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41-692: BCG may refer to: Astronomy [ edit ] Blue compact galaxy Brightest cluster galaxy Medicine and biochemistry [ edit ] BCG vaccine (Bacillus Calmette–Guérin), for tuberculosis Ballistocardiography , measuring heart forces Bromocresol green , a dye and pH indicator Companies [ edit ] Beijing Capital Group , China Boston Consulting Group Buffalo Creek and Gauley Railroad (BC&G), Clay County, West Virginia, US Other uses [ edit ] BCG matrix , for product line analysis Billy Gillispie , American basketball coach Bolt , carrier group in

82-422: A spheroidal galactic bulge around the galactic core. However, some stars inhabit a spheroidal halo or galactic spheroid , a type of galactic halo . The orbital behaviour of these stars is disputed, but they may exhibit retrograde and/or highly inclined orbits, or not move in regular orbits at all. Halo stars may be acquired from small galaxies which fall into and merge with the spiral galaxy—for example,

123-588: A supermassive black hole at their centers. In our own galaxy, for instance, the object called Sagittarius A* is a supermassive black hole. There are many lines of evidence for the existence of black holes in spiral galaxy centers, including the presence of active nuclei in some spiral galaxies, and dynamical measurements that find large compact central masses in galaxies such as Messier 106 . Bar-shaped elongations of stars are observed in roughly two-thirds of all spiral galaxies. Their presence may be either strong or weak. In edge-on spiral (and lenticular) galaxies,

164-425: A density wave, it gets squeezed and makes new stars, some of which are short-lived blue stars that light the arms. The first acceptable theory for the spiral structure was devised by C. C. Lin and Frank Shu in 1964, attempting to explain the large-scale structure of spirals in terms of a small-amplitude wave propagating with fixed angular velocity, that revolves around the galaxy at a speed different from that of

205-641: A firearm British Comedy Guide Big City Greens , show on the Disney Channel "Birth control glasses", common dysphemism for GI glasses Topics referred to by the same term [REDACTED] This disambiguation page lists articles associated with the title BCG . If an internal link led you here, you may wish to change the link to point directly to the intended article. Retrieved from " https://en.wikipedia.org/w/index.php?title=BCG&oldid=1144192076 " Category : Disambiguation pages Hidden categories: Short description

246-406: A flat, rotating disk containing stars , gas and dust , and a central concentration of stars known as the bulge . These are often surrounded by a much fainter halo of stars, many of which reside in globular clusters . Spiral galaxies are named by their spiral structures that extend from the center into the galactic disc. The spiral arms are sites of ongoing star formation and are brighter than

287-413: A large role in driving starbursts. Galaxies in the midst of a starburst frequently show tidal tails , an indication of a close encounter with another galaxy, or are in the midst of a merger. Turbulence, along with variations of time and space, cause the dense gas within a galaxy to compress and rapidly increase star formation. The efficiency at which the galaxy forms also increases its SFR . These changes in

328-480: A quarter 2.5 billion years ago, until present, where over two-thirds of the galaxies in the visible universe ( Hubble volume ) have bars. 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

369-527: 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 (such as M81/M82), a collision with another galaxy (such as the Antennae), or by another process that forces material into the centre of the galaxy (such as a stellar bar). The inside of the starburst is quite an extreme environment. The large amounts of gas mean that massive stars are formed. Young, hot stars ionize

410-438: 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

451-640: Is an extremely old spiral galaxy located in the Abell 1689 galaxy cluster in the Virgo constellation. A1689B11 is 11 billion light years from the Earth, forming 2.6 billion years after the Big Bang. In June 2019, citizen scientists through Galaxy Zoo reported that the usual Hubble classification , particularly concerning spiral galaxies , may not be supported, and may need updating. The pioneer of studies of

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492-433: Is clear that the elliptical orbits come close together in certain areas to give the effect of arms. Stars therefore do not remain forever in the position that we now see them in, but pass through the arms as they travel in their orbits. The following hypotheses exist for star formation caused by density waves: Spiral arms appear visually brighter because they contain both young stars and more massive and luminous stars than

533-497: Is different from Wikidata All article disambiguation pages All disambiguation pages Blue compact galaxy For example, the star formation rate of the Milky Way galaxy is approximately 3 M ☉ /yr, while starburst galaxies can experience star formation rates of 100 M ☉ /yr or more. In a starburst galaxy, the rate of star formation is so large that the galaxy consumes all of its gas reservoir, from which

574-403: Is difficult since starburst galaxies do not represent a specific type in and of themselves. Starbursts can occur in disk galaxies , and irregular galaxies often exhibit knots of starburst spread throughout the irregular galaxy. Nevertheless, astronomers typically classify starburst galaxies based on their most distinct observational characteristics. Some of the categorizations include: Firstly,

615-524: Is the central value; it is useful to define: R o p t = 3.2 h {\displaystyle R_{opt}=3.2h} as the size of the stellar disk, whose luminosity is L t o t = 2 π I 0 h 2 {\displaystyle L_{tot}=2\pi I_{0}h^{2}} . The spiral galaxies light profiles, in terms of the coordinate R / h {\displaystyle R/h} , do not depend on galaxy luminosity. Before it

656-560: Is the oldest and most distant known spiral galaxy, as of 2024.The galaxy has a redshift of 4.4, meaning its light took 12.4 billion years to reach Earth. The oldest grand design spiral galaxy on file is BX442 . At eleven billion years old, it is more than two billion years older than any previous discovery. Researchers believe the galaxy's shape is caused by the gravitational influence of a companion dwarf galaxy . Computer models based on that assumption indicate that BX442's spiral structure will last about 100 million years. A1689B11

697-555: The Sagittarius Dwarf Spheroidal Galaxy is in the process of merging with the Milky Way and observations show that some stars in the halo of the Milky Way have been acquired from it. Unlike the galactic disc, the halo seems to be free of dust , and in further contrast, stars in the galactic halo are of Population II , much older and with much lower metallicity than their Population I cousins in

738-554: 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. Spiral galaxies may consist of several distinct components: The relative importance, in terms of mass, brightness and size, of the different components varies from galaxy to galaxy. Spiral arms are regions of stars that extend from

779-415: The bulge of Sa and SBa galaxies tends to be large. In contrast, the bulges of Sc and SBc galaxies are much smaller and are composed of young, blue Population I stars . Some bulges have similar properties to those of elliptical galaxies (scaled down to lower mass and luminosity); others simply appear as higher density centers of disks, with properties similar to disk galaxies. Many bulges are thought to host

820-499: The center of barred and unbarred spiral galaxies . These long, thin regions resemble a spiral and thus give spiral galaxies their name. Naturally, different classifications of spiral galaxies have distinct arm-structures. Sc and SBc galaxies, for instance, have very "loose" arms, whereas Sa and SBa galaxies have tightly wrapped arms (with reference to the Hubble sequence). Either way, spiral arms contain many young, blue stars (due to

861-421: The expanding universe. M82 is the archetypal starburst galaxy. Its high level of star formation is due to a close encounter with the nearby spiral M81. Maps of the regions made with radio telescopes show large streams of neutral hydrogen connecting the two galaxies, also as a result of the encounter. Radio images of the central regions of M82 also show a large number of young supernova remnants, left behind when

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902-587: The first drawing of Andromeda Galaxy 's spiral structure. In 1852 Stephen Alexander supposed that Milky Way is also a spiral nebula. The question of whether such objects were separate galaxies independent of the Milky Way, or a type of nebula existing within our own galaxy, was the subject of the Great Debate of 1920, between Heber Curtis of Lick Observatory and Harlow Shapley of Mount Wilson Observatory . Beginning in 1923, Edwin Hubble observed Cepheid variables in several spiral nebulae, including

943-531: The galactic disc (but similar to those in the galactic bulge). The galactic halo also contains many globular clusters. The motion of halo stars does bring them through the disc on occasion, and a number of small red dwarfs close to the Sun are thought to belong to the galactic halo, for example Kapteyn's Star and Groombridge 1830 . Due to their irregular movement around the center of the galaxy, these stars often display unusually high proper motion . BRI 1335-0417

984-404: The galaxy rotates. The arm would, after a few galactic rotations, become increasingly curved and wind around the galaxy ever tighter. This is called the winding problem . Measurements in the late 1960s showed that the orbital velocity of stars in spiral galaxies with respect to their distance from the galactic center is indeed higher than expected from Newtonian dynamics but still cannot explain

1025-411: The galaxy's gas and stars. They suggested that the spiral arms were manifestations of spiral density waves – they assumed that the stars travel in slightly elliptical orbits, and that the orientations of their orbits is correlated i.e. the ellipses vary in their orientation (one to another) in a smooth way with increasing distance from the galactic center. This is illustrated in the diagram to the right. It

1066-440: The gas (mainly hydrogen ) around them, creating H II regions . Groups of hot stars are known as OB associations . These stars burn bright and fast, and are quite likely to explode at the end of their lives as supernovae . After the supernova explosion, the ejected material expands and becomes a supernova remnant . These remnants interact with the surrounding environment within the starburst (the interstellar medium ) and can be

1107-501: The high mass density and the high rate of star formation), which make the arms so bright. A bulge is a large, tightly packed group of stars. The term refers to the central group of stars found in most spiral galaxies, often defined as the excess of stellar light above the inward extrapolation of the outer (exponential) disk light. Using the Hubble classification, the bulge of Sa galaxies is usually composed of Population II stars , which are old, red stars with low metal content. Further,

1148-514: The more massive stars created in the starburst came to the end of their lives. The Antennae is another starburst system, detailed by a Hubble picture, released in 1997. Spiral galaxy Spiral galaxies form a class of galaxy originally described by Edwin Hubble in his 1936 work The Realm of the Nebulae and, as such, form part of the Hubble sequence . Most spiral galaxies consist of

1189-418: The presence of the bar can sometimes be discerned by the out-of-plane X-shaped or (peanut shell)-shaped structures which typically have a maximum visibility at half the length of the in-plane bar. The bulk of the stars in a spiral galaxy are located either close to a single plane (the galactic plane ) in more or less conventional circular orbits around the center of the galaxy (the Galactic Center ), or in

1230-427: The rate of star formation also led to variations with depletion time, and power a starburst with its own galactic mechanisms rather than merging with another galaxy. 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

1271-412: The rest of the galaxy. As massive stars evolve far more quickly, their demise tends to leave a darker background of fainter stars immediately behind the density waves. This make the density waves much more prominent. Spiral arms simply appear to pass through the older established stars as they travel in their galactic orbits, so they also do not necessarily follow the arms. As stars move through an arm,

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1312-462: The rotation of the Galaxy and the formation of the spiral arms was Bertil Lindblad in 1925. He realized that the idea of stars arranged permanently in a spiral shape was untenable. Since the angular speed of rotation of the galactic disk varies with distance from the centre of the galaxy (via a standard solar system type of gravitational model), a radial arm (like a spoke) would quickly become curved as

1353-476: The site of naturally occurring masers . Studying nearby starburst galaxies can help us determine the history of galaxy formation and evolution. Large numbers of the most distant galaxies seen, for example, in the Hubble Deep Field are known to be starbursts, but they are too far away to be studied in any detail. Observing nearby examples and exploring their characteristics can give us an idea of what

1394-417: The so-called "Andromeda Nebula" , proving that they are, in fact, entire galaxies outside our own. The term spiral nebula has since fallen out of use. The Milky Way was once considered an ordinary spiral galaxy. Astronomers first began to suspect that the Milky Way is a barred spiral galaxy in the 1960s. Their suspicions were confirmed by Spitzer Space Telescope observations in 2005, which showed that

1435-568: The space velocity of each stellar system is modified by the gravitational force of the local higher density. Also the newly created stars do not remain forever fixed in the position within the spiral arms, where the average space velocity returns to normal after the stars depart on the other side of the arm. Charles Francis and Erik Anderson showed from observations of motions of over 20,000 local stars (within 300 parsecs) that stars do move along spiral arms, and described how mutual gravity between stars causes orbits to align on logarithmic spirals. When

1476-422: The stability of the spiral structure. Since the 1970s, there have been two leading hypotheses or models for the spiral structures of galaxies: These different hypotheses are not mutually exclusive, as they may explain different types of spiral arms. Bertil Lindblad proposed that the arms represent regions of enhanced density (density waves) that rotate more slowly than the galaxy's stars and gas. As gas enters

1517-647: 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, and one that typically 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 (the Antennae Galaxies), and IC 10 . Starburst galaxies are defined by these three interrelated factors: Commonly used definitions include: Mergers and tidal interactions between gas-rich galaxies play

1558-472: The surrounding disc because of the young, hot OB stars that inhabit them. Roughly two-thirds of all spirals are observed to have an additional component in the form of a bar-like structure, extending from the central bulge, at the ends of which the spiral arms begin. The proportion of barred spirals relative to barless spirals has likely changed over the history of the universe , with only about 10% containing bars about 8 billion years ago, to roughly

1599-422: The theory is applied to gas, collisions between gas clouds generate the molecular clouds in which new stars form, and evolution towards grand-design bisymmetric spirals is explained. The stars in spirals are distributed in thin disks radial with intensity profiles such that with h {\displaystyle h} being the disk scale-length; I 0 {\displaystyle I_{0}}

1640-506: Was happening in the early universe as the light we see from these distant galaxies left them when the universe was much younger (see redshift ). However, starburst galaxies seem to be quite rare in our local universe, and are more common further away – indicating that there were more of them billions of years ago. All galaxies were closer together then, and therefore more likely to be influenced by each other's gravity. More frequent encounters produced more starbursts as galactic forms evolved with

1681-572: Was understood that spiral galaxies existed outside of our Milky Way galaxy, they were often referred to as spiral nebulae , due to Lord Rosse , whose telescope Leviathan was the first to reveal the spiral structure of galaxies. In 1845 he discovered the spiral structure of M51, a galaxy nicknamed later as the " Whirlpool Galaxy ", and his drawings of it closely resemble modern photographs. In 1846 and in 1849 Lord Rosse identified similar pattern in Messier 99 and Messier 33 respectively. In 1850 he made

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