to several billion stars, a small number compared to the Milky Way's 200–400 billion stars. The Large Magellanic Cloud, which orbits the Milky Way and contains over 30 billion stars, is sometimes classified as a dwarf galaxy. Dwarf galaxies' formation and activity are thought to be influenced by interactions with larger galaxies. Astronomers identify numerous types of dwarf galaxies, based on their composition. Current theory states that most galaxies, including dwarf galaxies, form in association with dark matter, or from gas that contains metals. However, NASA's Galaxy Evolution Explorer space probe identified new dwarf galaxies forming out of gases with low metallicity; these galaxies were located in the Leo Ring, a cloud of hydrogen and helium around two massive galaxies in the constellation Leo. Because of their small size, dwarf galaxies have been observed being pulled toward and ripped by neighbouring spiral galaxies, resulting in galaxy merger.
There are many dwarf galaxies in the Local Group. A 2007 paper has suggested that many dwarf galaxies were created by galactic tides during the early evolutions of the Milky Way and Andromeda. Tidal dwarf galaxies are produced when their gravitational masses interact. Streams of galactic material are pulled away from the parent galaxies and the halos of dark matter that surround them. A 2018 study suggests that some local dwarf galaxies formed early, during the Dark Ages within the first billion years after the big bang. More than 20 known dwarf galaxies orbit the Milky Way, recent observations have led astronomers to believe the largest globular cluster in the Milky Way, Omega Centauri, is in fact the core of a dwarf galaxy with a black hole at its centre, at some time absorbed by the Milky Way. Elliptical galaxy: dwarf elliptical galaxy Dwarf spheroidal galaxy: Once a subtype of dwarf ellipticals, now regarded as a distinct type Irregular galaxy: dwarf irregular galaxy Spiral galaxy: dwarf spiral galaxy Magellanic type dwarfs Blue compact dwarf galaxies Ultra-compact dwarf galaxies In astronomy, a blue compact dwarf galaxy is a small galaxy which contains large clusters of young, massive stars.
These stars, the brightest of which are blue, cause the galaxy. Most BCD galaxies are classified as dwarf irregular galaxies or as dwarf lenticular galaxies; because they are composed of star clusters, BCD galaxies lack a uniform shape. They consume gas intensely, which causes their stars to become violent when forming. BCD galaxies cool in the process of forming new stars; the galaxies' stars are all formed at different time periods, so the galaxies have time to cool and to build up matter to form new stars. As time passes, this star formation changes the shape of the galaxies. Nearby examples include NGC 1705, NGC 2915, NGC 3353 and UGCA 281. Ultra-compact dwarf galaxies are a class of compact galaxies with high stellar densities, discovered in the 2000s, they are thought to be on the order of 200 light years across. It is theorised that these are the cores of nucleated dwarf elliptical galaxies that have been stripped of gas and outlying stars by tidal interactions, travelling through the hearts of rich clusters.
UCDs have been found in the Virgo Cluster, Fornax Cluster, Abell 1689, the Coma Cluster, amongst others. In particular, an unprecedentedly large sample of ~ 100 UCDs has been found in the core region of the Virgo cluster by the Next Generation Virgo Cluster Survey team; the first relatively robust studies of the global properties of Virgo UCDs suggest that UCDs have distinct dynamical and structural properties from normal globular clusters. An extreme example of UCD is M60-UCD1, about 54 million light years away, which contains 200 million solar masses within a 160 light year radius. M59-UCD3 is the same size as M60-UCD1 with a half-light radius, rh, of 20 parsecs but is 40% more luminous with an apparent relative magnitude of −14.6. This makes M59-UCD3 the densest known galaxy. Based on stellar orbital velocities, two UCD in the Virgo Cluster are claimed to have supermassive black holes weighing 13% and 18% of the galaxies' masses. Galaxy morphological classification – System for categorizing galaxies based on appearance List of nearest galaxies Pea galaxy – Possibly a type of Luminous Blue Compact Galaxy, undergoing high rates of star formation Milky Way Satellite Galaxies SPACE.com article on "hobbit galaxies" Science article on "hobbit galaxies"