The Andromeda Galaxy known as Messier 31, M31, or NGC 224, is a spiral galaxy 780 kiloparsecs from Earth, the nearest major galaxy to the Milky Way. Its name stems from the area of the Earth's sky; the virial mass of the Andromeda Galaxy is of the same order of magnitude as that of the Milky Way, at a trillion solar masses. The mass of either galaxy is difficult to estimate with any accuracy, but it was long thought that the Andromeda Galaxy is more massive than the Milky Way by a margin of some 25% to 50%; this has been called into question by a 2018 study which cited a lower estimate on the mass of the Andromeda Galaxy, combined with preliminary reports on a 2019 study estimating a higher mass of the Milky Way. The Andromeda Galaxy has a diameter of about 220,000 light-years, making it the largest member of the Local Group at least in terms of extension, if not mass; the number of stars contained in the Andromeda Galaxy is estimated at one trillion, or twice the number estimated for the Milky Way.
The Milky Way and Andromeda galaxies are expected to collide in ~4.5 billion years, merging to form a giant elliptical galaxy or a large disc galaxy. With an apparent magnitude of 3.4, the Andromeda Galaxy is among the brightest of the Messier objects making it visible to the naked eye from Earth on moonless nights when viewed from areas with moderate light pollution. Around the year 964, the Persian astronomer Abd al-Rahman al-Sufi described the Andromeda Galaxy, in his Book of Fixed Stars as a "nebulous smear". Star charts of that period labeled it as the Little Cloud. In 1612, the German astronomer Simon Marius gave an early description of the Andromeda Galaxy based on telescopic observations; the German philosopher Immanuel Kant in 1755 in his work Universal Natural History and Theory of the Heavens conjectured that the blurry spot was an island universe. In 1764, Charles Messier cataloged Andromeda as object M31 and incorrectly credited Marius as the discoverer despite it being visible to the naked eye.
In 1785, the astronomer William Herschel noted a faint reddish hue in the core region of Andromeda. He believed Andromeda to be the nearest of all the "great nebulae", based on the color and magnitude of the nebula, he incorrectly guessed that it is no more than 2,000 times the distance of Sirius. In 1850, William Parsons, 3rd Earl of Rosse and made the first drawing of Andromeda's spiral structure. In 1864, William Huggins noted; the spectra of Andromeda displays a continuum of frequencies, superimposed with dark absorption lines that help identify the chemical composition of an object. Andromeda's spectrum is similar to the spectra of individual stars, from this, it was deduced that Andromeda has a stellar nature. In 1885, a supernova was seen in the first and so far only one observed in that galaxy. At the time Andromeda was considered to be a nearby object, so the cause was thought to be a much less luminous and unrelated event called a nova, was named accordingly. In 1887, Isaac Roberts took the first photographs of Andromeda, still thought to be a nebula within our galaxy.
Roberts mistook Andromeda and similar spiral nebulae as solar systems being formed. In 1912, Vesto Slipher used spectroscopy to measure the radial velocity of Andromeda with respect to our Solar System—the largest velocity yet measured, at 300 kilometres per second. In 1917, Heber Curtis observed a nova within Andromeda. Searching the photographic record, 11 more novae were discovered. Curtis noticed that these novae were, on average, 10 magnitudes fainter than those that occurred elsewhere in the sky; as a result, he was able to come up with a distance estimate of 500,000 light-years. He became a proponent of the so-called "island universes" hypothesis, which held that spiral nebulae were independent galaxies. In 1920, the Great Debate between Harlow Shapley and Curtis took place concerning the nature of the Milky Way, spiral nebulae, the dimensions of the Universe. To support his claim of the Great Andromeda Nebula being, in fact, an external galaxy, Curtis noted the appearance of dark lanes within Andromeda which resembled the dust clouds in our own galaxy, as well as historical observations of Andromeda Galaxy's significant Doppler shift.
In 1922 Ernst Öpik presented a method to estimate the distance of Andromeda using the measured velocities of its stars. His result placed the Andromeda Nebula far outside our galaxy at a distance of about 450,000 parsecs. Edwin Hubble settled the debate in 1925 when he identified extragalactic Cepheid variable stars for the first time on astronomical photos of Andromeda; these were made using the 2.5-metre Hooker telescope, they enabled the distance of Great Andromeda Nebula to be determined. His measurement demonstrated conclusively that this feature was not a cluster of stars and gas within our own galaxy, but an separate galaxy located a significant distance from the Milky Way. In 1943, Walter Baade was the first person to resolve stars in the central region of the Andromeda Galaxy. Baade identified two distinct populations of stars based on their metallicity, naming the young, high-velocity stars in the disk Type I and the older, red stars in the bulge Type II; this nomenclature was subsequently adopted for stars within the Milky Way, elsewhere.
Baade discovered that there were two types of Cepheid variables, which resulted in a doubling of the distance estimate to Andromeda, as well as the remainder o
Observational astronomy is a division of astronomy, concerned with recording data about the observable universe, in contrast with theoretical astronomy, concerned with calculating the measurable implications of physical models. It is the practice and study of observing celestial objects with the use of telescopes and other astronomical instruments; as a science, the study of astronomy is somewhat hindered in that direct experiments with the properties of the distant universe are not possible. However, this is compensated by the fact that astronomers have a vast number of visible examples of stellar phenomena that can be examined; this allows for observational data to be plotted on graphs, general trends recorded. Nearby examples of specific phenomena, such as variable stars, can be used to infer the behavior of more distant representatives; those distant yardsticks can be employed to measure other phenomena in that neighborhood, including the distance to a galaxy. Galileo Galilei recorded what he saw.
Since that time, observational astronomy has made steady advances with each improvement in telescope technology. A traditional division of observational astronomy is based on the region of the electromagnetic spectrum observed: Optical astronomy is the part of astronomy that uses optical instruments to observe light from near-infrared to near-ultraviolet wavelengths. Visible-light astronomy, using wavelengths detectable with the human eyes, falls in the middle of this spectrum. Infrared astronomy deals with the analysis of infrared radiation; the most common tool is the reflecting telescope, but with a detector sensitive to infrared wavelengths. Space telescopes are used at certain wavelengths where the atmosphere is opaque, or to eliminate noise. Radio astronomy detects radiation of millimetre to decametre wavelength; the receivers are similar to those used in radio broadcast transmission but much more sensitive. See Radio telescopes. High-energy astronomy includes X-ray astronomy, gamma-ray astronomy, extreme UV astronomy.
Occultation astronomy is the observation of the instant one celestial object occults or eclipses another. Multi-chord asteroid occultation observations measure the profile of the asteroid to the kilometre level. In addition to using electromagnetic radiation, modern astrophysicists can make observations using neutrinos, cosmic rays or gravitational waves. Observing a source using multiple methods is known as multi-messenger astronomy. Optical and radio astronomy can be performed with ground-based observatories, because the atmosphere is transparent at the wavelengths being detected. Observatories are located at high altitudes so as to minimise the absorption and distortion caused by the Earth's atmosphere; some wavelengths of infrared light are absorbed by water vapor, so many infrared observatories are located in dry places at high altitude, or in space. The atmosphere is opaque at the wavelengths used by X-ray astronomy, gamma-ray astronomy, UV astronomy and far infrared astronomy, so observations must be carried out from balloons or space observatories.
Powerful gamma rays can, however be detected by the large air showers they produce, the study of cosmic rays is a expanding branch of astronomy. For much of the history of observational astronomy all observation was performed in the visual spectrum with optical telescopes. While the Earth's atmosphere is transparent in this portion of the electromagnetic spectrum, most telescope work is still dependent on seeing conditions and air transparency, is restricted to the night time; the seeing conditions depend on thermal variations in the air. Locations that are cloudy or suffer from atmospheric turbulence limit the resolution of observations; the presence of the full Moon can brighten up the sky with scattered light, hindering observation of faint objects. For observation purposes, the optimal location for an optical telescope is undoubtedly in outer space. There the telescope can make observations without being affected by the atmosphere. However, at present it remains costly to lift telescopes into orbit.
Thus the next best locations are certain mountain peaks that have a high number of cloudless days and possess good atmospheric conditions. The peaks of the islands of Mauna Kea, Hawaii and La Palma possess these properties, as to a lesser extent do inland sites such as Llano de Chajnantor, Cerro Tololo and La Silla in Chile; these observatory locations have attracted an assemblage of powerful telescopes, totalling many billion US dollars of investment. The darkness of the night sky is an important factor in optical astronomy. With the size of cities and human populated areas expanding, the amount of artificial light at night has increased; these artificial lights produce a diffuse background illumination that makes observation of faint astronomical features difficult without special filters. In a few locations such as the state of Arizona and in the United Kingdom, this has led to campaigns for the reduction of light pollution; the use of hoods around street lights not only improves the amount of light directed toward the ground, but helps reduce the light directed toward the sky.
Atmospheric effects can hinder the resolution of a telescope. Without some means of correcting for the blurring effect of the shifting atmosphere, teles
In mathematics, a spiral is a curve which emanates from a point, moving farther away as it revolves around the point. Two major definitions of "spiral" in the American Heritage Dictionary are: a curve on a plane that winds around a fixed center point at a continuously increasing or decreasing distance from the point. A three-dimensional curve that turns around an axis at a constant or continuously varying distance while moving parallel to the axis; the first definition describes a planar curve, that extends in both of the perpendicular directions within its plane. In another example, the "center lines" of the arms of a spiral galaxy trace logarithmic spirals; the second definition includes two kinds of 3-dimensional relatives of spirals: a conical or volute spring, the vortex, created when water is draining in a sink is described as a spiral, or as a conical helix. Quite explicitly, definition 2 includes a cylindrical coil spring and a strand of DNA, both of which are quite helical, so that "helix" is a more useful description than "spiral" for each of them.
In the side picture, the black curve at the bottom is an Archimedean spiral, while the green curve is a helix. The curve shown in red is a conic helix. A two-dimensional spiral may be described most using polar coordinates, where the radius r is a monotonic continuous function of angle θ; the circle would be regarded as a degenerate case. Some of the most important sorts of two-dimensional spirals include: The Archimedean spiral: r = a + b ⋅ θ The Euler spiral, Cornu spiral or clothoid Fermat's spiral: r = θ 1 / 2 The hyperbolic spiral: r = a / θ The lituus: r = θ − 1 / 2 The logarithmic spiral: r = a ⋅ e b θ. For example, a conic helix may be defined as a spiral on a conic surface, with the distance to the apex an exponential function of θ; the helix and vortex can be viewed as a kind of three-dimensional spiral. For a helix with thickness, see spring. A rhumb line is the curve on a sphere traced by a ship with constant bearing; the loxodrome has an infinite number of revolutions, with the separation between them decreasing as the curve approaches either of the poles, unlike an Archimedean spiral which maintains uniform line-spacing regardless of radius.
The study of spirals in nature has a long history. Christopher Wren observed. D'Arcy Wentworth Thompson's On Form gives extensive treatment to these spirals, he describes how shells are formed by rotating a closed curve around a fixed axis: the shape of the curve remains fixed but its size grows in a geometric progression. In some shells, such as Nautilus and ammonites, the generating curve revolves in a plane perpendicular to the axis and the shell will form a planar discoid shape. In others it follows a skew path forming a helico-spiral pattern. Thompson studied spirals occurring in horns, teeth and plants. A model for the pattern of florets in the head of a sunflower was proposed by H. Vogel; this has the form θ = n × 137.5 ∘, r = c n where n is the index number of the floret and c is a constant scaling factor, is a form of Fermat's spiral. The angle 137.5° is the golden angle, related to the golden ratio and gives a close packing of florets. Spirals in plants and animals are described as whorls.
This is the name given to spiral shaped fingerprints. When potassium sulfate is heated in water and subjected to swirling in a beaker, the crystals form a multi-arm spiral structure when allowed to settle Potassium sulfate forms a spiral structure in solution. A spiral like form has been found in Mezine, Ukraine, as part of a decorative object dated to 10,000 BCE; the spiral and triple spiral motif is a Neolithic symbol in Europe. The Celtic symbol the triple spiral is in fact a pre-Celtic symbol, it is carved into the rock of a stone lozenge near the main entra
Messier 81 is a spiral galaxy about 12 million light-years away, with a diameter of 90,000 light years, about half the size of the Milky Way, in the constellation Ursa Major. Due to its proximity to Earth, large size, active galactic nucleus, Messier 81 has been studied extensively by professional astronomers; the galaxy's large size and high brightness makes it a popular target for amateur astronomers. Messier 81 was first discovered by Johann Elert Bode on December 31, 1774; the galaxy is sometimes referred to as "Bode's Galaxy". In 1779, Pierre Méchain and Charles Messier reidentified Bode's object, subsequently listed in the Messier Catalogue. Messier 81 is located 10° northwest of Alpha Ursae Majoris along with several other galaxies in the Messier 81 Group. Messier 81 and Messier 82 can both be viewed using binoculars and small telescopes; the two objects are not observable to the unaided eye, although experienced amateur astronomers may be able to see Messier 81 under exceptional observing conditions with a dark sky.
Telescopes with apertures of 8 inches or larger are needed to distinguish structures in the galaxy. Its far northern declination makes it visible for observers in the northern hemisphere, it is not visible to most observers in the southern hemisphere, except those in a narrow latitude range south of the equator. Most of the emission at infrared wavelengths originates from interstellar dust; this interstellar dust is found within the galaxy's spiral arms, it has been shown to be associated with star formation regions. The general explanation is that the hot, short-lived blue stars that are found within star formation regions are effective at heating the dust and thus enhancing the infrared dust emission from these regions. Only one supernova has been detected in Messier 81; the supernova, named SN 1993J, was discovered on 28 March 1993 by F. García in Spain. At the time, it was the second brightest supernova observed in the 20th century; the spectral characteristics of the supernova changed over time.
It looked more like a type II supernova with strong hydrogen spectral line emission, but the hydrogen lines faded and strong helium spectral lines appeared, making the supernova look more like a type Ib. Moreover, the variations in SN 1993J's luminosity over time were not like the variations observed in other type II supernova but did resemble the variations observed in type Ib supernovae. Hence, the supernova has been classified as a type IIb, a transitory class between type II and type Ib; the scientific results from this supernova suggested that type Ib and Ic supernovae were formed through the explosions of giant stars through processes similar to those taking place in type II supernovae. The supernova was used to estimate a distance of 8.5 ± 1.3 Mly to Messier 81. As a local galaxy, the Central Bureau for Astronomical Telegrams tracks novae in M81 along with M31 and M33. Messier 81 is the largest galaxy in the M81 Group, a group of 34 galaxies located in the constellation Ursa Major. At 11.7 Mly from the Earth, it makes this group and the Local Group, containing the Milky Way, relative neighbors in the Virgo Supercluster.
Gravitational interactions of M81 with M82 and NGC 3077 have stripped hydrogen gas away from all three galaxies, forming gaseous filamentary structures in the group. Moreover, these interactions have allowed interstellar gas to fall into the centers of M82 and NGC 3077, leading to vigorous star formation or starburst activity there. List of galaxies List of Messier objects M81 in fiction Messier object New General Catalogue StarDate: M81 Fact Sheet M81, SEDS Messier pages SST: Messier 81 NASA Astronomy Picture of the Day: Bright Galaxy M81 NightSkyInfo.com - M81, Bode's Galaxy Messier 81 on WikiSky: DSS2, SDSS, GALEX, IRAS, Hydrogen α, X-Ray, Sky Map and images ESA/Hubble images of M81 Galaxy Messier 81 Spitzer Image Gallery Helkit Observatory Deep image of the M81 Area Bode's Galaxy at Constellation Guide
Amateur astronomy is a hobby where participants enjoy observing or imaging celestial objects in the sky using the unaided eye, binoculars, or telescopes. Though scientific research may not be their primary goal, some amateur astronomers make contributions in doing citizen science, such as by monitoring variable stars, double stars sunspots, or occultations of stars by the Moon or asteroids, or by discovering transient astronomical events, such as comets, galactic novae or supernovae in other galaxies. Amateur astronomers do not use the field of astronomy as their primary source of income or support, have no professional degree in astrophysics or advanced academic training in the subject. Most amateurs are beginners or hobbyists, while others have a high degree of experience in astronomy and may assist and work alongside professional astronomers. Many astronomers have studied the sky throughout history in an amateur framework. Amateur astronomers view the sky at night, when most celestial objects and astronomical events are visible, but others observe during the daytime by viewing the Sun and solar eclipses.
Some just look at the sky using nothing more than their eyes or binoculars, but more dedicated amateurs use portable telescopes or telescopes situated in their private or club observatories. Amateurs can join as members of amateur astronomical societies, which can advise, educate or guide them towards ways of finding and observing celestial objects. Collectively, amateur astronomers observe a variety of celestial phenomena. Common targets of amateur astronomers include the Moon, stars, meteor showers, a variety of deep sky objects such as star clusters and nebulae. Many amateurs like to specialise in observing particular objects, types of objects, or types of events which interest them. One branch of amateur astronomy, amateur astrophotography, involves the taking of photos of the night sky. Astrophotography has become more popular with the introduction of far easier to use equipment including, digital cameras, DSLR cameras and sophisticated purpose built high quality CCD cameras. Most amateur astronomers work at visible wavelengths, but a small minority experiment with wavelengths outside the visible spectrum.
An early pioneer of radio astronomy was Grote Reber, an amateur astronomer who constructed the first purpose built radio telescope in the late 1930s to follow up on the discovery of radio wavelength emissions from space by Karl Jansky. Non-visual amateur astronomy includes the use of infrared filters on conventional telescopes, the use of radio telescopes; some amateur astronomers use home-made radio telescopes, while others use radio telescopes that were built for astronomical research but have since been made available for use by amateurs. The One-Mile Telescope is one such example. Amateur astronomers use a range of instruments to study the sky, depending on a combination of their interests and resources. Methods include looking at the night sky with the naked eye, using binoculars, using a variety of optical telescopes of varying power and quality, as well as additional sophisticated equipment, such as cameras, to study light from the sky in both the visual and non-visual parts of the spectrum.
Commercial telescopes are available and used, but it is common for amateur astronomers to build their own custom telescopes. Some people focus on amateur telescope making as their primary interest within the hobby of amateur astronomy. Although specialized and experienced amateur astronomers tend to acquire more specialized and more powerful equipment over time simple equipment is preferred for certain tasks. Binoculars, for instance, although of lower power than the majority of telescopes tend to provide a wider field of view, preferable for looking at some objects in the night sky. Amateur astronomers use star charts that, depending on experience and intentions, may range from simple planispheres through to detailed charts of specific areas of the night sky. A range of astronomy software is available and used by amateur astronomers, including software that generates maps of the sky, software to assist with astrophotography, observation scheduling software, software to perform various calculations pertaining to astronomical phenomena.
Amateur astronomers like to keep records of their observations, which takes the form of an observing log. Observing logs record details about which objects were observed and when, as well as describing the details that were seen. Sketching is sometimes used within logs, photographic records of observations have been used in recent times; the information gathered is used to help studies and interactions between amateur astronomers in yearly gatherings. Although not professional information or credible, it is a way for the hobby lovers to share their new sightings and experiences; the Internet is an essential tool of amateur astronomers. The popularity of imaging among amateurs has led to large numbers of web sites being written by individuals about their images and equipment. Much of the social interaction of amateur astronomy occurs on mailing lists or discussion groups. Discussion group servers host numerous astronomy lists. A great deal of the commerce of amateur astronomy, the buying and selling of equipment, occurs online.
Many amateurs use online tools to plan their nightly observing sessions, using tools such as the Clear Sky Chart. Wh
A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas and dark matter. The word galaxy is derived from the Greek galaxias "milky", a reference to the Milky Way. Galaxies range in size from dwarfs with just a few hundred million stars to giants with one hundred trillion stars, each orbiting its galaxy's center of mass. Galaxies are categorized according to their visual morphology as spiral, or irregular. Many galaxies are thought to have supermassive black holes at their centers; the Milky Way's central black hole, known as Sagittarius A*, has a mass four million times greater than the Sun. As of March 2016, GN-z11 is the oldest and most distant observed galaxy with a comoving distance of 32 billion light-years from Earth, observed as it existed just 400 million years after the Big Bang. Research released in 2016 revised the number of galaxies in the observable universe from a previous estimate of 200 billion to a suggested 2 trillion or more, containing more stars than all the grains of sand on planet Earth.
Most of the galaxies are 1,000 to 100,000 parsecs in diameter and separated by distances on the order of millions of parsecs. For comparison, the Milky Way has a diameter of at least 30,000 parsecs and is separated from the Andromeda Galaxy, its nearest large neighbor, by 780,000 parsecs; the space between galaxies is filled with a tenuous gas having an average density of less than one atom per cubic meter. The majority of galaxies are gravitationally organized into groups and superclusters; the Milky Way is part of the Local Group, dominated by it and the Andromeda Galaxy and is part of the Virgo Supercluster. At the largest scale, these associations are arranged into sheets and filaments surrounded by immense voids; the largest structure of galaxies yet recognised is a cluster of superclusters, named Laniakea, which contains the Virgo supercluster. The origin of the word galaxy derives from the Greek term for the Milky Way, galaxias, or kyklos galaktikos due to its appearance as a "milky" band of light in the sky.
In Greek mythology, Zeus places his son born by a mortal woman, the infant Heracles, on Hera's 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 realizes she is nursing an unknown baby: she pushes the baby away, some of her milk spills, it produces the faint band of light known as the Milky Way. In the astronomical literature, the capitalized word "Galaxy" is 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." Galaxies were discovered telescopically and were known as spiral nebulae. Most 18th to 19th Century astronomers considered them as either unresolved star clusters or anagalactic nebulae, were just thought as a part of the Milky Way, but their true composition and natures remained a mystery. Observations using larger telescopes of a few nearby bright galaxies, like the Andromeda Galaxy, began resolving them into huge conglomerations of stars, but based on the apparent faintness and sheer population of stars, the true distances of these objects placed them well beyond the Milky Way.
For this reason they were popularly called island universes, but this term fell into disuse, as the word universe implied the entirety of existence. Instead, they 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, the Sombrero Galaxy. Astronomers work with numbers from certain catalogues, such as the Messier catalogue, the NGC, the IC, the CGCG, the MCG and UGC. 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 spiral galaxy having the number 109 in the catalogue of Messier, having the designations NGC 3992, UGC 6937, CGCG 269-023, MCG +09-20-044, PGC 37617; the realization that we live in a galaxy, one among many galaxies, parallels major discoveries that were made about the Milky Way and other nebulae. The Greek philosopher Democritus proposed that the bright band on the night sky known as the Milky Way might consist of distant stars.
Aristotle, believed the Milky Way to be caused by "the ignition of the fiery exhalation of some stars that were large and close together" and that the "ignition takes place in the upper part of the atmosphere, in the region of the World, continuous with the heavenly motions." The Neoplatonist philosopher Olympiodorus the Younger was critical of this view, arguing that if the Milky Way is sublunary it should appear different at different times and places on Earth, that it should have parallax, which it does not. In his view, the Milky Way is celestial. According to Mohani Mohamed, the Arabian astronomer Alhazen made the first attempt at observing and measuring the Milky Way's parallax, he thus "determined that because the Milky Way had no parallax, it must be remote from the Earth, not belonging to the atmosphere." The Persian astronomer al-Bīrūnī
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