The Carina–Sagittarius Arm (also known as the Sagittarius Arm or Sagittarius–Carina Arm , labeled -I ) is generally thought to be a minor spiral arm of the Milky Way galaxy . Each spiral arm is a long, diffuse curving streamer of stars that radiates from the Galactic Center . These gigantic structures are often composed of billions of stars and thousands of gas clouds. The Carina–Sagittarius Arm is one of the most pronounced arms in our galaxy as many HII regions , young stars and giant molecular clouds are concentrated in it.
28-646: The Milky Way is a barred spiral galaxy , consisting of a central crossbar and bulge from which two major and several minor spiral arms radiate outwards. This arm lies between two major spiral arms, the Scutum–Centaurus Arm , the near part of which is visible looking inward , i.e. toward the Galactic Center with the rest beyond the galactic central bulge, and the Perseus Arm , similar in size and shape but locally much closer looking outward, away from
56-665: A barred lenticular galaxy . A new type, SBm, was subsequently created to describe somewhat irregular barred spirals , such as the Magellanic Clouds , which were once classified as irregular galaxies, but have since been found to contain barred spiral structures. Among other types in Hubble's classifications for the galaxies are the spiral galaxy, elliptical galaxy and irregular galaxy. Although theoretical models of galaxy formation and evolution had not previously expected galaxies becoming stable enough to host bars very early in
84-399: A disturbance in the orbital resonances of stars in the bar structure leads to an inward collapse in which the bar becomes thicker and shorter though the exact mechanism behind this buckling instability remains hotly debated. Barred spiral galaxies with high mass accumulated in their center thus tend to have short, stubby bars. Such buckling phenomena are significantly suppressed and delayed by
112-426: A sign of galaxies reaching full maturity as the "formative years" end. A 2008 investigation found that only 20 percent of the spiral galaxies in the distant past possessed bars, compared with about 65 percent of their local counterparts. The general classification is "SB" (spiral barred). The sub-categories are based on how open or tight the arms of the spiral are. SBa types feature tightly bound arms. SBc types are at
140-401: Is located, is classified as a barred spiral galaxy. Edwin Hubble classified spiral galaxies of this type as "SB" (spiral, barred) in his Hubble sequence and arranged them into sub-categories based on how open the arms of the spiral are. SBa types feature tightly bound arms, while SBc types are at the other extreme and have loosely bound arms. SBb-type galaxies lie in between the two. SB0 is
168-544: Is rotating at Ω g p {\displaystyle \Omega _{gp}} , the spiral arms appear to be at rest). The stars within the arms are not necessarily stationary, though at a certain distance from the center, R c {\displaystyle R_{c}} , the corotation radius, the stars and the density waves move together. Inside that radius, stars move more quickly ( Ω > Ω g p {\displaystyle \Omega >\Omega _{gp}} ) than
196-708: The Scutum-Centaurus Arm and Perseus Arm . This suggests that the Carina–Sagittarius Arm is a minor arm, along with the Norma Arm (Outer Arm). These two appear to be mostly concentrations of gas, sparsely sprinkled with pockets of newly formed stars. A number of Messier objects and other objects visible through an amateur's telescope or binoculars are found in the Sagittarius Arm (here listed approximately in order from east to west along
224-484: The Southern Pinwheel Galaxy . Bars are thought to be temporary phenomena in the lives of spiral galaxies; the bar structures decay over time, transforming galaxies from barred spirals to more "regular" spiral patterns. Past a certain size the accumulated mass of the bar compromises the stability of the overall bar structure. Simulations show that many bars likely experience a "buckling" event in which
252-675: The Galactocentric azimuth, around −2 and 65 degrees). The results were that the spiral pitch angle of the arms is 7.3 ± 1.5 degrees, and the half-width of the arms of the Milky Way were found to be 0.2 kpc. The nearest part to the Sun is around 1.4 ± 0.2 kpc away. In 2008, infrared observations with the Spitzer Space Telescope showed that the Carina–Sagittarius Arm has a relative paucity of young stars, in contrast with
280-471: The ILR, the extra density in the spiral arms pulls more often than the epicyclic rate of the stars, and the stars are thus unable to react and move in such a way as to "reinforce the spiral density enhancement". The density wave theory also explains a number of other observations that have been made about spiral galaxies. For example, "the ordering of H I clouds and dust bands on the inner edges of spiral arms,
308-469: The arm): Barred spiral galaxy A barred spiral galaxy is a spiral galaxy with a central bar-shaped structure composed of stars . Bars are found in about two thirds of all spiral galaxies in the local universe, and generally affect both the motions of stars and interstellar gas within spiral galaxies and can affect spiral arms as well. The Milky Way Galaxy , where the Solar System
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#1732772727122336-420: The arms were not material in nature, but instead made up of areas of greater density, similar to a traffic jam on a highway. The cars move through the traffic jam: the density of cars increases in the middle of it. The traffic jam itself, however, moves more slowly. In the galaxy, stars, gas, dust, and other components move through the density waves, are compressed, and then move out of them. More specifically,
364-596: The bright, immediately obvious extent of the Milky Way in a perfect observational sky. It is named for its proximity to the Sagittarius and Carina constellations as seen in the night sky from Earth, in the direction of the Galactic Center . The arm dissipates near its middle, shortly after reaching its maximal angle, viewed from the Solar System, from the Galactic Center of about 80°. Extending from
392-405: The density wave theory argues that the "gravitational attraction between stars at different radii" prevents the so-called winding problem, and actually maintains the spiral pattern. The rotation speed of the arms is defined to be Ω g p {\displaystyle \Omega _{gp}} , the global pattern speed. (Thus, within a certain non-inertial reference frame , which
420-450: The density waves. The hot OB stars that are created ionize the gas of the interstellar medium , and form H II regions. These stars have relatively short lifetimes, however, and expire before fully leaving the density wave. The smaller, redder stars do leave the wave, and become distributed throughout the galactic disk. Density waves have also been described as pressurizing gas clouds and thereby catalyzing star formation. Beginning in
448-466: The existence of young, massive stars and H II regions throughout the arms, and an abundance of old, red stars in the remainder of the disk". When clouds of gas and dust enter into a density wave and are compressed, the rate of star formation increases as some clouds meet the Jeans criterion , and collapse to form new stars. Since star formation does not happen immediately, the stars are slightly behind
476-399: The galaxy may be compressed and cause shock waves periodically. Theoretically, the formation of a global spiral pattern is treated as an instability of the stellar disk caused by the self-gravity , as opposed to tidal interactions . The mathematical formulation of the theory has also been extended to other astrophysical disk systems, such as Saturn's rings . Originally, astronomers had
504-697: The galaxy's central bar is the Sagittarius Arm (Sagittarius bar). Beyond the dissipated zone it is the Carina Arm . A study was done with the measured parallaxes and motions of 10 regions in the Sagittarius arm where massive stars are formed. Data was gathered using the BeSSeL Survey with the VLBA , and the results were synthesized to discover the physical properties of these sections (called
532-428: The gravitational attraction between stars can only maintain the spiral structure if the frequency at which a star passes through the arms is less than the epicyclic frequency , κ ( R ) {\displaystyle \kappa (R)} , of the star. This means that a long-lived spiral structure will only exist between the inner and outer Lindblad resonance (ILR, OLR, respectively), which are defined as
560-413: The idea of long-lived quasistatic spiral structure (QSSS hypothesis). In this hypothesis, the spiral pattern rotates with a particular angular frequency (pattern speed), whereas the stars in the galactic disk orbit at varying speeds , which depend on their distance to the galaxy center . The presence of spiral density waves in galaxies has implications on star formation , since the gas orbiting around
588-423: The idea that the arms of a spiral galaxy were material. However, if this were the case, then the arms would become more and more tightly wound, since the matter nearer to the center of the galaxy rotates faster than the matter at the edge of the galaxy. The arms would become indistinguishable from the rest of the galaxy after only a few orbits. This is called the winding problem. Lin & Shu proposed in 1964 that
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#1732772727122616-449: The inner stars. This effect builds over time to stars orbiting farther out, which creates a self-perpetuating bar structure. The bar structure is believed to act as a type of stellar nursery , channeling gas inwards from the spiral arms through orbital resonance , fueling star birth in the vicinity of its center. This process is also thought to explain why many barred spiral galaxies have active galactic nuclei , such as that seen in
644-501: The late 1970s, Peter Goldreich , Frank Shu , and others applied density wave theory to the rings of Saturn. Saturn's rings (particularly the A Ring ) contain a great many spiral density waves and spiral bending waves excited by Lindblad resonances and vertical resonances (respectively) with Saturn's moons . The physics are largely the same as with galaxies, though spiral waves in Saturn's rings are much more tightly wound (extending
672-490: The other extreme and have loosely bound arms. SBb galaxies lie in between. SBm describes somewhat irregular barred spirals. SB0 is a barred lenticular galaxy . of barred Magellanic spiral Density wave theory Density wave theory or the Lin–Shu density wave theory is a theory proposed by C.C. Lin and Frank Shu in the mid-1960s to explain the spiral arm structure of spiral galaxies . The Lin–Shu theory introduces
700-409: The presence of a supermassive black hole in the galactic center but occur nonetheless. Since so many spiral galaxies have bar structures, it is likely that they are recurring phenomena in spiral galaxy development. The oscillating evolutionary cycle from spiral galaxy to barred spiral galaxy is thought to take on average about two billion years. Recent studies have confirmed the idea that bars are
728-458: The radii such that: Ω ( R ) = Ω g p + κ / m {\displaystyle \Omega (R)=\Omega _{gp}+\kappa /m} and Ω ( R ) = Ω g p − κ / m {\displaystyle \Omega (R)=\Omega _{gp}-\kappa /m} , respectively. Past the OLR and within
756-441: The spiral arms, and outside, stars move more slowly ( Ω < Ω g p {\displaystyle \Omega <\Omega _{gp}} ). For an m -armed spiral, a star at radius R from the center will move through the structure with a frequency m ( Ω g p − Ω ( R ) ) {\displaystyle m(\Omega _{gp}-\Omega (R))} . So,
784-410: The universe's history, evidence has recently emerged of the existence of numerous spiral galaxies in the early universe. Barred galaxies are apparently predominant, with surveys showing that up to two-thirds of all spiral galaxies develop a bar. The creation of the bar is generally thought to be the result of a density wave radiating from the center of the galaxy whose effects reshape the orbits of
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