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28-407: Bulges that have properties similar to those of elliptical galaxies are often called "classical bulges" due to their similarity to the historic view of bulges. These bulges are composed primarily of stars that are older, Population II stars , and hence have a reddish hue (see stellar evolution ). These stars are also in orbits that are essentially random compared to the plane of the galaxy, giving

56-487: A galaxy. If this happens that would increase the density at the center of the galaxy, and 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, then that many galaxies have not experienced a merger since the formation of their disk. This would then mean that current theories of galaxy formation and evolution greatly over-predict

84-557: A spherical galaxy with a equal to b , the number is 0, and the Hubble type is E0. While the limit in the literature is about E7, it has been known since 1966 that the E4 to E7 galaxies are misclassified lenticular galaxies with disks inclined at different angles to our line of sight. This has been confirmed through spectral observations revealing the rotation of their stellar disks. Hubble recognized that his shape classification depends both on

112-469: A subset of the "early-type" galaxy population. Most elliptical galaxies are composed of older, low-mass stars , with a sparse interstellar medium , and they tend to be surrounded by large numbers of globular clusters . Star formation activity in elliptical galaxies is typically minimal; they may, however, undergo brief periods of star formation when merging with other galaxies. Elliptical galaxies are believed to make up approximately 10–15% of galaxies in

140-426: A typical globular cluster , but contain a considerable amount of dark matter not present in clusters. Most of these small galaxies may not be related to other ellipticals. The Hubble classification of elliptical galaxies contains an integer that describes how elongated the galaxy image is. The classification is determined by the ratio of the major ( a ) to the minor ( b ) axes of the galaxy's isophotes : Thus for

168-506: Is a type of galaxy with an approximately ellipsoidal shape and a smooth, nearly featureless image. They are one of the four main classes of galaxy described by Edwin Hubble in his Hubble sequence and 1936 work The Realm of the Nebulae , along with spiral and lenticular galaxies. Elliptical (E) galaxies are, together with lenticular galaxies (S0) with their large-scale disks, and ES galaxies with their intermediate scale disks,

196-409: Is redder and metal-rich, and another that is bluer and metal-poor. The dynamical properties of elliptical galaxies and the bulges of disk galaxies are similar, suggesting that they may be formed by the same physical processes, although this remains controversial. The luminosity profiles of both elliptical galaxies and bulges are well fit by Sersic's law , and a range of scaling relations between

224-411: Is very little interstellar matter (neither gas nor dust), which results in low rates of star formation , few open star clusters , and few young stars; rather elliptical galaxies are dominated by old stellar populations , giving them red colors. Large elliptical galaxies typically have an extensive system of globular clusters . They generally have two distinct populations of globular clusters: one that

252-476: The M–sigma relation which relates the velocity dispersion of the surrounding stars to the mass of the black hole at the center. Elliptical galaxies are preferentially found in galaxy clusters and in compact groups of galaxies . Unlike flat spiral galaxies with organization and structure, elliptical galaxies are more three-dimensional, without much structure, and their stars are in somewhat random orbits around

280-519: The Virgo Supercluster , and they are not the dominant type of galaxy in the universe overall. They are preferentially found close to the centers of galaxy clusters . Elliptical galaxies range in size from dwarf ellipticals with tens of millions of stars, to supergiants of over one hundred trillion stars that dominate their galaxy clusters. Originally, Edwin Hubble hypothesized that elliptical galaxies evolved into spiral galaxies, which

308-453: The "disky" normal and dwarf ellipticals , which contain disks. This is, however, an abuse of the nomenclature, as there are two types of early-type galaxy, those with disks and those without. Given the existence of ES galaxies with intermediate-scale disks, it is reasonable to expect that there is a continuity from E to ES, and onto the S0 galaxies with their large-scale stellar disks that dominate

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336-664: The X-shape of the bulge. The X-shape makes up 45% of the mass of the bulge in the Milky Way. The boxy/peanut bulges are in fact the bar of a galaxy seen edge-on. Other edge-on galaxies can also show a boxy/peanut bar sometimes with an X-shape. Most bulges and pseudo-bulges are thought to host a central relativistic compact mass, which is traditionally assumed to be a supermassive black hole . Such black holes by definition cannot be observed directly (light cannot escape them), but various pieces of evidence suggest their existence, both in

364-417: The bulge a distinct spherical form. Due to the lack of dust and gases, bulges tend to have almost 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

392-419: The bulges of spiral galaxies and in the centers of ellipticals. The masses of the black holes correlate tightly 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

420-411: The center. The largest galaxies are supergiant ellipticals, or type-cD galaxies . Elliptical galaxies vary greatly in both size and mass with diameters ranging from 3,000 light years to more than 700,000 light years, and masses from 10 to nearly 10 solar masses. This range is much broader for this galaxy type than for any other. The smallest, the dwarf elliptical galaxies , may be no larger than

448-412: The disk, and are not the product of a merging process. When left alone, disk galaxies can rearrange their stars and gas (as a response to instabilities). The products of this process (called secular evolution) are often observed in such galaxies; both spiral disks and galactic bars can result from secular evolution of galaxy disks. Secular evolution is also expected to send gas and stars to the center of

476-406: The elliptical galaxies' structural parameters unify the population. Every massive elliptical galaxy contains a supermassive black hole at its center. Observations of 46 elliptical galaxies, 20 classical bulges, and 22 pseudobulges show that each contain a black hole at the center. The mass of the black hole is tightly correlated with the mass of the galaxy, evidenced through correlations such as

504-492: The galaxy cluster within which they reside than the centrally-located giant galaxy. In recent years, evidence has shown that a reasonable proportion (~25%) of early-type (E, ES and S0) galaxies have residual gas reservoirs and low-level star formation. Herschel Space Observatory researchers have speculated that the central black holes in elliptical galaxies keep the gas from cooling enough for star formation. VVV Survey Too Many Requests If you report this error to

532-461: The galaxy's far outskirts, and the winding angle of the spiral arms. Until recently 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 strictly essential to the initial seeding and growth of massive black holes. Elliptical galaxies An elliptical galaxy

560-412: The intrinsic shape of the galaxy, as well as the angle with which the galaxy is observed. Hence, some galaxies with Hubble type E0 are actually elongated. It is sometimes said that there are two physical types of ellipticals: the giant ellipticals with slightly "boxy"-shaped isophotes, whose shapes result from random motion which is greater in some directions than in others (anisotropic random motion); and

588-553: The last 8 billion years. In contrast, about two thirds of galaxies in dense galaxy clusters (such as the Virgo Cluster ) 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 often referred to as pseudobulges or disky-bulges. These bulges have stars that are not orbiting randomly, but rather orbit in an ordered fashion in

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616-456: The light at large radii. Dwarf spheroidal galaxies appear to be a distinct class: their properties are more similar to those of irregulars and late spiral-type galaxies. At the large end of the elliptical spectrum, there is further division, beyond Hubble's classification. Beyond gE giant ellipticals, lies D-galaxies and cD-galaxies . These are similar to their smaller brethren, but more diffuse, with large haloes that may as much belong to

644-487: The light of the bulge in which they reside. Typically the rate at which new stars are formed in pseudobulges is similar to the rate at which stars form in disk galaxies. Sometimes bulges contain nuclear rings that are forming stars at much higher rate (per area) than is typically found in outer disks, as shown in NGC 4314 (see photo). Properties such as spiral structure and young stars suggest that some bulges did not form through

672-518: The number of mergers in the past few billion years. Edge-on galaxies can sometimes have a boxy/peanut bulge with an X-shape. The boxy nature of the Milky Way bulge was revealed by the COBE satellite and later confirmed with the VVV survey with the help of red clump stars. The VVV survey also found two overlapping populations of red clump stars and an X-shape of the bulge. The WISE satellite later confirmed

700-560: The same plane as the stars in the outer disk. This contrasts greatly with elliptical galaxies. Subsequent studies (using the Hubble Space Telescope ) show that the bulges of many galaxies are not devoid of dust, but rather show a varied and complex structure. This structure often looks similar to a spiral galaxy , but is much smaller. Giant spiral galaxies are typically 2–100 times the size of those spirals that exist in bulges. Where they exist, these central spirals dominate

728-524: 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 extremely gas-rich mergers that happened more recently than those mergers that formed classical bulges (within the last 5 billion years). However, it is difficult for disks to survive the merging process, casting doubt on this scenario. Many astronomers suggest that bulges that appear similar to disks form outside of

756-503: Was gas-rich, the tidal forces can also cause inflows to the newly merged galaxy nucleus. Following a major merger , gas clouds are more likely to convert into stars, due to shocks (see star formation ). 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 roughly constant for at least

784-465: Was later discovered to be false, although the accretion of gas and smaller galaxies may build a disk around a pre-existing ellipsoidal structure. Stars found inside of elliptical galaxies are on average much older than stars found in spiral galaxies. Elliptical galaxies are characterized by several properties that make them distinct from other classes of galaxy. They are spherical or ovoid masses of stars, starved of star-making gases. Furthermore, there

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