Together with irregular galaxies, spiral galaxies make up approximately 60 percent of the galaxies in the local universe.
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But dwarf irregular galaxies are likely to have formed from lightweight hydrogen and helium that remain pristine from when the universe was young.
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These events occur disproportionately often in dwarf irregular galaxies, which are thought to be similar to some of the earliest galaxies that populated the universe.
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They hope to localize more bursts to see whether they usually live in dwarf irregular galaxies, and whether they all appear alongside steady radio sources, both of which would support the newborn-magnetar theory.
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Most galaxies are either disc-shaped spirals or egg-shaped ellipticals, but there are some oddballs as well, such as lenticular galaxies (kind of a cross between spirals and ellipticals), irregular galaxies (which don't really have a discernable shape or structure), and extremely-low density galaxies known as ultra diffuse objects.
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Some irregular galaxies were once spiral or elliptical galaxies but were deformed by an uneven external gravitational force. Irregular galaxies may contain abundant amounts of gas and dust.Faulkes Telescope Educational Guide - Galaxies - Irregulars This is not necessarily true for dwarf irregulars.Walter, F. et al. Astophys J 661, 102 - 114, 2007 Irregular galaxies are commonly small, about one tenth the mass of the Milky Way galaxy.
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Strings of red H II regions delineate the arms of the Whirlpool Galaxy. H II regions are found only in spiral galaxies like the Milky Way and irregular galaxies. They are not seen in elliptical galaxies. In irregular galaxies, they may be dispersed throughout the galaxy, but in spirals they are most abundant within the spiral arms.
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Hunter's BS is from the University of Arizona (1975) and her PhD in astronomy from the University of Illinois in 1982. Her thesis was on the star-forming properties of irregular galaxies. It earned an award from the Astronomical Society of the Pacific.
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Deidre A. Hunter is an American astronomer at Lowell Observatory. Her primary research area is tiny irregular galaxies — their origins, evolution and star production, and the shapes that are formed. She uses many parts of the electromagnetic spectrum, and includes spectroscopy in her approach.
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Most galaxies are organized into distinct shapes that allow for classification schemes. They are commonly divided into spiral, elliptical and Irregular galaxies. As the name suggests, an elliptical galaxy has the cross-sectional shape of an ellipse. The stars move along random orbits with no preferred direction.
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Science of Earth Systems. Cengage Learning. p. 107. . Irregular galaxies do not fall into any of the regular classes of the Hubble sequence, and they are often chaotic in appearance, with neither a nuclear bulge nor any trace of spiral arm structure.Morgan, W. W. & Mayall, N. U. (1957).
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In the end, supernova explosions and strong stellar winds from the most massive stars in the resulting star cluster will disperse the gases of the H II region, leaving behind a cluster of stars which have formed, such as the Pleiades. H II regions can be observed at considerable distances in the universe, and the study of extragalactic H II regions is important in determining the distance and chemical composition of galaxies. Spiral and irregular galaxies contain many H II regions, while elliptical galaxies are almost devoid of them. In spiral galaxies, including our Milky Way, H II regions are concentrated in the spiral arms, while in irregular galaxies they are distributed chaotically.
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In contrast, dwarf elliptical galaxies, dwarf irregular galaxies, and the dwarf versions of Magellanic type galaxies (which may be considered transitory between spiral and irregular in terms of morphology) are very common. It is suggested that dwarf spiral galaxies can transform into dwarf elliptical galaxies, especially in dense cluster environments.
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Only when the available gas is nearly consumed or dispersed does the starburst activity end. Starbursts are often associated with merging or interacting galaxies. The prototype example of such a starburst-forming interaction is M82, which experienced a close encounter with the larger M81. Irregular galaxies often exhibit spaced knots of starburst activity.
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Magellanic spiral galaxies are (usually) dwarf galaxies which are classified as the type Sm (and SAm, SBm, SABm). They are galaxies with one single spiral arm, and are named after their prototype, the Large Magellanic Cloud, an SBm galaxy. They can be considered to be intermediate between dwarf spiral galaxies and irregular galaxies.
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M74 is the brightest member of the M74 Group, a group of 5–7 galaxies that also includes the peculiar spiral galaxy NGC 660 and a few irregular galaxies. Although different group identification methods may consistently identify many of the same member galaxies in this group, the exact group membership is still uncertain.
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Blue compact dwarf PGC 51017. In astronomy, a blue compact dwarf galaxy (BCD galaxy) is a small galaxy which contains large clusters of young, hot, massive stars. These stars, the brightest of which are blue, causes the galaxy itself to appear blue in colour. Most BCD galaxies are also classified as dwarf irregular galaxies or as dwarf lenticular galaxies.
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SAm galaxies are a type of unbarred spiral galaxy, while SBm are a type of barred spiral galaxy.Linda S. Sparke, John Sill Gallagher, Galaxies in the Universe: An Introduction, 2ed., Cambridge University Press, 2007, SABm are a type of intermediate spiral galaxy. Type Sm and Im galaxies have also been categorized as irregular galaxies with some structure (type Irr-1).
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Spiral galaxies are typically surrounded by a halo of older stars. Both the Milky Way and one of our nearest galaxy neighbors, the Andromeda Galaxy, are spiral galaxies. Irregular galaxies are chaotic in appearance, and are neither spiral nor elliptical. About a quarter of all galaxies are irregular, and the peculiar shapes of such galaxies may be the result of gravitational interaction.
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A barred irregular galaxy is an irregular version of a barred spiral galaxy. Examples include the Large Magellanic Cloud and NGC 6822.Norbert Przybilla, Quantitative Spectroscopy of Supergiants, Munich, 2002 Some barred irregular galaxies (like the Large Magellanic Cloud) may be dwarf spiral galaxies, which have been distorted into an irregular shape by tidal interactions with a more massive neighbor.
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Shells are generally considered to have formed after the accretion of a smaller galaxy by a massive one. It has weak radio wave emission. NGC 7196 is the foremost member of a galaxy group known as the NGC 7196 group, which also includes NGC 7200 and some dwarf elliptical and irregular galaxies. In the same galaxy cloud lies NGC 7168.
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Sm galaxies are typically disrupted and asymmetric.citeBase; Neutral Hydrogen in the Interacting Magellanic Spirals NGC 4618/4625 ; Stephanie J. Bush; Eric M. Wilcots; (accessed 1 March 2009) dSm galaxies are dwarf spiral galaxies or dwarf irregular galaxies, depending on categorization scheme. The Magellanic spiral classification was introduced by Gerard de Vaucouleurs, along with Magellanic irregular (Im), when he revamped the Hubble classification of galaxies.
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The M74 Group (also known as the NGC 628 Group) is a small group of galaxies in the constellation Pisces. The face-on spiral galaxy M74 (NGC 628) is the brightest galaxy within the group. Other members include the peculiar spiral galaxy NGC 660 and several smaller irregular galaxies . The M74 Group is one of many galaxy groups that lie within the Virgo Supercluster.
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The Milky Way is a barred spiral, although the bar itself is difficult to observe from Earth's current position within the galactic disc. The most convincing evidence for the stars forming a bar in the galactic center comes from several recent surveys, including the Spitzer Space Telescope. Together with irregular galaxies, spiral galaxies make up approximately 60% of galaxies in today's universe. They are mostly found in low-density regions and are rare in the centers of galaxy clusters.
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The Magellanic Cloud galaxies were once classified as irregular galaxies. The Large Magellanic Cloud has since been re-classified as type SBm Corso, G. and Buscombe, W. The Observatory, 90, 229 - 233 (1970) On the spiral structure of the Large Magellanic Cloud a type of barred spiral galaxy, the barred Magellanic spiral type. The Small Magellanic Cloud remains classified as an irregular galaxy of type Im under current Galaxy morphological classification, although it does contain a bar structure.
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Due to redshift the most distant objects in the HDF (Lyman-break galaxies) are not actually visible in the Hubble images; they can only be detected in images of the HDF taken at longer wavelengths by ground- based telescopes.Ferguson et al. (1999), p.105 The HDF galaxies contained a considerably larger proportion of disturbed and irregular galaxies than the local universe; galaxy collisions and mergers were more common in the young universe as it was much smaller than today.
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Arp 299 (parts of it are also known as IC 694 and NGC 3690) is a pair of colliding galaxies approximately 134 million light-years away in the constellation Ursa Major. Both of the galaxies involved in the collision are barred irregular galaxies. It is not completely clear which object is historically called IC 694. According to some sources, the small appendage more than an arcminute northwest of the main pair is actually IC 694, not the primary (eastern) companion.
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Buat attended École normale supérieure Paris-Saclay from 1982 to 1986, after which she did her Master of Advanced Studies in astrophysics at the Paris Observatory. She wrote her PhD thesis, entitled Ultraviolet emission of spiral and irregular galaxies. Interpretation in terms of star formation at the Paris Diderot University under the supervision of Jean-Michel Deharveng in 1989. In 2003, Buat was appointed professor at the Aix-Marseille University, before which she worked at the university as an assistant professor.
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O type main-sequence stars and the most massive of the B type blue-white stars become supergiants. Due to their extreme masses, they have short lifespans, between 30 million years and a few hundred thousand years. They are mainly observed in young galactic structures such as open clusters, the arms of spiral galaxies, and in irregular galaxies. They are less abundant in spiral galaxy bulges and are rarely observed in elliptical galaxies, or globular clusters, which are composed mainly of old stars.
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It is believed that giant elliptical galaxies form when spirals and irregular galaxies collide. The wealth of galaxies at different stages of their evolution also allowed astronomers to estimate the variation in the rate of star formation over the lifetime of the Universe. While estimates of the redshifts of HDF galaxies are somewhat crude, astronomers believe that star formation was occurring at its maximum rate 8–10 billion years ago, and has decreased by a factor of about 10 since then.Connolly et al.
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The of elliptical galaxies paints them as galaxies where star formation finished after an initial burst at high-redshift, leaving them to shine with only their aging stars. 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, hot stars in their spiral arms. Very little star formation is thought to occur in elliptical galaxies, because of their lack of gas compared to spiral or irregular galaxies.
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This can result in a migration to the main body of the galaxy and a loss of cluster members through internal close encounters. Open clusters generally survive for a few hundred million years, with the most massive ones surviving for a few billion years. In contrast, the more massive globular clusters of stars exert a stronger gravitational attraction on their members, and can survive for longer. Open clusters have been found only in spiral and irregular galaxies, in which active star formation is occurring.
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Leo T is a dwarf galaxy situated in the Leo constellation and discovered in 2006 in the data obtained by Sloan Digital Sky Survey. The galaxy is located at the distance of about 420 kpc from the Sun and moves away from the Sun with the velocity of about 35 km/s. The velocity with respect to the Milky Way is around −60 km/s implying a slow infall onto the Milky Way. Leo T is classified as a transitional object ('T' in the name) between dwarf spheroidal galaxies (dSph) and dwarf irregular galaxies (dIrr).
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This consensus is based largely on two lines of evidence. First, long gamma-ray bursts are found without exception in systems with abundant recent star formation, such as in irregular galaxies and in the arms of spiral galaxies. This is strong evidence of a link to massive stars, which evolve and die within a few hundred million years and are never found in regions where star formation has long ceased. This does not necessarily prove the collapsar model (other models also predict an association with star formation) but does provide significant support.
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However, Leo A is nearly unique among irregular galaxies in that more than 90% of its stars formed more recently than 8 billion years ago, suggesting a rather unusual evolutionary history. The presence of RR Lyrae variables shows that the galaxy has an old stellar population that is up to 10 billion years in age. Leo A is at a distance of about 2.5 million light-years, and is a member of the Local Group of galaxies. The neutral hydrogen in this galaxy occupies in a volume similar to its optical extent, and is distributed in a squashed, uneven ring.
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Its luminosity is about . Like most irregular galaxies, NGC 5204 is relatively rich in gas and dust, although it lacks any prominent nebulas or broad areas of vigorous star formation. Despite its relatively diffuse distribution of stars, the galaxy does have several clusters of hot, young stars that are thought to be the location of most of the eleven known X-ray sources. The galaxy also appears to have a larger than normal dark matter component as the estimated mass of its visible portion does not adequately explain the observed rotation curve of the individual stars, even very close to its center.
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LEDA 2108986 has accreted a gas disk which counter-rotates relative to its stellar disk. It also displays a young spiral pattern within this stellar disk. The presence of such faint disk structures and rotation within some dwarf early-type galaxies in galaxy clusters has often been heralded as evidence that they were once late-type spiral or dwarf irregular galaxies prior to experiencing a cluster-induced transformation, known as galaxy harassment. The extreme isolation of LEDA 2108986 is proof that dwarf early- type galaxies can be built by accretion events, as opposed to disk-stripping scenarios within the "galaxy harassment" model.
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This is the largest observed disk in the nearby universe and it is a similar size to the disk of Malin 1. This disk appears asymmetric, with the east side showing a rapid decline in column density between 100 and 120 kpc, while the decline in the west side is smoother. Inside the disk are observed regions with higher brightness. They are compact, measuring less than 10kpc across and have a mass similar to that of dwarf irregular galaxies, and they hold about 10% of the galactic HI. It has been suggested that these clumps of HI are the remnants of accreted galaxies.
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The spectrograph was to be used instead to study extended, low-surface-brightness gaseous nebulae or irregular galaxies. Mayall's thesis advisor, William Hammond Wright, and the then head of the Lick stellar spectroscopy program, Joseph Haines Moore, encouraged him to develop his spectrograph. The device was constructed by the Lick Observatory's own workshop, and proved to be more efficient for extended, low- surface-brightness objects, particularly in the ultraviolet part of the spectrum, thus confirming the expectations of Mayall. With Wright's strong encouragement, Mayall had used fused quartz to make ultraviolet transmitting optics, whereas the Mt. Wilson spectrographs used heavy glass lenses and prisms, which absorb ultraviolet radiation.
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As the name suggests, such galaxies are irregular and chaotic in appearance, lacking central bulges and spiral arms — they are distinctly different from spirals or ellipticals. It is thought that irregular galaxies may once have been spirals or ellipticals, but became distorted over time through external gravitational forces during interactions or mergers with other galaxies. Dwarf irregulars in particular are important to our overall understanding of galactic evolution, as they are thought to be similar to the first galaxies that formed in the Universe. IC 4710 lies in the southern constellation of Pavo (The Peacock), which also contains the spiral galaxy which is mimic to Milky Way, NGC 6744.
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In fact, current evidence suggests the opposite: the early Universe appears to be dominated by spiral and irregular galaxies. In the currently favored picture of galaxy formation, present-day ellipticals formed as a result of mergers between these earlier building blocks; while some lenticular galaxies may have formed this way, others may have accreted their disks around pre-existing spheroids.Graham, Alister W.; Dullo, Bililign T.; Savorgnan, Giulia A. D. (2015), Hiding in Plain Sight: An Abundance of Compact Massive Spheroids in the Local Universe Some lenticular galaxies may also be evolved spiral galaxies, whose gas has been stripped away leaving no fuel for continued star formation, although the galaxy LEDA 2108986 opens the debate on this.
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NGC 346, an open cluster in the Small Magellanic Cloud There are over 1,000 known open clusters in our galaxy, but the true total may be up to ten times higher than that. In spiral galaxies, open clusters are largely found in the spiral arms where gas densities are highest and so most star formation occurs, and clusters usually disperse before they have had time to travel beyond their spiral arm. Open clusters are strongly concentrated close to the galactic plane, with a scale height in our galaxy of about 180 light years, compared to a galactic radius of approximately 50,000 light years. In irregular galaxies, open clusters may be found throughout the galaxy, although their concentration is highest where the gas density is highest.
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The two are 88,000 light-years apart and lie around 18 million light-years away. These two plus another four nearby dwarf irregular galaxies constitute the NGC 672 group, and all six appear to have had a burst of star formation in the last ten million years. The group is thought connected to another group of six galaxies known as the NGC 784 group, named for its principal galaxy, the barred spiral NGC 784. Together with two isolated dwarf galaxies, these fourteen appear to be moving in a common direction and constitute a group possibly located on a dark matter filament. 3C 48 was the first quasar ever to be observed, although its true identity was not uncovered until after that of 3C 273 in 1963.
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Two very different glowing gas clouds in the Large Magellanic Cloud Like many irregular galaxies, the LMC is rich in gas and dust, and is currently undergoing vigorous star formation activity. It holds the Tarantula Nebula, the most active star-forming region in the Local Group. NGC 1783 is one of the biggest globular clusters in the Large Magellanic Cloud The LMC has a wide range of galactic objects and phenomena that make it known as an "astronomical treasure-house, a great celestial laboratory for the study of the growth and evolution of the stars", per Robert Burnham Jr. Surveys of the galaxy have found roughly 60 globular clusters, 400 "planetary nebulae", and 700 open clusters, along with hundreds of thousands of giant and supergiant stars.Burnham (1978), 840–848.
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Elliptical and lenticular galaxies are commonly referred to together as “early-type” galaxies, while spirals and irregular galaxies are referred to as “late types”. This nomenclature is the source of the common, but erroneous, belief that the Hubble sequence was intended to reflect a supposed evolutionary sequence, from elliptical galaxies through lenticulars to either barred or regular spirals. In fact, Hubble was clear from the beginning that no such interpretation was implied: > The nomenclature, it is emphasized, refers to position in the sequence, and > temporal connotations are made at one's peril. The entire classification is > purely empirical and without prejudice to theories of evolution... The evolutionary picture appears to be lent weight by the fact that the disks of spiral galaxies are observed to be home to many young stars and regions of active star formation, while elliptical galaxies are composed of predominantly old stellar populations.
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Venn was born and raised in Toronto, Canada, receiving her BSc from the University of Toronto in Physics & Astronomy in 1987. She then received her PhD in Astronomy from the University of Texas at Austin in 1994 working with Christopher Sneden and David L. Lambert on the evolution of massive stars, and pursued postdoctoral research at the Max Planck Institute for Astrophysics/Universitäts-Sternwarte_München working with Rolf-Peter Kudritzki on the properties of massive stars in the Magellanic Clouds and other Local Group dwarf irregular galaxies. She held a Clare Boothe Luce Professorship in Physics & Astronomy at Macalester College from 1996–2004, then moved to the University of Victoria as a Canada Research Chair (Tier II) in Exploration and Understanding of Space and Professor of Physics & Astronomy. She has served on numerous science advisory, time allocation, and funding committees, and currently represents Canada on the Thirty Meter Telescope Board of Governors.
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In contrast to star formation in density-wave theories, which are limited to disk-shaped galaxies and produce global spiral patterns, SSPSF applies equally well to spirals, to irregular galaxies and to any local concentrations of gas in elliptical galaxies. The effect may be envisioned as an "SIR infection model" in a differentially rotating disk, the host galaxy. The SIR model (perhaps most popularly familiar in the form of Conway's Game of Life) is applied to star formation propagating through the galaxy: Each generation of stars in a neighborhood includes some massive ones whose stellar winds and, soon, supernovae, produce shock waves in the gas (Susceptible material). These lead to collapsing nearby gas clouds, which produce the next generation of stars (Infection propagation); but in the immediate neighborhood, all initially available gas is used, so no further stars are born there for some period of time despite the shocks (Recovery from infection).
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Tully and Fisher used optical luminosity, but subsequent work showed the relation to be tighter when defined using microwave to infrared (K band) radiation (a good proxy for stellar mass), and even tighter when luminosity is replaced by the galaxy's total baryonic mass (the sum of its mass in stars and gas). This latter form of the relation is known as the Baryonic Tully–Fisher relation (BTFR), and states that baryonic mass is proportional to velocity to the power of roughly 3.5–4.S. Torres-Flores, B. Epinat, P. Amram, H. Plana, C. Mendes de Oliveira (2011), "GHASP: an Hα kinematic survey of spiral and irregular galaxies -- IX. The NIR, stellar and baryonic Tully–Fisher relations", The TFR can be used to estimate the distance to spiral galaxies by allowing the luminosity of a galaxy to be derived from its directly measurable line width. The distance can then be found by comparing the luminosity to the apparent brightness.
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This has important uses all over astronomy, from detecting binary stars, exoplanets, compact objects such as neutron stars and black holes (by the motion of hydrogen in accretion disks around them), identifying groups of objects with similar motions and presumably origins (moving groups, star clusters, galaxy clusters, and debris from collisions), determining distances (actually redshifts) of galaxies or quasars, and identifying unfamiliar objects by analysis of their spectrum. Balmer lines can appear as absorption or emission lines in a spectrum, depending on the nature of the object observed. In stars, the Balmer lines are usually seen in absorption, and they are "strongest" in stars with a surface temperature of about 10,000 kelvins (spectral type A). In the spectra of most spiral and irregular galaxies, active galactic nuclei, H II regions and planetary nebulae, the Balmer lines are emission lines. In stellar spectra, the H-epsilon line (transition 7→2, 397.007 nm) is often mixed in with another absorption line caused by ionized calcium known as "H" (the original designation given by Joseph von Fraunhofer).
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