1.
Galaxy
–
A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The word galaxy is derived from the Greek galaxias, literally milky, Galaxies range in size from dwarfs with just a few billion stars to giants with one hundred trillion stars, each orbiting its galaxys center of mass. Galaxies are categorized according to their morphology as elliptical, spiral. Many galaxies are thought to have holes at their active centers. The Milky Ways central black hole, known as Sagittarius A*, has a four million times greater than the Sun. Recent estimates of the number of galaxies in the observable universe range from 200 billion to 2 trillion or more, most of the galaxies are 1,000 to 100,000 parsecs in diameter and separated by distances on the order of millions of parsecs. The space between galaxies is filled with a gas having an average density of less than one atom per cubic meter. The majority of galaxies are organized into groups, clusters. At the largest scale, these associations are generally arranged into sheets and filaments surrounded by immense voids. In Greek mythology, Zeus places his son born by a mortal woman, the infant Heracles, on Heras 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 then realizes she is nursing a baby, she pushes the baby away, some of her milk spills and. In the astronomical literature, the capitalized word Galaxy is often 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. However, the word Universe was later understood to mean the entirety of existence, so this expression fell into disuse and the objects instead 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, and the Sombrero Galaxy. Astronomers work with numbers from certain catalogues, such as the Messier catalogue, the NGC, the IC, the CGCG, 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 galaxy having the number 109 in the catalogue of Messier, but also codes NGC3992, UGC6937, CGCG 269-023, MCG +09-20-044. One of the arguments to do so is that these impressive objects deserve better than uninspired codes, for instance, Bodifee and Berger propose the informal, descriptive name Callimorphus Ursae Majoris for the well-formed barred galaxy Messier 109 in Ursa Major
Galaxy
–
NGC 4414, a typical spiral galaxy in the
constellation Coma Berenices, is about 55,000
light-years in diameter and approximately 60 million light-years away from Earth
Galaxy
–
A
fish-eye mosaic of the Milky Way arching at a high inclination across the night sky, shot from a dark-sky location in Chile
Galaxy
–
Photograph of the "Great Andromeda Nebula" from 1899, later identified as the
Andromeda Galaxy
Galaxy
–
NGC 3923 Elliptical Shell Galaxy-Hubble Space Telescope photograph
2.
Spiral galaxy
–
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
Spiral galaxy
–
An example of a spiral galaxy, the
Pinwheel Galaxy (also known as Messier 101 or NGC 5457)
Spiral galaxy
–
Barred spiral galaxy UGC 12158.
Spiral galaxy
–
NGC 1300 in
infrared light.
Spiral galaxy
–
Spiral galaxy NGC 1345
3.
Elliptical galaxy
–
An elliptical galaxy is a type of galaxy having an approximately ellipsoidal shape and a smooth, nearly featureless brightness profile. Unlike flat spiral galaxies with organization and structure, they are more three-dimensional, without much structure and they are one of the three main classes of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae, along with spiral and lenticular galaxies. Elliptical galaxies range in shape from spherical to highly flat. Originally Edwin Hubble hypothesized that elliptical galaxies evolved into spiral galaxies, stars found inside of elliptical galaxies are much older than stars found in spiral galaxies. Elliptical galaxies are believed to make up approximately 10%–15% of galaxies in the Virgo Supercluster and they are preferentially found close to the centers of galaxy clusters. Elliptical galaxies are also called early-type galaxies, due to their location in the Hubble sequence, elliptical galaxies are characterized by several properties that make them distinct from other classes of galaxy. They are spherical or 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 galaxies is predominantly radial, unlike the disks of spiral galaxies. Large elliptical galaxies typically have a system of globular clusters. The dynamical properties of galaxies and the bulges of disk galaxies are similar, suggesting that they may be formed by the same physical processes. The luminosity profiles of both elliptical galaxies and bulges are well fit by Sersics law, every massive elliptical galaxy contains a supermassive black hole at its center. Observations of 46 elliptical galaxies,20 classical bulges, and 22 pseudobulges show that each contain a black hole at the center, elliptical galaxies are preferentially found in galaxy clusters and in compact groups of galaxies. The traditional portrait of elliptical galaxies paints them as galaxies where star formation finished after an initial burst at high-redshift, elliptical galaxies typically appear yellow-red, which is in contrast to the distinct blue tinge of most spiral galaxies. In spirals, this blue color emanates largely from the young, very little star formation is thought to occur in elliptical galaxies, because of their lack of gas compared to spiral or irregular galaxies. However, in recent years, evidence has shown that a proportion of these galaxies have residual gas reservoirs. Researchers with the Herschel Space Observatory have speculated that the black holes in elliptical galaxies keep the gas from cooling enough for star formation. Elliptical galaxies vary greatly in size and mass with diameters ranging from 3000 lightyears to more than 700,000 lightyears. This range is broader for this galaxy type than for any other
Elliptical galaxy
–
The giant elliptical galaxy ESO 325-G004
Elliptical galaxy
–
The central galaxy in this image is a gigantic elliptical galaxy designated 4C 73.08.
Elliptical galaxy
–
The brilliant central object is a supergiant elliptical galaxy, the dominant member of a
galaxy cluster with the name MACSJ1423.8+2404. Note the
gravitational lensing.
Elliptical galaxy
–
Elliptical galaxy IC 2006.
4.
Hubble sequence
–
The Hubble sequence is a morphological classification scheme for galaxies invented by Edwin Hubble in 1926. It is often known colloquially as the Hubble tuning fork diagram because of the shape in which it is traditionally represented, hubble’s scheme divides regular galaxies into 3 broad classes – ellipticals, lenticulars and spirals – based on their visual appearance. A fourth class contains galaxies with an irregular appearance, to this day, the Hubble sequence is the most commonly used system for classifying galaxies, both in professional astronomical research and in amateur astronomy. On the left lie the ellipticals, elliptical galaxies have smooth, featureless light distributions and appear as ellipses in photographic images. They are denoted by the letter E, followed by an integer n representing their degree of ellipticity in the sky, the ellipticity increases from left to right on the Hubble diagram, with near-circular galaxies situated on the very left of the diagram. It is important to note that the ellipticity of a galaxy on the sky is only related to the true 3-dimensional shape. Observationally, the most flattened elliptical galaxies have ellipticities e =0.7 and this is consistent with their being truly ellipsoidal structures rather than disks viewed at a range of angles. Examples of elliptical galaxies, M49, M59, M60, M87, on the right of the Hubble sequence diagram are two parallel branches encompassing the spiral galaxies. A spiral galaxy consists of a disk, with stars forming a spiral structure. Roughly half of all spirals are observed to have a bar-like structure, extending from the central bulge. In the tuning-fork diagram, the regular spirals occupy the upper branch and are denoted by the letter S, while the lower branch contains the barred spirals, both type of spirals are further subdivided according to the detailed appearance of their spiral structures. The basic spiral types can be extended to enable finer distinctions of appearance, for example, spiral galaxies whose appearance is intermediate between two of the above classes are often identified by appending two lower-case letters to the main galaxy type. Our own Milky Way is generally classed as SBb, making it a barred spiral with well-defined arms, however, this classification is somewhat uncertain since we can only infer how our galaxy would appear to an outside observer. Examples of regular galaxies, M31, M74, M81, M104, M51a, NGC300. Examples of barred spiral galaxies, M91, M95, NGC1097, NGC1300, NGC1672, NGC2536, NGC2903. At the centre of the Hubble tuning fork, where the two branches and the elliptical branch join, lies an intermediate class of galaxies known as lenticulars. These galaxies consist of a central bulge, similar in appearance to an elliptical galaxy, surrounded by an extended. Unlike spiral galaxies, the disks of galaxies have no visible spiral structure and are not actively forming stars in any significant quantity
Hubble sequence
–
Tuning-fork style diagram of the Hubble sequence
Hubble sequence
–
The giant elliptical galaxy ESO 325-G004.
Hubble sequence
–
The
Pinwheel Galaxy (Messier 101/NGC 5457): a spiral galaxy classified as type Scd on the Hubble sequence
Hubble sequence
–
The barred spiral galaxy
NGC 1300: a type SBbc
5.
Bulge (astronomy)
–
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
Bulge (astronomy)
–
Artist's impression of the central bulge of the
Milky Way.
Bulge (astronomy)
–
An image of
Messier 81, a galaxy with a classical bulge. Notice that the spiral structure ends at the onset of the bulge.
Bulge (astronomy)
–
Astronomers refer to the distinctive spiral-like bulge of galaxies such as ESO 498-G5 as disc-type bulges, or pseudobulges.
Bulge (astronomy)
–
A Hubble Space Telescope image of the central region of
NGC 4314, a galaxy with a star-forming nuclear ring.
6.
Spiral arm
–
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
Spiral arm
–
An example of a spiral galaxy, the
Pinwheel Galaxy (also known as Messier 101 or NGC 5457)
Spiral arm
–
Barred spiral galaxy UGC 12158.
Spiral arm
–
NGC 1300 in
infrared light.
Spiral arm
–
Spiral galaxy NGC 1345
7.
Structure
–
Structure is an arrangement and organization of interrelated elements in a material object or system, or the object or system so organized. Material structures include man-made objects such as buildings and machines and natural objects such as biological organisms, abstract structures include data structures in computer science and musical form. Types of structure include a hierarchy, a network featuring many-to-many links, buildings, aircraft, skeletons, anthills, beaver dams and salt domes are all examples of load-bearing structures. The results of construction are divided into buildings and non-building structures, the effects of loads on physical structures are determined through structural analysis, which is one of the tasks of structural engineering. The structural elements can be classified as one-dimensional, two-dimensional, or three-dimensional, the latter was the main option available to early structures such as Chichen Itza. Two-dimensional elements with a third dimension have little of either. The structure elements are combined in structural systems, the majority of everyday load-bearing structures are section-active structures like frames, which are primarily composed of one-dimensional structures. In biology, structures exist at all levels of organization, ranging hierarchically from the atomic and molecular to the cellular, tissue, organ, organismic, population, usually, a higher-level structure is composed of multiple copies of a lower-level structure. Structural biology is concerned with the structure of macromolecules, particularly proteins. The function of molecules is determined by their shape as well as their composition. Protein structure has a four-level hierarchy, the primary structure is the sequence of amino acids that make it up. It has a backbone made up of a repeated sequence of a nitrogen. The secondary structure consists of repeated patterns determined by hydrogen bonding, the two basic types are the α-helix and the β-pleated sheet. The tertiary structure is a back and forth bending of the chain. Chemical structure refers to both molecular geometry and electronic structure, the structure can be represented by a variety of diagrams called structural formulas. Lewis structures use a dot notation to represent the valence electrons for an atom, bonds between atoms can be represented by lines with one line for each pair of electrons that is shared. In a simplified version of such a diagram, called a skeletal formula, only carbon-carbon bonds, atoms in a crystal have a structure that involves repetition of a basic unit called a unit cell. The atoms can be modeled as points on a lattice, and one can explore the effect of symmetry operations that include rotations about a point, reflections about a symmetry planes, and translations
Structure
–
Gothic quadripartite cross-ribbed vaults of the
Saint-Séverin church in Paris
Structure
–
The structure of a
DNA molecule is essential to its function.
8.
Magellanic spiral
–
Magellanic spiral galaxies are dwarf galaxies which are classified as the type Sm. They are galaxies with one single spiral arm, and are named after their prototype, the Large Magellanic Cloud and they can be considered to be intermediate between dwarf spiral galaxies and irregular galaxies. SAm galaxies are a type of unbarred spiral galaxy, while SBm are a type of barred spiral galaxy, sABm are a type of intermediate spiral galaxy. Type Sm and Im galaxies have also been categorized as irregular galaxies with some structure, Sm galaxies are typically disrupted and asymmetric. DSm galaxies are spiral galaxies or dwarf irregular galaxies, depending on categorization scheme. The Magellanic spiral classification was introduced by Gerard de Vaucouleurs, along with Magellanic irregular, Large Magellanic Cloud Small Magellanic Cloud NGC1311 NGC4618 NGC4236 NGC55 NGC4214 NGC3109 NGC4625 NGC5713 NGC5204 NGC2552 Galaxy morphological classification
Magellanic spiral
–
Large Magellanic Cloud
9.
Metallicity
–
In astronomy and physical cosmology, the metallicity or Z is the fraction of mass of a star or other kind of astronomical object that is not in hydrogen or helium. Most of the matter in the universe is in the form of hydrogen and helium, so astronomers use the word metals as a convenient short term for all elements except hydrogen. This usage is distinct from the physical definition of a solid metal. In cosmological terms, the universe is chemically evolving, according to the Big Bang Theory, the early universe first consisted of hydrogen and helium, with trace amounts of lithium and beryllium, but no heavier elements. It is believed that older generations of stars generally have lower metallicities than those of younger generations and these became commonly known as Population I and Population II stars. A third stellar population was introduced in 1978, known as Population III stars and these extremely metal-poor stars were theorised to have been the first-born stars created in the universe. Measurements have demonstrated the connection between a stars metallicity and gas giant planets, like Jupiter and Saturn, the more metals in a star and thus its planetary system and proplyd, the more likely the system may have gas giant planets and rocky planets. Current models show that the metallicity along with the planetary system temperature and distance from the star are key to planet. Metallicity also affects a stars color temperature, metal poor stars are bluer and metal rich stars are redder. The Sun, with 8 planets and 5 planetesimals, is used as the reference, other stars are noted with a positive or negative value. A star with a =0.0 has the iron abundance as the Sun. A star with =−1.0 has one tenth heavy elements of found in the Sun. At =+1, the element abundance is 10 times the Suns value. The survey of population of stars shows that older stars have less metallicity. Stellar composition, as determined by spectroscopy, is simply defined by the parameters X, Y and Z. Here X is the percentage of hydrogen, Y is the fractional percentage of helium. It is simply defined as, X + Y + Z =1.00 In most stars, nebulae and other sources, hydrogen. The hydrogen mass fraction is generally expressed as X ≡ m H M where M is the mass of the system
Metallicity
–
The globular cluster
M80. Stars in globular clusters are mainly older metal-poor members of Population II.
10.
Hubble Ultra-Deep Field
–
The Hubble Ultra-Deep Field is an image of a small region of space in the constellation Fornax, containing an estimated 10,000 galaxies. It is composited from Hubble Space Telescope data accumulated over a period from September 24,2003, looking back approximately 13 billion years it has been used to search for galaxies that existed at that time. The HUDF image was taken in a section of the sky with a low density of stars in the near-field, allowing much better viewing of dimmer. In August and September 2009, the Hubbles Deep Field was expanded using the channel of the recently attached Wide Field Camera 3. When combined with existing HUDF data, astronomers were able to identify a new list of very distant galaxies. Located southwest of Orion in the southern-hemisphere constellation Fornax, the image is 2.4 arcminutes to an edge. The image is oriented so that the left corner points toward north on the celestial sphere. On September 25,2012, NASA released a further refined version of the Ultra-Deep Field dubbed the eXtreme Deep Field. The XDF reveals galaxies that span back 13.2 billion years in time, on June 3,2014, NASA released the Hubble Ultra-Deep Field image composed of, for the first time, the full range of ultraviolet to near-infrared light. In the years since the original Hubble Deep Field, the Hubble Deep Field South, a workshop on how to best carry out surveys with the ACS was held at STScI in late 2002. At the workshop Massimo Stiavelli advocated an Ultra Deep Field as a way to study the objects responsible for the reionization of the Universe, unlike the Deep Fields, the HUDF does not lie in Hubbles Continuous Viewing Zone. As with the fields, this one was required to contain very little emission from our galaxy, with little Zodiacal dust.8 galaxy. The coordinates of the field are right ascension 3h 32m 39. 0s, the field is 200 arcseconds to a side, with a total area of 11 square arcminutes, and lies in the constellation of Fornax. Four filters were used on the ACS, centered on 435,606,775 and 850 nm and these wavelength ranges match those used by the GOODS sample, allowing direct comparison between the two. As with the Deep Fields, the HUDF used Directors Discretionary Time, the observations were done in two sessions, from September 23 to October 28,2003, and December 4,2003, to January 15,2004. The total exposure time is just under 1 million seconds, from 400 orbits, in total,800 ACS exposures were taken over the course of 11.3 days,2 every orbit, and NICMOS observed for 4.5 days. To observe the sky to the same sensitivity, the HST would need to observe continuously for a million years. The sensitivity of the ACS limits its capability of detecting galaxies at high redshift to about 6
Hubble Ultra-Deep Field
–
Hubble Ultra-Deep Field (HUDF) image (full range of
ultraviolet to
near-infrared light) includes some of the most distant
galaxies to have been imaged by an
optical telescope, existing shortly after the
Big Bang (June 2014). (This photo has the same field of view as the 2012 XDF, but NASA reverted to the older moniker of HUDF with the photo's release.)
Hubble Ultra-Deep Field
–
The original NASA release, containing galaxies of various ages, sizes, shapes, and colors. The smallest, reddest galaxies, of which there are approximately 10,000, are some of the most distant galaxies to have been imaged by an optical telescope, probably existing shortly after the
Big Bang.
Hubble Ultra-Deep Field
–
Location of the Hubble Ultra-Deep Field on the sky
Hubble Ultra-Deep Field
–
Diagram illustrating comparative sampling distance of the HUDF and the earlier
Hubble Deep Field.
11.
Spiral galaxies
–
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
Spiral galaxies
–
An example of a spiral galaxy, the
Pinwheel Galaxy (also known as Messier 101 or NGC 5457)
Spiral galaxies
–
Barred spiral galaxy UGC 12158.
Spiral galaxies
–
NGC 1300 in
infrared light.
Spiral galaxies
–
Spiral galaxy NGC 1345
12.
Magellanic Clouds
–
The Magellanic Clouds are two irregular dwarf galaxies visible from the southern hemisphere, they are members of the Local Group and are orbiting the Milky Way galaxy. Because they both show signs of a bar structure, they are reclassified as Magellanic spiral galaxies. Collectively they were known to Māori of New Zealand as Nga Patari-Kaihau or as Te Reporepo and were used as predictors of winds. The Magellanic Clouds have been known since the first millennium in Western Asia, the first preserved mention of the Large Magellanic Cloud is by the Persian astronomer Al Sufi. In Europe, the Clouds were first observed by Italian explorers Peter Martyr dAnghiera, subsequently, they were reported by Antonio Pigafetta, who accompanied the expedition of Ferdinand Magellan on its circumnavigation of the world in 1519–1522. However, naming the clouds after Magellan did not become widespread until much later, in Bayers Uranometria they are designated as nubecula major and nubecula minor. In the 1756 star map of the French astronomer Lacaille, they are designated as le Grand Nuage and le Petit Nuage. In Sri Lanka, from ancient times, these clouds have been referred to as the Maha Mera Paruwathaya meaning the great mountain, roughly 21° apart in the night sky, the true distance between them is roughly 75,000 light-years. Until the discovery of the Sagittarius Dwarf Elliptical Galaxy in 1994, the LMC lies about 160,000 light years away, while the SMC is around 200,000. The LMC is about twice the diameter of the SMC, for comparison, the Milky Way is about 100,000 ly across. Observation and theoretical evidence suggest that the Magellanic Clouds have both been greatly distorted by tidal interaction with the Milky Way as they close to it. Streams of neutral hydrogen connect them to the Milky Way and to each other and their gravity has affected the Milky Way as well, distorting the outer parts of the galactic disk. Aside from their different structure and lower mass, they differ from our galaxy in two major ways, first, they are gas-rich, a higher fraction of their mass is hydrogen and helium compared to the Milky Way. They are also more metal-poor than the Milky Way, the youngest stars in the LMC and SMC have a metallicity of 0.5 and 0.25 times solar, respectively. Both are noted for their nebulae and young populations, but as in our own galaxy their stars range from the very young to the very old. The Large Magellanic Cloud was host galaxy to a supernova, the brightest observed in four centuries. Measurements with the Hubble Space Telescope, announced in 2006, suggest the Magellanic Clouds may be moving too fast to be long term companions of the Milky Way. They suggest the reason for this is due to a past interaction with the LMC splitting the SMC, and they have dubbed this smaller remnant the Mini Magellanic Cloud
Magellanic Clouds
–
The Large and Small Magellanic Clouds
Magellanic Clouds
–
ALMA antennae bathed in red light. In the background there is the southern Milky Way on the left and the Magellanic Clouds at the top.
Magellanic Clouds
–
The
Large Magellanic Cloud (LMC)
Magellanic Clouds
–
Part of the
Small Magellanic Cloud (SMC)
13.
Large Magellanic Cloud
–
The Large Magellanic Cloud is a satellite galaxy of the Milky Way. The LMC has a diameter of about 14,000 light-years, the LMC is the fourth-largest galaxy in the Local Group, after the Andromeda Galaxy, the Milky Way, and the Triangulum Galaxy. The LMC is classified as a Magellanic spiral and it contains a very prominent bar in its center, suggesting that it may have been a barred dwarf spiral galaxy before its spiral arms were disrupted, likely by the Milky Ways gravity. The LMCs present irregular appearance is likely the result of interactions with both the Milky Way and the Small Magellanic Cloud. The first recorded mention of the Large Magellanic Cloud was by the Persian astronomer Abd al-Rahman al-Sufi Shirazi, the next recorded observation was in 1503–4 by Amerigo Vespucci in a letter about his third voyage. In this letter he mentions three Canopes, two bright and one obscure, bright refers to the two Magellanic Clouds, and obscure refers to the Coalsack, ferdinand Magellan sighted the LMC on his voyage in 1519, and his writings brought the LMC into common Western knowledge. The galaxy now bears his name, measurements with the Hubble Space Telescope, announced in 2006, suggest the Large and Small Magellanic Clouds may be moving too fast to be orbiting the Milky Way. The Large Magellanic Cloud is usually considered an irregular galaxy, however, it shows signs of a bar structure, and is often reclassified as a Magellanic-type dwarf spiral galaxy. The Large Magellanic Cloud has a prominent central bar and a spiral arm, the central bar seems to be warped so that the east and west ends are nearer the Milky Way than the middle. In 2014, measurements from the Hubble Space Telescope made it possible to determine that the LMC has a period of 250 million years. The LMC was long considered to be a galaxy that could be assumed to lie at a single distance from the Solar System. However, in 1986, Caldwell and Coulson found that field Cepheid variables in the northeast portion of the LMC lie closer to the Milky Way than Cepheids in the southwest portion. More recently, this inclined geometry for field stars in the LMC has been confirmed via observations of Cepheids, core helium-burning red clump stars, all three of these papers find an inclination of ~35°, where a face-on galaxy has an inclination of 0°. Further work on the structure of the LMC using the kinematics of stars showed that the LMCs disk is both thick and flared. These results were confirmed by Grocholski et al. who calculated distances to a number of clusters, the distance to the LMC has been calculated using a variety of standard candles, with Cepheid variables being one of the most popular. Cepheids have been shown to have a relationship between their absolute luminosity and the period over which their brightness varies, however, Cepheids appear to suffer from a metallicity effect, where Cepheids of different metallicities have different period–luminosity relations. Unfortunately, the Cepheids in the Milky Way typically used to calibrate the period–luminosity relation are more rich than those found in the LMC. Modern 8-meter-class optical telescopes have discovered eclipsing binaries throughout the Local Group, parameters of these systems can be measured without mass or compositional assumptions
Large Magellanic Cloud
–
The Large Magellanic Cloud
Large Magellanic Cloud
–
Small part of the Large Magellanic Cloud
Large Magellanic Cloud
–
Location of the Large Magellanic Cloud with respect to the
Milky Way and other satellite galaxies
Large Magellanic Cloud
–
Two very different glowing gas clouds in the Large Magellanic Cloud
14.
Barred spiral galaxy
–
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
Barred spiral galaxy
–
NGC 1300, viewed nearly face-on;
Hubble Space Telescope image
Barred spiral galaxy
–
Milky Way Galaxy Spiral Arms - based on
WISE data.
Barred spiral galaxy
–
NGC 2787
Barred spiral galaxy
–
NGC 4314
15.
Small Magellanic Cloud
–
The Small Magellanic Cloud, or Nebucula Minor, is a dwarf galaxy near the Milky Way. It is classified as an irregular galaxy. It has a diameter of about 7,000 light-years, contains several hundred million stars, the SMC contains a central bar structure and it is speculated that it was once a barred spiral galaxy that was disrupted by the Milky Way to become somewhat irregular. At a distance of about 200,000 light-years, it is one of the Milky Ways nearest neighbors and it is also one of the most distant objects that can be seen with the naked eye. With a mean declination of approximately −73 degrees, it can only be viewed from the Southern Hemisphere, since it has a very low surface brightness, it is best viewed from a dark site away from city lights. It forms a pair with the Large Magellanic Cloud, which lies a further 20 degrees to the east, in the southern hemisphere, the Magellanic clouds have long been included in the lore of native inhabitants, including south sea islanders and indigenous Australians. Persian astronomer Al Sufi labelled the larger of the two clouds as Al Bakr, the White Ox, european sailors may have first noticed the clouds during the Middle Ages when they were used for navigation. Portuguese and Dutch sailors called them the Cape Clouds, a name that was retained for several centuries, during the circumnavigation of the Earth by Ferdinand Magellan in 1519–22, they were described by Antonio Pigafetta as dim clusters of stars. In Johann Bayers celestial atlas Uranometria, published in 1603, he named the smaller cloud, in Latin, Nubecula means a little cloud. Between 1834 and 1838, John Frederick William Herschel made observations of the skies with his 14-inch reflector from the Royal Observatory at the Cape of Good Hope. While observing the Nubecula Minor, he described it as a mass of light with an oval shape. Within the area of this cloud he catalogued a concentration of 37 nebulae, in 1891, Harvard College Observatory opened an observing station at Arequipa in Peru. Between 1893 and 1906, under the direction of Solon Bailey, henrietta Swan Leavitt, an astronomer at the Harvard College Observatory, used the plates from Arequipa to study the variations in relative luminosity of stars in the SMC. This important period-luminosity relation allowed the distance to any other variable to be estimated in terms of the distance to the SMC. Hence, once the distance to the SMC was known with greater accuracy, using this period-luminosity relation, in 1913 the distance to the SMC was first estimated by Ejnar Hertzsprung. First he measured thirteen nearby cepheid variables to find the magnitude of a variable with a period of one day. By comparing this to the periodicity of the variables as measured by Leavitt and this later proved to be a gross underestimate of the true distance, but it did demonstrate the potential usefulness of this technique. Announced in 2006, measurements with the Hubble Space Telescope suggest the Large, there is a bridge of gas connecting the Small Magellanic Cloud with the Large Magellanic Cloud, which is evidence of tidal interaction between the galaxies
Small Magellanic Cloud
–
Small Magellanic Cloud. Source: Digitized Sky Survey 2
Small Magellanic Cloud
–
Panoramic Large and Small Magellanic Clouds as seen from
ESO 's
VLT observation site. The galaxies are on the left side of the image.
Small Magellanic Cloud
–
Location
16.
Galaxy morphological classification
–
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
Galaxy morphological classification
–
Tuning-fork style diagram of the Hubble sequence
Galaxy morphological classification
–
The Hubble sequence throughout the universe's history.
Galaxy morphological classification
Galaxy morphological classification
–
NGC 6782: a spiral galaxy (type SB(r)0/a) with three rings of different radii, as well as a bar.
17.
Lynx (constellation)
–
Lynx, named after the animal, is a constellation in the northern sky that was introduced in the 17th century by Johannes Hevelius. It is a faint constellation with its brightest stars forming a zigzag line, the orange giant Alpha Lyncis is the brightest star in the constellation, while the semiregular variable star Y Lyncis is a target for amateur astronomers. Six star systems have found to contain planets. Those of 6 Lyncis and HD75898 were discovered by the Doppler method, polish astronomer Johannes Hevelius formed the constellation in the 17th century from 19 faint stars that he observed with the unaided eye between the constellations Ursa Major and Auriga. Naming it Lynx because of its faintness, he challenged future stargazers to see it, Hevelius gave it the alternate name of Tiger in his catalogue as well, but kept the former name only in his atlas. English astronomer John Flamsteed adopted the constellation in his catalogue, published in 1712, Lynx is bordered by Camelopardalis to the north, Auriga to the west, Gemini to the southwest, Cancer to the south, Leo to the east and Ursa Major to the northeast. Covering 545.4 square degrees and 1. 322% of the sky, it ranks 28th of the 88 constellations in size. The three-letter abbreviation for the constellation, as adopted by the International Astronomical Union in 1922, is Lyn, the official constellation boundaries, as set by Eugène Delporte in 1930, are defined by a polygon of 20 segments. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 06h 16m 13. 76s and 09h 42m 50. 22s, while the coordinates are between +32. 97° and +61. 96°. On dark nights, the stars can be seen as a crooked line extending roughly between Camelopardalis and Leo, and north of the bright star Castor. Lynx is most readily observed from the winter to late summer to northern hemisphere observers. The whole constellation is visible to observers north of latitude 28°S, english astronomer Francis Baily gave a single star a Bayer designation—Alpha Lyncis—while Flamsteed numbered 44 stars, though several lie across the boundary in Ursa Major. Overall, there are 97 stars within the constellations borders brighter than or equal to apparent magnitude 6.5, the brightest star in this constellation is Alpha Lyncis, with an apparent magnitude of 3.14. It is a giant of spectral type K7III located 203 ±2 light-years distant from Earth. Around twice as massive as the Sun, it has exhausted the hydrogen at its core and has evolved away from the main sequence, the star has swollen to about 55 times the Suns radius and it is emitting roughly 673 times the luminosity of the Sun. The stellar atmosphere has cooled, giving it a surface temperature of 3,880 K, the only star with a proper name is Alsciaukat, also known as 31 Lyncis, located 380 ±10 light-years from Earth. This star is also a giant with around twice the Suns mass that has swollen. It is anywhere from 59 to 75 times as wide as the Sun, Alsciaukat is also a variable star, ranging in brightness by 0.05 magnitude over 25 to 30 days from its baseline magnitude of 4.25
Lynx (constellation)
–
Illustration from
Urania's Mirror (1825). The obsolete constellation
Telescopium Herschelii is to its right.
Lynx (constellation)
–
List of stars in Lynx
18.
Ursa Major
–
Ursa Major is a constellation in the northern celestial hemisphere. One of the 48 constellations listed by Ptolemy, it one of the 88 modern constellations. It can be throughout the year in most of the northern hemisphere. The Big Dipper and the constellation as a whole have mythological significance in world cultures. Ursa Major occupies an area in the northern celestial hemisphere. Its also the namesake of its family, which includes all the constellations it borders except for Leo. The three-letter abbreviation for the constellation, as adopted by the IAU in 1922, is UMa, the Big Dipper is an asterism within Ursa Major composed of seven bright stars that together comprise one of the best-known patterns in the sky. β Ursae Majoris, called Merak, with a magnitude of 2.37, γ Ursae Majoris, known as either Phecda or Phad, with a magnitude of 2.44. δ Ursae Majoris, or Megrez, meaning root of the tail, Alioth is the brightest star of Ursa Major and the 33rd-brightest in the sky, with a magnitude of 1.76. It is also the brightest of the peculiar A stars, magnetic stars whose chemical elements are either depleted or enhanced, ζ Ursae Majoris, Mizar, the second star in from the end of the handle of the Big Dipper, and the constellations fourth-brightest star. Mizar, which means girdle, forms a double star, with its optical companion Alcor. The ability to resolve the two stars with the eye is often quoted as a test of eyesight, although even people with quite poor eyesight can see the two stars. η Ursae Majoris, known as either Alkaid or Benetnash, both meaning the end of the tail, with a magnitude of 1.85, Alkaid is the third-brightest star of Ursa Major. Except for Dubhe and Alkaid, the stars of the Big Dipper all have proper motions heading toward a point in Sagittarius. A few other such stars have been identified, and together they are called the Ursa Major Moving Group, the stars Merak and Dubhe are known as the pointer stars because they are helpful for finding Polaris, also known as the North Star or Pole Star. By visually tracing a line from Merak through Dubhe and continuing, ones eye will land on Polaris, W Ursae Majoris is the prototype of a class of contact binary variable stars, and ranges between 7. 75m and 8. 48m. 47 Ursae Majoris is a Sun-like star with a three-planet system,47 Ursae Majoris b, discovered in 1996, orbits every 1078 days and is 2.53 times the mass of Jupiter. 47 Ursae Majoris c, discovered in 2001, orbits every 2391 days and is 0.54 times the mass of Jupiter
Ursa Major
–
Visible at latitudes between +
90 ° and −
30 °. Best visible at 21:00 (9 p.m.) during the month of April. The
Big Dipper or Plough.
Ursa Major
–
List of stars in Ursa Major
Ursa Major
–
The constellation Ursa Major as it can be seen by the unaided eye.
Ursa Major
–
Ursa Major and Ursa Minor in relation to Polaris
19.
Dwarf galaxy
–
A dwarf galaxy is a small galaxy composed of about 100 million up to several billion stars, a small number compared to the Milky Ways 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 galaxies, based on their shape. Current theory states that most galaxies, including galaxies, form in association with dark matter. However, NASAs Galaxy Evolution Explorer space probe identified new dwarf galaxies forming out of gases with low metallicity and 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 size, dwarf galaxies have been observed being pulled toward and ripped by neighbouring spiral galaxies. There are many galaxies in the Local Group, these small galaxies frequently orbit larger galaxies, such as the Milky Way, the Andromeda Galaxy. A2007 paper has suggested that dwarf galaxies were created by galactic tides during the early evolutions of the Milky Way. Tidal dwarf galaxies are produced when galaxies collide and their gravitational masses interact, streams of galactic material are pulled away from the parent galaxies and the halos of dark matter that surround them. These stars, the brightest of which are blue, cause the galaxy itself to appear blue in colour, most BCD galaxies are also classified as dwarf irregular galaxies or as dwarf lenticular galaxies. Because they are composed of 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 NGC1705, NGC2915, NGC3353 and UGCA281. Ultra-compact dwarf galaxies are a class of compact galaxies with very high stellar populations. They are thought to be on the order of 200 light years across, UCDs have been found in the Virgo Cluster, Fornax Cluster, Abell 1689, and the Coma Cluster, amongst others. In particular, a large sample of ~100 UCDs has been found in the core region of the Virgo cluster by the Next Generation Virgo Cluster Survey team
Dwarf galaxy
–
The
Large Magellanic Cloud, a satellite galaxy of the
Milky Way. Picture taken by
Hubble.
Dwarf galaxy
–
Dwarf galaxy DDO 68.
Dwarf galaxy
–
The
Phoenix Dwarf Galaxy is a dwarf irregular galaxy, featuring younger stars in its inner regions and older ones at its outskirts.
Dwarf galaxy
–
UGC 11411 is a galaxy known as an irregular blue compact dwarf (BCD) galaxy.
20.
Peculiar galaxy
–
A peculiar galaxy is a galaxy which is unusual in its size, shape, or composition. They can be irregular in shape due to the immense gravitational forces which act on them during encounters with other galaxies. They are the result of recent mergers between two or more normal galaxies, specifically, Peculiar galaxies may be from galaxy mergers and collisions that happened in the recent past. Peculiar galaxies are of size to regular sized Spiral and Elliptical galaxies. In addition, peculiar galaxies can be used by astronomers in order to determine the structure of regular galaxies, astronomers have identified two types of peculiar galaxies, interacting galaxies and active galactic nuclei. Peculiar galaxies are designated by p or pec in some catalogs and they are snapshots of galaxy mergers in the first millions of years after the collision. Thats when the galaxy is in an active state and still maintains some common features of the host galaxies. They constitute between 5% and 10% of the galaxy population, although most ‘normal’ galaxies will show peculiar features if examined carefully. Peculiar galaxies show a diversity of form. Many peculiar galaxies are experiencing episodes of enhanced star formation, called starbursts and they also show a greater tendency to host ACTIVE GALACTIC NUCLEI compared with the normal galaxy population Peculiar galaxies have been mapped by Halton Arp in his 1966 Atlas of Peculiar Galaxies
Peculiar galaxy
–
ESO 162-17 is a peculiar galaxy located about 40 million light-years away in the constellation of Carina.
21.
International Standard Book Number
–
The International Standard Book Number is a unique numeric commercial book identifier. An ISBN is assigned to each edition and variation of a book, for example, an e-book, a paperback and a hardcover edition of the same book would each have a different ISBN. The ISBN is 13 digits long if assigned on or after 1 January 2007, the method of assigning an ISBN is nation-based and varies from country to country, often depending on how large the publishing industry is within a country. The initial ISBN configuration of recognition was generated in 1967 based upon the 9-digit Standard Book Numbering created in 1966, the 10-digit ISBN format was developed by the International Organization for Standardization and was published in 1970 as international standard ISO2108. Occasionally, a book may appear without a printed ISBN if it is printed privately or the author does not follow the usual ISBN procedure, however, this can be rectified later. Another identifier, the International Standard Serial Number, identifies periodical publications such as magazines, the ISBN configuration of recognition was generated in 1967 in the United Kingdom by David Whitaker and in 1968 in the US by Emery Koltay. The 10-digit ISBN format was developed by the International Organization for Standardization and was published in 1970 as international standard ISO2108, the United Kingdom continued to use the 9-digit SBN code until 1974. The ISO on-line facility only refers back to 1978, an SBN may be converted to an ISBN by prefixing the digit 0. For example, the edition of Mr. J. G. Reeder Returns, published by Hodder in 1965, has SBN340013818 -340 indicating the publisher,01381 their serial number. This can be converted to ISBN 0-340-01381-8, the check digit does not need to be re-calculated, since 1 January 2007, ISBNs have contained 13 digits, a format that is compatible with Bookland European Article Number EAN-13s. An ISBN is assigned to each edition and variation of a book, for example, an ebook, a paperback, and a hardcover edition of the same book would each have a different ISBN. The ISBN is 13 digits long if assigned on or after 1 January 2007, a 13-digit ISBN can be separated into its parts, and when this is done it is customary to separate the parts with hyphens or spaces. Separating the parts of a 10-digit ISBN is also done with either hyphens or spaces, figuring out how to correctly separate a given ISBN number is complicated, because most of the parts do not use a fixed number of digits. ISBN issuance is country-specific, in that ISBNs are issued by the ISBN registration agency that is responsible for country or territory regardless of the publication language. Some ISBN registration agencies are based in national libraries or within ministries of culture, in other cases, the ISBN registration service is provided by organisations such as bibliographic data providers that are not government funded. In Canada, ISBNs are issued at no cost with the purpose of encouraging Canadian culture. In the United Kingdom, United States, and some countries, where the service is provided by non-government-funded organisations. Australia, ISBNs are issued by the library services agency Thorpe-Bowker
International Standard Book Number
–
A 13-digit ISBN, 978-3-16-148410-0, as represented by an
EAN-13 bar code
22.
Lenticular galaxy
–
A lenticular galaxy is a type of galaxy intermediate between an elliptical and a spiral galaxy in galaxy morphological classification schemes. Lenticular galaxies are galaxies that have used up or lost most of their interstellar matter. They may, however, retain significant dust in their disks, as a result, they consist mainly of aging stars. Because of their spiral arms, if they are inclined face-on it is often difficult to distinguish between them and elliptical galaxies. Despite the morphological differences, lenticular and elliptical galaxies share common properties like spectral features, both can be considered early-type galaxies that are passively evolving, at least in the local part of the Universe. Lenticular galaxies are unique in that they have a visible disk component as well as a prominent bulge component and they have much higher bulge-to-disk ratios than typical spirals and do not have the canonical spiral arm structure of late-type galaxies, yet may exhibit a central bar. This bulge dominance can be seen in the ratio distribution of a lenticular galaxy sample. The distribution for lenticular galaxies rises steadily in the range 0.25 to 0.85 whereas the distribution for spirals is essentially flat in that same range, larger axial ratios can be explained by observing face-on disk galaxies or by having a sample of spheroidal galaxies. Imagine looking at two disk galaxies edge-on, one with a bulge and one without a bulge, the galaxy with a prominent bulge will have a larger edge-on axial ratio compared to the galaxy without a bulge based on the definition of axial ratio. Thus a sample of galaxies with prominent spheroidal components will have more galaxies at larger axial ratios. The fact that the galaxy distribution rises with increasing observed axial ratio implies that lenticulars are dominated by a central bulge component. Lenticular galaxies are often considered to be a poorly understood transition state between spiral and elliptical galaxies, which results in their placement on the Hubble sequence. This results from lenticulars having both prominent disk and bulge components, the disk component is usually featureless, which precludes a classification system similar to spiral galaxies. As the bulge component is spherical, elliptical galaxy classifications are also unsuitable. Lenticular galaxies are divided into subclasses based upon either the amount of dust present or the prominence of a central bar. While ellipticals and spirals tend to have somewhat well-defined Sérsic profiles, the disk component of lenticular galaxies often has a very flat surface brightness distribution, especially in the outermost regions of the disk. Also, there is typically an observed truncation in the brightness of lenticular galaxies at ~4 scale radii of the disk. These features are consistent with the structure of spiral galaxies
Lenticular galaxy
–
NGC 2787 is an example of a lenticular galaxy with visible dust absorption. While this galaxy has been classified as an S0 galaxy, one can see the difficulty in differentiating between spirals, ellipticals, and lenticulars. Credit:
HST
Lenticular galaxy
–
The
Spindle Galaxy (NGC 5866), a lenticular galaxy in the
Draco constellation. This image shows that lenticular galaxies may retain a considerable amount of dust in their disk. There is little to no gas and thus they are considered deficient in
interstellar matter.
Lenticular galaxy
–
Hubble image of ESO 381-12.
Lenticular galaxy
–
NGC 4866 is a lenticular galaxy located in the constellation of Virgo.
23.
Flocculent spiral galaxy
–
A flocculent spiral galaxy is a type of spiral galaxy. Unlike the well-defined spiral architecture of a grand design spiral galaxy, flocculent galaxies are patchy, approximately 30% of spirals are flocculent, 10% are grand design, and the rest are referred to as multi-armed. The multiple-arm type is sometimes grouped into the flocculent category, the prototypical flocculent spiral is NGC2841. PDF A Near-Infrared Atlas of Spiral Galaxies, Debra Meloy Elmegreen,1981, doi,10. 1086/190757, Bibcode, 1981ApJS.47. 229E COSMOS astronomy encyclopedia - Flocculent Spiral
Flocculent spiral galaxy
–
NGC 4414, a flocculent spiral
Flocculent spiral galaxy
–
NGC 4414
Flocculent spiral galaxy
–
NGC 2841
Flocculent spiral galaxy
–
NGC 7793
24.
Grand design spiral galaxy
–
A grand design spiral galaxy is a type of spiral galaxy with prominent and well-defined spiral arms, as opposed to multi-arm and flocculent spirals which have subtler structural features. The spiral arms of a grand design galaxy extend clearly around the galaxy through many radians, as of 2002, approximately 10 percent of all currently known spiral galaxies are classified as grand design type spirals, including M81, M51, M74, M100, and M101. Density wave theory is the explanation for the well-defined structure of grand design spirals. According to this theory, the arms are created inside density waves that turn around the galaxy at different speeds from the stars in the galaxys disk. Stars are clumped in these regions due to gravitational attraction towards the dense material. This causes material to clump in the dense regions, at the center of these galaxies are supermassive black holes
Grand design spiral galaxy
–
A
Spitzer Space Telescope image of
Messier 81, a grand design spiral.
25.
Intermediate spiral galaxy
–
An intermediate spiral galaxy is a galaxy that is in between the classifications of a barred spiral galaxy and an unbarred spiral galaxy. It is designated as SAB in the galaxy morphological classification scheme, by definition, a galaxy is a congregation of stars held together by gravity. The first intermediate spiral galaxy discovered is the Milky Way, by Galileo and he was the first person with a telescope powerful enough to make such a discovery. Before Galileo, it was thought that all objects in the sky were either the planets in the Solar System, moons, comets. Until the beginning on the century, astronomers did not know the size of the Universe. Nothing was resolved at the debate, neither side was able to provide evidence to prove their side correct over their opponent. In 1923, Edwin Hubble resolved the matter with a photograph that he took of the Andromeda Galaxy, what he found in his photograph was a very bright light source pulsing at a certain rate, a Cepheid variable, located outside the Milky Way. This can be used to determine the distance to it, Hubble proved that the Universe was full of galaxies, and disproved that the Milky Way was the extent of the Universe. There are many types of galaxies in the Universe, elliptical, barred spiral galaxies, they vary in shape and size, a galaxy starts out as a giant cloud of cold gas. The cloud of gas must be close to zero, if the gas cloud is too hot, the atoms will have too much kinetic energy. When a cloud reaches a mass of about 109−1011 times the mass of the Sun, for an intermediate spiral galaxy to form, the gas cloud must be rotating, and as the gas coalesces, the gas cloud will flatten out to form a disk shape. The mass remaining constant, to comply with the laws which govern the conservation of angular momentum. At the very center of the galaxy, the gas will condense so much under pressure that a supermassive black hole will form. All spiral galaxies have a hole at the center of their galaxy, called the galactic nucleus. The Central Bulge contains Population II stars, which are old stars, devoid of metals, for this reason the central bulge glows an orange/red color. The disk of the galaxy is full of star formation, and Population I stars, star formation takes place in the spiral arms of the galaxy. The rotation velocity of the galaxy is nearly uniform throughout the entire disk, the reason for this is due to an even bigger halo of postulated dark matter that extends beyond the size of the galaxy. This dark matter is to date made up of a substance which radiates no light
Intermediate spiral galaxy
–
NGC 253 is an intermediate spiral galaxy located in the
constellation Sculptor.
Intermediate spiral galaxy
–
Under the de Vaucouleurs classification system, SAB-galaxies are intermediate between SA-galaxies and SB-galaxies
Intermediate spiral galaxy
–
Messier 65
Intermediate spiral galaxy
–
NGC 4725
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