A starburst galaxy is a galaxy undergoing an exceptionally high rate of star formation, as compared to the long-term average rate of star formation in the galaxy or the star formation rate observed in most other galaxies. For example, the star formation rate of the Milky Way galaxy is 3 M☉/yr, starburst galaxies can experience star formation rates that are more than a factor of 100 times greater. In a starburst galaxy, the rate of star formation is so large that the galaxy will consume all of its gas reservoir, from which the stars are forming, on a timescale much shorter than the age of the galaxy; as such, the starburst nature of a galaxy is a phase, one that occupies a brief period of a galaxy's evolution. The majority of starburst galaxies are in the midst of a merger or close encounter with another galaxy. Starburst galaxies include M82, NGC 4038/NGC 4039, IC 10. Starburst galaxies are defined by these three interrelated factors: The rate at which the galaxy is converting gas into stars.
The available quantity of gas from which stars can be formed. A comparison of the timescale on which star formation will consume the available gas with the age or rotation period of the galaxy. Used definitions include: Continued star-formation where the current SFR would exhaust the available gas reservoir in much less than the age of the Universe. Continued star-formation where the current SFR would exhaust the available gas reservoir in much less than the dynamical timescale of the galaxy; the current SFR, normalised by the past-averaged SFR, is much greater than unity. This ratio is referred to as the "birthrate parameter". Mergers and tidal interactions between gas-rich galaxies play a large role in driving starbursts. Galaxies in the midst of a starburst show tidal tails, an indication of a close encounter with another galaxy, or are in the midst of a merger. Interactions between galaxies that do not merge can trigger unstable rotation modes, such as the bar instability, which causes gas to be funneled towards the nucleus and ignites bursts of star formation near the galactic nucleus.
It has been shown that there is a strong correlation between the lopsidedness of a galaxy and the youth of its stellar population, with more lopsided galaxies having younger central stellar populations. As lopsidedness can be caused by tidal interactions and mergers between galaxies, this result gives further evidence that mergers and tidal interactions can induce central star formation in a galaxy and drive a starburst. Classifying types of starburst galaxies is difficult since starburst galaxies do not represent a specific type in and of themselves. Starbursts can occur in disk galaxies, irregular galaxies exhibit knots of starburst spread throughout the irregular galaxy. Astronomers classify starburst galaxies based on their most distinct observational characteristics; some of the categorizations include: Blue compact galaxies. These galaxies are low mass, low metallicity, dust-free objects; because they are dust-free and contain a large number of hot, young stars, they are blue in optical and ultraviolet colours.
It was thought that BCGs were genuinely young galaxies in the process of forming their first generation of stars, thus explaining their low metal content. However, old stellar populations have been found in most BCGs, it is thought that efficient mixing may explain the apparent lack of dust and metals. Most BCGs close interactions. Well-studied BCGs include IZw18, ESO338-IG04 and Haro11. Blue compact dwarf galaxies are small compact galaxies. Green Pea galaxies are small compact galaxies resembling primordial starbursts, they were found by citizen scientists taking part in the Galaxy Zoo project. Luminous infrared galaxies. Ultra-luminous Infrared Galaxies ULIRGs); these galaxies are extremely dusty objects. The ultraviolet radiation produced by the obscured star-formation is absorbed by the dust and reradiated in the infrared spectrum at wavelengths of around 100 micrometres; this explains the extreme red colours associated with ULIRGs. It is not known for sure that the UV radiation is produced purely by star-formation, some astronomers believe ULIRGs to be powered by active galactic nuclei.
X-ray observations of many ULIRGs that penetrate the dust suggest that many starburst galaxies are double-cored systems, lending support to the hypothesis that ULIRGs are powered by star-formation triggered by major mergers. Well-studied ULIRGs include Arp 220. Hyperluminous Infrared galaxies, sometimes called Submillimeter galaxies. Wolf-Rayet galaxies, galaxies where a large portion of the bright stars are Wolf-Rayet stars; the Wolf-Rayet phase is a short-lived phase in the life of massive stars 10% of the total life-time of these stars and as such any galaxy is to contain few of these. However, because the stars are both luminous and have distinctive spectral features, it is possible to identify these stars in the spectra of entire galaxies and doing so allows good constraints to be placed on the properties of the starbursts in these galaxies. Firstly, a starburst galaxy must have a large supply of gas available to form stars; the burst itself may be triggered by a close encounter with another galaxy, a collision with another galaxy, or by another process which forces material into the centre of the galaxy.
The inside of the starburst is quite an extreme environment. The large amount
NGC 4036 is the New General Catalogue identifier for a lenticular galaxy in the northern circumpolar constellation of Ursa Major. In the Carnegie Atlas of Galaxies, it is described as being "characterized by an irregular pattern of dust lanes threaded through the disc in an'embryonic' spiral pattern indicating a mixed S0/Sa form." It is located near the Big Dipper, a little to the north of the mid-way point between the stars Alpha Ursae Majoris and Delta Ursae Majoris. With a visual magnitude of 10.7, it can be dimly viewed using a 4 in aperture telescope. The visual dimensions of this galaxy are 2.703 × 1.027 arc minutes, with the major axis having a position angle of 85°. The galaxy is being viewed nearly edge-on, with an inclination of 18° to the line of sight from the Earth, it is moving away from us with a heliocentric radial velocity of 1,445 km/s. The estimated visual luminosity of the galaxy is 4.2×1010 L⊙, or 42 billion times the brightness of the Sun. Photographs show a wound spiral pattern in the galactic disk, with three dust lanes—the southern side of the galaxy appears dimmer due to the dust configuration.
The mass of the free gas and dust in the galaxy is about 1.7×109 M⊙ and 4.4×105 M⊙ with around 7×104 M⊙ of the gas being in an ionized state. This is a type of active galaxy known as a LINER, which means that it shows emission lines of ionized gas in the region of its nucleus. Chemically, the stars at the center of the nucleus have a higher metallicity than in the neighboring regions, it appears. The galactic bulge itself appears triaxial with the ionized gas concentrated near the center of this bulge region. Images of the galaxy with the Hubble Space Telescope show a wispy disk of dust on all sides of the nucleus, with what appears to be the tendrilous remains of an ionization cone leading away 4 arc seconds to the northeast; such features are common in Seyfert galaxies. On July 23, 2007, Type Ia supernova SN 2007gi was discovered by Koichi Itagaki near the central bulge of this galaxy, it reached peak magnitude around August 14 steadily declined in brightness thereafter. Materials identified in the spectrum included silicon and sulfur moving outward at velocities of around 15,500 km/s.
This is a 50% higher velocity than what is observed with supernovae of this type. NGC 4036 is a member of the LGG 266 galaxy group, along with NGC 4041, IC 758, UGC 7009, UGC 7019, it is located just 17 arc minutes from NGC 4041, the two form a pair with a projected separation of around 470 kly
Atlas of Peculiar Galaxies
The Atlas of Peculiar Galaxies is a catalog of peculiar galaxies produced by Halton Arp in 1966. A total of 338 galaxies are presented in the atlas, published in 1966 by the California Institute of Technology; the primary goal of the catalog was to present photographs of examples of the different kinds of peculiar structures found among galaxies. Arp realized that the reason why galaxies formed into spiral or elliptical shapes was not well understood, he perceived peculiar galaxies as small "experiments" that astronomers could use to understand the physical processes that distort spiral or elliptical galaxies. With this atlas, astronomers had a sample of peculiar galaxies; the atlas does not present a complete overview of every peculiar galaxy in the sky but instead provides examples of the different phenomena as observed in nearby galaxies. Because little was known at the time of publication about the physical processes that caused the different shapes, the galaxies in the atlas are sorted based on their appearance.
Objects 1–101 are individual peculiar spiral galaxies or spiral galaxies that have small companions. Objects 102 -- 145 are elliptical-like galaxies. Individual or groups of galaxies with neither elliptical nor spiral shapes are listed as objects 146–268. Objects 269–327 are double galaxies. Objects that do not fit into any of the above categories are listed as objects 332–338. Most objects are best known by their other designations, but a few galaxies are best known by their Arp numbers. Today, the physical processes that lead to the peculiarities seen in the Arp atlas are thought to be well understood. A large number of the objects have been interpreted as interacting galaxies, including M51, Arp 220, the Antennae Galaxies. A few of the galaxies are dwarf galaxies that do not have enough mass to produce enough gravity to allow the galaxies to form any cohesive structure. NGC 1569 is an example of one of the dwarf galaxies in the atlas. A few other galaxies are radio galaxies; these objects contain active galactic nuclei.
The atlas includes the nearby radio galaxies M87 and Centaurus A. Many of the peculiar associations present in the catalogue have been interpreted as galaxy mergers, though Arp refuted the idea, rather, that apparent associations were prime examples of ejections, he writes in "Seeing Red": For me, the whole lesson of the Atlas of Peculiar Galaxies was that galaxies are ejected material. The merger mania seems to be a first guess based on a cursory look at galaxies; the Atlas of Peculiar Galaxies contained a interesting class of galaxies called spirals with companions on the ends of arms. How had they gotten there? Not by accidental collisions or by the beginning of a merger process, fashionably used to "explain" everything in the extragalactic realm; these are dwarf galaxies or poorly defined spiral galaxies that have low surface brightnesses. Low surface brightness galaxies are quite common; the exception is NGC 2857, an Sc spiral galaxy. This category contains spiral galaxies with arms; this category contains spiral galaxies with arms.
Some spiral arm segments may appear detached because dust lanes in the spiral arms obscure the arms' starlight. Other spiral arms may appear segmented because of the presence of bright star clusters in the spiral arms. Most spiral galaxies contain two defined spiral arms, or they contain only fuzzy filamentary spiral structures. Galaxies with three well-defined spiral arms are rare. One-armed spiral galaxies are rare. In this case, the single spiral arm may be formed by a gravitational interaction with another galaxy; the spiral arms in these galaxies have an asymmetric appearance. One spiral arm may appear to be brighter than the other. In the photographic plates produced by Arp, the bright arm would look dark or "heavy". While most of these galaxies are asymmetric spiral galaxies, NGC 6365 is an interacting pair of galaxies where one of the two galaxies is viewed edge-on and just happens to lie where the spiral arm for the other face-on galaxy would be visible; these are galaxies. Some objects, such as IC 167, are ordinary spiral galaxies viewed from an unusual angle.
Other objects, such as UGC 10770, are interacting pairs of galaxies with tidal tails that look similar to spiral arms. Many of these spiral galaxies are interacting with the low surface brightness galaxies in the field of view. In some cases, however, it may be difficult to determine whether the companion is physically near the spiral galaxy or whether the companion is a foreground/background source or a source on the edge of the spiral galaxy. Again, many of these spiral galaxies are interacting with companion galaxies, although some of the identified companion galaxies may be foreground/background sources or bright star clusters within the individual galaxies. Galaxies in this category are always interacting sources; the most famous of these objects is the Whirlpool galaxy, composed of a spiral galaxy NGC 5194, interacting with a smaller elliptical galaxy NGC 5195. The inter
The apparent magnitude of an astronomical object is a number, a measure of its brightness as seen by an observer on Earth. The magnitude scale is logarithmic. A difference of 1 in magnitude corresponds to a change in brightness by a factor of 5√100, or about 2.512. The brighter an object appears, the lower its magnitude value, with the brightest astronomical objects having negative apparent magnitudes: for example Sirius at −1.46. The measurement of apparent magnitudes or brightnesses of celestial objects is known as photometry. Apparent magnitudes are used to quantify the brightness of sources at ultraviolet and infrared wavelengths. An apparent magnitude is measured in a specific passband corresponding to some photometric system such as the UBV system. In standard astronomical notation, an apparent magnitude in the V filter band would be denoted either as mV or simply as V, as in "mV = 15" or "V = 15" to describe a 15th-magnitude object; the scale used to indicate magnitude originates in the Hellenistic practice of dividing stars visible to the naked eye into six magnitudes.
The brightest stars in the night sky were said to be of first magnitude, whereas the faintest were of sixth magnitude, the limit of human visual perception. Each grade of magnitude was considered twice the brightness of the following grade, although that ratio was subjective as no photodetectors existed; this rather crude scale for the brightness of stars was popularized by Ptolemy in his Almagest and is believed to have originated with Hipparchus. In 1856, Norman Robert Pogson formalized the system by defining a first magnitude star as a star, 100 times as bright as a sixth-magnitude star, thereby establishing the logarithmic scale still in use today; this implies that a star of magnitude m is about 2.512 times as bright as a star of magnitude m + 1. This figure, the fifth root of 100, became known as Pogson's Ratio; the zero point of Pogson's scale was defined by assigning Polaris a magnitude of 2. Astronomers discovered that Polaris is variable, so they switched to Vega as the standard reference star, assigning the brightness of Vega as the definition of zero magnitude at any specified wavelength.
Apart from small corrections, the brightness of Vega still serves as the definition of zero magnitude for visible and near infrared wavelengths, where its spectral energy distribution approximates that of a black body for a temperature of 11000 K. However, with the advent of infrared astronomy it was revealed that Vega's radiation includes an Infrared excess due to a circumstellar disk consisting of dust at warm temperatures. At shorter wavelengths, there is negligible emission from dust at these temperatures. However, in order to properly extend the magnitude scale further into the infrared, this peculiarity of Vega should not affect the definition of the magnitude scale. Therefore, the magnitude scale was extrapolated to all wavelengths on the basis of the black-body radiation curve for an ideal stellar surface at 11000 K uncontaminated by circumstellar radiation. On this basis the spectral irradiance for the zero magnitude point, as a function of wavelength, can be computed. Small deviations are specified between systems using measurement apparatuses developed independently so that data obtained by different astronomers can be properly compared, but of greater practical importance is the definition of magnitude not at a single wavelength but applying to the response of standard spectral filters used in photometry over various wavelength bands.
With the modern magnitude systems, brightness over a wide range is specified according to the logarithmic definition detailed below, using this zero reference. In practice such apparent magnitudes do not exceed 30; the brightness of Vega is exceeded by four stars in the night sky at visible wavelengths as well as the bright planets Venus and Jupiter, these must be described by negative magnitudes. For example, the brightest star of the celestial sphere, has an apparent magnitude of −1.4 in the visible. Negative magnitudes for other bright astronomical objects can be found in the table below. Astronomers have developed other photometric zeropoint systems as alternatives to the Vega system; the most used is the AB magnitude system, in which photometric zeropoints are based on a hypothetical reference spectrum having constant flux per unit frequency interval, rather than using a stellar spectrum or blackbody curve as the reference. The AB magnitude zeropoint is defined such that an object's AB and Vega-based magnitudes will be equal in the V filter band.
As the amount of light received by a telescope is reduced by transmission through the Earth's atmosphere, any measurement of apparent magnitude is corrected for what it would have been as seen from above the atmosphere. The dimmer an object appears, the higher the numerical value given to its apparent magnitude, with a difference of 5 magnitudes corresponding to a brightness factor of 100. Therefore, the apparent magnitude m, in the spectral band x, would be given by m x = − 5 log 100 , more expressed in terms of common logarithms as m x
A supernova is an event that occurs upon the death of certain types of stars. Supernovae are more energetic than novae. In Latin, nova means "new", referring astronomically to what appears to be a temporary new bright star. Adding the prefix "super-" distinguishes supernovae from ordinary novae, which are far less luminous; the word supernova was coined by Walter Baade and Fritz Zwicky in 1931. Only three Milky Way, naked-eye supernova events have been observed during the last thousand years, though many have been seen in other galaxies; the most recent directly observed supernova in the Milky Way was Kepler's Supernova in 1604, but two more recent supernova remnants have been found. Statistical observations of supernovae in other galaxies suggest they occur on average about three times every century in the Milky Way, that any galactic supernova would certainly be observable with modern astronomical telescopes. Supernovae may expel much, if not all, of the material away from a star at velocities up to 30,000 km/s or 10% of the speed of light.
This drives an expanding and fast-moving shock wave into the surrounding interstellar medium, in turn, sweeping up an expanding shell of gas and dust, observed as a supernova remnant. Supernovae create and eject the bulk of the chemical elements produced by nucleosynthesis. Supernovae play a significant role in enriching the interstellar medium with the heavier atomic mass chemical elements. Furthermore, the expanding shock waves from supernovae can trigger the formation of new stars. Supernova remnants are expected to accelerate a large fraction of galactic primary cosmic rays, but direct evidence for cosmic ray production was found only in a few of them so far, they are potentially strong galactic sources of gravitational waves. Theoretical studies indicate that most supernovae are triggered by one of two basic mechanisms: the sudden re-ignition of nuclear fusion in a degenerate star or the sudden gravitational collapse of a massive star's core. In the first instance, a degenerate white dwarf may accumulate sufficient material from a binary companion, either through accretion or via a merger, to raise its core temperature enough to trigger runaway nuclear fusion disrupting the star.
In the second case, the core of a massive star may undergo sudden gravitational collapse, releasing gravitational potential energy as a supernova. While some observed supernovae are more complex than these two simplified theories, the astrophysical collapse mechanics have been established and accepted by most astronomers for some time. Owing to the wide range of astrophysical consequences of these events, astronomers now deem supernova research, across the fields of stellar and galactic evolution, as an important area for investigation; the earliest recorded supernova HB9 was viewed by Indians 5,000-years ago and recorded in the oldest Star chart. The SN 185, was viewed by Chinese astronomers in 185 AD; the brightest recorded supernova was SN 1006, which occurred in 1006 AD and was described by observers across China, Iraq and Europe. The observed supernova SN 1054 produced the Crab Nebula. Supernovae SN 1572 and SN 1604, the latest to be observed with the naked eye in the Milky Way galaxy, had notable effects on the development of astronomy in Europe because they were used to argue against the Aristotelian idea that the universe beyond the Moon and planets was static and unchanging.
Johannes Kepler began observing SN 1604 at its peak on October 17, 1604, continued to make estimates of its brightness until it faded from naked eye view a year later. It was the second supernova to be observed in a generation. There is some evidence that the youngest galactic supernova, G1.9+0.3, occurred in the late 19th century more than Cassiopeia A from around 1680. Neither supernova was noted at the time. In the case of G1.9+0.3, high extinction along the plane of the galaxy could have dimmed the event sufficiently to go unnoticed. The situation for Cassiopeia A is less clear. Infrared light echos have been detected showing that it was a type IIb supernova and was not in a region of high extinction. Before the development of the telescope, only five supernovae were seen in the last millennium. Compared to a star's entire history, the visual appearance of a galactic supernova is brief spanning several months, so that the chances of observing one is once in a lifetime. Only a tiny fraction of the 100 billion stars in a typical galaxy have the capacity to become a supernova, restricted to either those having large mass or extraordinarily rare kinds of binary stars containing white dwarfs.
However and discovery of extragalactic supernovae are now far more common. The first such observation was of SN 1885A in the Andromeda galaxy. Today and professional astronomers are finding several hundred every year, some when near maximum brightness, others on old astronomical photographs or plates. American astronomers Rudolph Minkowski and Fritz Zwicky developed the modern supernova classification scheme beginning in 1941. During the 1960s, astronomers found that the maximum intensities of supernovae could be used as standard candles, hence indicators of astronomical distances; some of the most distant supernovae observed in 2003, appeared dimmer than expected. This supports the view. Techniques were developed for reconstructing supernovae events that have no written records of being observed; the date of the Cassiopeia A supernova event was determined from light echoes off nebulae, while the age of supernova remnant RX J0852.0-4622 was estimated from temperature
A constellation is a group of stars that forms an imaginary outline or pattern on the celestial sphere representing an animal, mythological person or creature, a god, or an inanimate object. The origins of the earliest constellations go back to prehistory. People used them to relate stories of their beliefs, creation, or mythology. Different cultures and countries adopted their own constellations, some of which lasted into the early 20th century before today's constellations were internationally recognized. Adoption of constellations has changed over time. Many have changed in shape; some became popular. Others were limited to single nations; the 48 traditional Western constellations are Greek. They are given in Aratus' work Phenomena and Ptolemy's Almagest, though their origin predates these works by several centuries. Constellations in the far southern sky were added from the 15th century until the mid-18th century when European explorers began traveling to the Southern Hemisphere. Twelve ancient constellations belong to the zodiac.
The origins of the zodiac remain uncertain. In 1928, the International Astronomical Union formally accepted 88 modern constellations, with contiguous boundaries that together cover the entire celestial sphere. Any given point in a celestial coordinate system lies in one of the modern constellations; some astronomical naming systems include the constellation where a given celestial object is found to convey its approximate location in the sky. The Flamsteed designation of a star, for example, consists of a number and the genitive form of the constellation name. Other star patterns or groups called asterisms are not constellations per se but are used by observers to navigate the night sky. Examples of bright asterisms include the Pleiades and Hyades within the constellation Taurus or Venus' Mirror in the constellation of Orion.. Some asterisms, like the False Cross, are split between two constellations; the word "constellation" comes from the Late Latin term cōnstellātiō, which can be translated as "set of stars".
The Ancient Greek word for constellation is ἄστρον. A more modern astronomical sense of the term "constellation" is as a recognisable pattern of stars whose appearance is associated with mythological characters or creatures, or earthbound animals, or objects, it can specifically denote the recognized 88 named constellations used today. Colloquial usage does not draw a sharp distinction between "constellations" and smaller "asterisms", yet the modern accepted astronomical constellations employ such a distinction. E.g. the Pleiades and the Hyades are both asterisms, each lies within the boundaries of the constellation of Taurus. Another example is the northern asterism known as the Big Dipper or the Plough, composed of the seven brightest stars within the area of the IAU-defined constellation of Ursa Major; the southern False Cross asterism includes portions of the constellations Carina and Vela and the Summer Triangle.. A constellation, viewed from a particular latitude on Earth, that never sets below the horizon is termed circumpolar.
From the North Pole or South Pole, all constellations south or north of the celestial equator are circumpolar. Depending on the definition, equatorial constellations may include those that lie between declinations 45° north and 45° south, or those that pass through the declination range of the ecliptic or zodiac ranging between 23½° north, the celestial equator, 23½° south. Although stars in constellations appear near each other in the sky, they lie at a variety of distances away from the Earth. Since stars have their own independent motions, all constellations will change over time. After tens to hundreds of thousands of years, familiar outlines will become unrecognizable. Astronomers can predict the past or future constellation outlines by measuring individual stars' common proper motions or cpm by accurate astrometry and their radial velocities by astronomical spectroscopy; the earliest evidence for the humankind's identification of constellations comes from Mesopotamian inscribed stones and clay writing tablets that date back to 3000 BC.
It seems that the bulk of the Mesopotamian constellations were created within a short interval from around 1300 to 1000 BC. Mesopotamian constellations appeared in many of the classical Greek constellations; the oldest Babylonian star catalogues of stars and constellations date back to the beginning in the Middle Bronze Age, most notably the Three Stars Each texts and the MUL. APIN, an expanded and revised version based on more accurate observation from around 1000 BC. However, the numerous Sumerian names in these catalogues suggest that they built on older, but otherwise unattested, Sumerian traditions of the Early Bronze Age; the classical Zodiac is a revision of Neo-Babylonian constellations from the 6th century BC. The Greeks adopted the Babylonian constellations in the 4th century BC. Twenty Ptolemaic constellations are from the Ancient Near East. Another ten have the same stars but different names. Biblical scholar, E. W. Bullinger interpreted some of the creatures mentioned in the books of Ezekiel and Revelation as the middle signs of the four quarters of the Zodiac, with the Lion as Leo, the Bull as Taurus, the Man representing Aquarius and the Eagle standing in for Scorpio.
The biblical Book of Job also
A satellite galaxy is a smaller companion galaxy that travels on bound orbits within the gravitational potential of a more massive and luminous host galaxy. Satellite galaxies and their constituents are bound to their host galaxy, in the same way that planets within our own solar system are gravitationally bound to the Sun. While most satellite galaxies are dwarf galaxies, satellite galaxies of large galaxy clusters can be much more massive. Moreover, satellite galaxies are not the only astronomical objects that are gravitationally bound to larger host galaxies. For this reason, astronomers have defined galaxies as gravitationally bound collections of stars that exhibit properties that cannot be explained by a combination of baryonic matter and Newton's laws of gravity. For example, measurements of the orbital speed of stars and gas within spiral galaxies result in a velocity curve that deviates from the theoretical prediction; this observation has motivated various explanations such as the theory of dark matter and modifications to Newtonian dynamics.
Therefore, despite being satellites of host galaxies, globular clusters should not be mistaken for satellite galaxies. Satellite galaxies are not only more extended and diffuse compared to globular clusters, but are enshrouded in massive dark matter halos that are thought to have been endowed to them during the formation process. Satellite galaxies lead tumultuous lives due to their chaotic interactions with both the larger host galaxy and other satellites. For example, the host galaxy is capable of disrupting the orbiting satellites via tidal and ram pressure stripping; these environmental effects can remove large amounts of cold gas from satellites, this can result in satellites becoming quiescent in the sense that they have ceased to form stars. Moreover, satellites can collide with their host galaxy resulting in a minor merger. On the other hand, satellites can merge with one another resulting in a major merger. Galaxies are composed of empty space, therefore galaxy mergers do not involve collisions between objects from one galaxy and objects from the other, these events result in much more massive galaxies.
Astronomers seek to constrain the rate at which both minor and major mergers occur to better understand the formation of gigantic structures of gravitationally bound conglomerations of galaxies such as galactic groups and clusters. Prior to the 20th century, the notion that galaxies existed beyond our Milky Way was not well established. In fact, the idea was so controversial at the time that it led to what is now heralded as the "Shapley-Curtis Great Debate" aptly named after the astronomers Harlow Shapley and Heber Doust Curtis that debated the nature of "nebulae" and the size of the Milky Way at the National Academy of Sciences on April 26, 1920. Shapley argued that the Milky Way was the entire universe and that all of the observed "nebulae" resided within this region. On the other hand, Curtis argued that the Milky way was much smaller and that the observed nebulae were in fact galaxies similar to our own Milky Way; this debate was not settled until late 1923 when the astronomer Edwin Hubble measured the distance to M31 using Cepheid Variable stars.
By measuring the period of these stars, Hubble was able to estimate their intrinsic luminosity and upon combining this with their measured apparent magnitude he estimated a distance of 300 kpc, an order-of-magnitude larger than the estimated size of the universe made by Shapley. This measurement verified that not only was the universe much larger than expected, but it demonstrated that the observed nebulae were distant galaxies with a wide range of morphologies. Despite Hubble's discovery that the universe was teeming with galaxies, a majority of the satellite galaxies of the Milky Way and the Local Group remained undetected until the advent of modern astronomical surveys such as the Sloan Digital Sky Survey and the Dark Energy Survey. In particular, the Milky Way is known to host 59 satellite galaxies, however two of these satellites known as the Large Magellanic Cloud and Small Magellanic Cloud have been observable in the Southern Hemisphere with the unaided eye since ancient times. Modern cosmological theories of galaxy formation and evolution predict a much larger number of satellite galaxies than what is observed.
However, more recent high resolution simulations have demonstrated that the current number of observed satellites pose no threat to the prevalent theory of galaxy formation. Spectroscopic and kinematic observations of satellite galaxies have yielded a wealth of information, used to study, among other things, the formation and evolution of galaxies, the environmental effects that enhance and diminish the rate of star formation within galaxies and the distribution of dark matter within the dark matter halo; as a result, satellite galaxies serve as a testing ground for prediction made by cosmological models. As mentioned above, satellite galaxies are categorized as dwarf galaxies and therefore follow a similar Hubble classification scheme as their host with the minor addition of a lowercase "d" in front of the various standard types to designate the dwarf galaxy status; these types include dwarf irre