Unbarred spiral galaxy
An unbarred spiral galaxy is a type of spiral galaxy without a central bar, or one, not a barred spiral galaxy. It is designated with an SA in the galaxy morphological classification scheme; the Sombrero Galaxy is an unbarred spiral galaxy. Barless spiral galaxies are one of three general types of spiral galaxies under the de Vaucouleurs system classification system, the other two being intermediate spiral galaxy and barred spiral galaxy. Under the Hubble tuning fork, it is one of two general types of spiral galaxy, the other being barred spirals
Metallicity
In astronomy, metallicity is used to describe the abundance of elements present in an object that are heavier than hydrogen or helium. Most of the physical 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 and helium"; this usage is distinct from the usual physical definition of a solid metal. For example and nebulae with high abundances of carbon, nitrogen and neon are called "metal-rich" in astrophysical terms though those elements are non-metals in chemistry; the presence of heavier elements hails from stellar nucleosynthesis, the theory that the majority of elements heavier than hydrogen and helium in the Universe are formed in the cores of stars as they evolve. Over time, stellar winds and supernovae deposit the metals into the surrounding environment, enriching the interstellar medium and providing recycling materials for the birth of new stars, it follows that older generations of stars, which formed in the metal-poor early Universe have lower metallicities than those of younger generations, which formed in a more metal-rich Universe.
Observed changes in the chemical abundances of different types of stars, based on the spectral peculiarities that were attributed to metallicity, led astronomer Walter Baade in 1944 to propose the existence of two different populations of stars. These became known as Population I and Population II stars. A third stellar population was introduced in 1978, known as Population III stars; these metal-poor stars were theorised to have been the "first-born" stars created in the Universe. Astronomers use several different methods to describe and approximate metal abundances, depending on the available tools and the object of interest; some methods include determining the fraction of mass, attributed to gas versus metals, or measuring the ratios of the number of atoms of two different elements as compared to the ratios found in the Sun. Stellar composition is simply defined by the parameters X, Y and Z. Here X is the mass fraction of hydrogen, Y is the mass fraction of helium, Z is the mass fraction of all the remaining chemical elements.
Thus X + Y + Z = 1.00. In most stars, nebulae, H II regions, other astronomical sources and helium are the two dominant elements; the hydrogen mass fraction is expressed as X ≡ m H / M, where M is the total mass of the system, m H is the fractional mass of the hydrogen it contains. The helium mass fraction is denoted as Y ≡ m He / M; the remainder of the elements are collectively referred to as "metals", the metallicity—the mass fraction of elements heavier than helium—can be calculated as Z = ∑ i > He m i M = 1 − X − Y. For the surface of the Sun, these parameters are measured to have the following values: Due to the effects of stellar evolution, neither the initial composition nor the present day bulk composition of the Sun is the same as its present-day surface composition; the overall stellar metallicity is defined using the total iron content of the star, as iron is among the easiest to measure with spectral observations in the visible spectrum. The abundance ratio is defined as the logarithm of the ratio of a star's iron abundance compared to that of the Sun and is expressed thus: = log 10 star − log 10 sun, where N Fe and N H are the number of iron and hydrogen atoms per unit of volume respectively.
The unit used for metallicity is the dex, contraction of "decimal exponent". By this formulation, stars with a higher metallicity than the Sun have a positive logarithmic value, whereas those with a lower metallicity than the Sun have a negative value. For example, stars with a value of +1 have 10 times the metallicity of the Sun. Young Population I stars have higher iron-to-hydrogen ratios than older Population II stars. Primordial Population III stars are estimated to have a metallicity of less than −6.0, that is, less than a millionth of the abundance of iron in the Sun. The same notation is used to express variations in abundances between other the individual elements as compared to solar proportions. For example, the notati
Pavo (constellation)
Pavo is a constellation in the southern sky whose name is Latin for "peacock." Pavo first appeared on a 35-cm diameter celestial globe published in 1598 in Amsterdam by Plancius and Jodocus Hondius and was depicted in Johann Bayer's star atlas Uranometria of 1603, was conceived by Petrus Plancius from the observations of Pieter Dirkszoon Keyser and Frederick de Houtman. French explorer and astronomer Nicolas-Louis de Lacaille gave its stars Bayer designations in 1756; the constellations Pavo, Grus and Tucana are collectively known as the "Southern Birds". The constellation's brightest member, Alpha Pavonis, is known as Peacock and appears as a 1.91-magnitude blue-white star, but is a spectroscopic binary. Delta Pavonis is a nearby Sun-like star. Six of the star systems in Pavo have been found to host planets, including HD 181433 with a super-earth, HD 172555 with evidence of a major interplanetary collision in the past few thousand years; the constellation contains NGC 6752, the third-brightest globular cluster in the sky, the spiral galaxy NGC 6744, which resembles the Milky Way but is twice as large.
Pavo displays an annual meteor shower known as the Delta Pavonids, whose radiant is near the star δ Pav. Pavo was one of the twelve constellations established by Petrus Plancius from the observations of the southern sky by explorers Pieter Dirkszoon Keyser and Frederick de Houtman, who had sailed on the first Dutch trading expedition, known as the Eerste Schipvaart, to the East Indies, it first appeared on a 35-cm diameter celestial globe published in 1598 in Amsterdam by Plancius with Jodocus Hondius. The first depiction of this constellation in a celestial atlas was in German cartographer Johann Bayer's Uranometria of 1603. De Houtman included it in his southern star catalogue the same year under the Dutch name De Pauww, "The Peacock". Pavo and the nearby constellations Phoenix and Tucana are collectively called the "Southern Birds". According to Mark Chartrand, former executive director of the National Space Institute, Plancius may not have been the first to designate this group of stars as a peacock: "In Greek myth the stars that are now the Peacock were Argos, builder of the ship Argo.
He was changed by the goddess Juno into a peacock and placed in the sky along with his ship." Indeed, the peacock "symboliz the starry firmament" for the Greeks, the goddess Hera was believed to drive through the heavens in a chariot drawn by peacocks. The peacock and the "Argus" nomenclature are prominent in a different myth, in which Io, a beautiful princess of Argos, was lusted after by Zeus. Zeus changed Io into a heifer to deceive his wife couple with her. Hera asked for the heifer as a gift. Zeus, unable to refuse such a reasonable request, reluctantly gave the heifer to Hera, who promptly banished Io and arranged for Argus Panoptes, a creature with one hundred eyes, to guard the now-pregnant Io from Zeus. Meanwhile, Zeus entreated Hermes to save Io. Hera adorned the tail of a peacock—her favorite bird—with Argus's eyes in his honor; as recounted in Ovid's Metamorphoses, the death of Argus Panoptes contains an explicit celestial reference: "Argus lay dead. Saturnia retrieved those eyes to set in place among the feathers of her bird and filled his tail with starry jewels."It is uncertain whether the Dutch astronomers had the Greek mythos in mind when creating Pavo but, in keeping with other constellations introduced by Plancius through Keyser and De Houtmann, the "peacock" in the new constellation referred to the green peacock, which the explorers would have encountered in the East Indies, rather than the blue peacock known to the ancient Greeks.
The Wardaman people of the Northern Territory in Australia saw the stars of Pavo and the neighbouring constellation Ara as flying foxes. Pavo is bordered by Telescopium to the north and Ara to the west, Octans to the south, Indus to the east and northeast. Covering 378 square degrees, it ranks 44th of the 88 modern constellations in size and covers 0.916% of the night sky. The three-letter abbreviation for the constellation, as adopted by the International Astronomical Union in 1922, is "Pav"; the official constellation boundaries, as set by Eugène Delporte in 1930, are defined by a polygon of 10 segments. In the equatorial coordinate system, the right ascension coordinates of these borders lie between 18h 10.4m and 21h 32.4m, while the declination coordinates are between −56.59° and −74.98°. As one of the deep southern constellations, it remains below the horizon at latitudes north of the 30th parallel in the Northern Hemisphere, is circumpolar at latitudes south of the 50th parallel in the Southern Hemisphere.
Although he depicted Pavo on his chart, Bayer did not assign its stars Bayer designations. French explorer and astronomer Nicolas-Louis de Lacaille labelled them Alpha to Omega in 1756, but omitted Psi and Xi, labelled two pairs of stars close together Mu and Phi Pavonis. In 1879, American astronomer Benjamin Gould designated a star Xi Pavonis as he felt its brightness warranted a name, but dropped Chi Pavonis due to its faintness. Lying near the constellation's northern border with Telescopium is Alpha Pavonis, the brightest star in Pavo, its proper name — Peacock — is an English translation of the constellation's name. It was assigned by the British Her Majesty's Nautical Almanac Office in the late 1930s. Alpha has an apparent magnitude of 1.91
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. The original release was combined from Hubble Space Telescope data accumulated over a period from September 24, 2003, through to January 16, 2004. Looking back 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 bright stars in the near-field, allowing much better viewing of dimmer, more distant objects. In August and September 2009, the HUDF field was observed at longer wavelengths using the infrared channel of the attached Wide Field Camera 3 instrument; 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 rectangular image is 2.4 arcminutes to an edge, or 3.4 arcminutes diagonally. This is one tenth of the angular diameter of a full moon viewed from Earth, smaller than a 1 mm by 1 mm square of paper held at 1 meter away, equal to one twenty-six-millionth of the total area of the sky.
The image is oriented. 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, revealing a galaxy theorized to be formed only 450 million years after the big bang event. 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. On January 23, 2019, the Instituto de Astrofísica de Canarias released an deeper version of the infrared images of the Hubble Ultra Deep Field obtained with the WFC3 instrument, named the ABYSS Hubble Ultra Deep Field; the new images improve the previous reduction of the WFC3/IR images, including careful sky background subtraction around the largest galaxies on the field of view. After this update, some galaxies were found to be twice as big as measured. In the years since the original Hubble Deep Field, the Hubble Deep Field South and the GOODS sample were analyzed, providing increased statistics at the high redshifts probed by the HDF.
When the Advanced Camera for Surveys detector was installed on the HST, it was realized that an ultra-deep field could observe galaxy formation out to higher redshifts than had been observed, as well as providing more information about galaxy formation at intermediate redshifts. 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. Following the workshop, the STScI Director Steven Beckwith decided to devote 400 orbits of Director's Discretionary time to the UDF and appointed Stiavelli as the lead of the Home Team implementing the observations. Unlike the Deep Fields, the HUDF does not lie in Hubble's Continuous Viewing Zone; the earlier observations, using the Wide Field and Planetary Camera 2 camera, were able to take advantage of the increased observing time on these zones by using wavelengths with higher noise to observe at times when earthshine contaminated the observations.
As with the earlier fields, this one was required to contain little emission from our galaxy, with little Zodiacal dust. The field was required to be in a range of declinations such that it could be observed both by southern hemisphere instruments, such as the Atacama Large Millimeter Array, northern hemisphere ones, such as those located on Hawaii, it was decided to observe a section of the Chandra Deep Field South, due to existing deep X-ray observations from Chandra X-ray Observatory and two interesting objects observed in the GOODS sample at the same location: a redshift 5.8 galaxy and a supernova. The coordinates of the field are right ascension 3h 32m 39.0s, declination −27° 47′ 29.1″. The field is 200 arcseconds to a side, with a total area of 11 square arcminutes, lies in the constellation of Fornax. Four filters were used on the ACS, centered on 435, 606, 775 and 850 nm, with exposure times set to give equal sensitivity in all filters; 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. In order to get the best resolution possible, the observations were dithered by pointing the telescope at different positions for each exposure—a process trialled with the Hubble Deep Field—so that the final image has a higher resolution than the pixels on their own would allow; the observations were done in two sessions, from September 23 to October 28, 2003, December 4, 2003, to January 15, 2004. The total exposure time is just under 1 million seconds, from 400 orbits, with a typical exposure time of 1200 seconds. In total, 800 ACS exposures were taken over the course of 11.3 days, 2 every orbit, NICMOS observed for 4.5 days. All the individual ACS exposures were processed and combined by Anton Koekemoer into a single set of scientifically useful images, each with a total exposure time ranging from 134,900 seconds to 347,100 seconds. To observe the whole 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 galax
Barred spiral galaxy
A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars. Bars are found in two thirds of all spiral galaxies. Bars affect both the motions of stars and interstellar gas within spiral galaxies and can affect spiral arms as well; the Milky Way Galaxy, where our own Solar System is located, is classified as a barred spiral galaxy. Edwin Hubble classified spiral galaxies of this type as "SB" in his Hubble sequence and arranged them into sub-categories based on how open the arms of the spiral are. SBa types feature bound arms, while SBc types are at the other extreme and have loosely bound arms. SBb-type galaxies lie in between the two. SB0 is a barred lenticular galaxy. A new type, SBm, was subsequently created to describe somewhat irregular barred spirals, such as the Magellanic Clouds, which were once classified as irregular galaxies, but have since been found to contain barred spiral structures. Among other types in Hubble's classifications for the galaxies are the spiral galaxy, elliptical galaxy and irregular galaxy.
Barred galaxies are predominant, with surveys showing that up to two-thirds of all spiral galaxies contain a bar. The current hypothesis is that the bar structure acts as a type of stellar nursery, fueling star birth at their centers; the bar is thought to act as a mechanism that channels gas inwards from the spiral arms through orbital resonance, in effect funneling the flow to create new stars. This process is thought to explain why many barred spiral galaxies have active galactic nuclei, such as that seen in the Southern Pinwheel Galaxy; the creation of the bar is thought to be the result of a density wave radiating from the center of the galaxy whose effects reshape the orbits of the inner stars. This effect builds over time to stars orbiting further out, which creates a self-perpetuating bar structure. Bars are thought to be temporary phenomena in the lives of spiral galaxies. Past a certain size the accumulated mass of the bar compromises the stability of the overall bar structure. Barred spiral galaxies with high mass accumulated in their center tend to have stubby bars.
Since so many spiral galaxies have bar structures, it is that they are recurring phenomena in spiral galaxy development. The oscillating evolutionary cycle from spiral galaxy to barred spiral galaxy is thought to take on the average about two billion years. Recent studies have confirmed the idea that bars are a sign of galaxies reaching full maturity as the "formative years" end. A 2008 investigation found that only 20 percent of the spiral galaxies in the distant past possessed bars, compared with about 65 percent of their local counterparts; the general classification is "SB". The sub-categories are based on how tight the arms of the spiral are. SBa types feature bound arms. SBc types are at the other extreme. SBb galaxies lie in between. SBm describes somewhat irregular barred spirals. SB0 is a barred lenticular galaxy. Galaxy morphological classification Galaxy formation and evolution Lenticular galaxy Firehose instability Britt, Robert Roy. "Milky Way’s Central Structure Seen with Fresh Clarity."
SPACE.com 16 August 2005. An article about the Spitzer Space Telescope's Milky Way discovery Devitt, Terry. "Galactic survey reveals a new look for the Milky Way." 16 August 2005. The original press release regarding the article above, from the Univ. of Wisconsin'Barred' Spiral Galaxy Pic Highlights Stellar Birth." SPACE.com 2 March 2001. Hastings and Jane Hastings. Classifying Galaxies: Barred Spirals, 1995. "Astronomers Find Multiple Generations of Star Formation in Central Starburst Ring of a Barred Spiral Galaxy." January 15, 2000. A press release concerning NGC 1326 Barred spirals come and go Sky & Telescope April 2002. "ESO Provides An Infrared Portrait of the Barred Spiral Galaxy Messier 83." November 29, 2001. A press release from the European Southern Observatory. Horton, Adam. "Spitzer NGC 1291 barred spiral galaxy seen in infrared." 22 October 2014
Bulge (astronomy)
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
Disc galaxy
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
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