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Encyclopedia > Spiral galaxy
An example of a spiral galaxy, the Pinwheel Galaxy (also known as Messier 101 or NGC 5457)
An example of a spiral galaxy, the Pinwheel Galaxy (also known as Messier 101 or NGC 5457)

A spiral galaxy is a galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble in his 1936 work “The Realm of the Nebulae”[1] and, as such, forms part of the Hubble sequence. Spiral galaxies consist of a flat, rotating disk of stars, gas and dust, and a central concentration of stars known as the bulge. These are surrounded by a much fainter halo of stars, many of which reside in globular clusters. Image File history File linksMetadata Download high resolution version (6000x4690, 9703 KB) Summary Description: the galaxy Messier 101 (M101, also known as NGC 5457 and also nicknamed the Pinwheel Galaxy) lies in the northern circumpolar constellation, Ursa Major (The Great Bear), at a distance of 25 million light-years from... Image File history File linksMetadata Download high resolution version (6000x4690, 9703 KB) Summary Description: the galaxy Messier 101 (M101, also known as NGC 5457 and also nicknamed the Pinwheel Galaxy) lies in the northern circumpolar constellation, Ursa Major (The Great Bear), at a distance of 25 million light-years from... The Pinwheel Galaxy (also known as Messier Object 101, or NGC 5457) is a spiral galaxy in the constellation Ursa Major. ... For other uses, see Galaxy (disambiguation). ... Astronomers classify galaxies based on their overall shape (elliptical, spiral or barred spiral) and further by the specific properties of the individual galaxy (for example degree of ellipse, number of spirals or definition of bar). ... Edwin Powell Hubble (November 20, 1889 – September 28, 1953) was an American astronomer. ... Year 1936 (MCMXXXVI) was a leap year starting on Wednesday (link will display the full calendar) of the Gregorian calendar. ... It has been suggested that this article or section be merged with Galaxy classification. ... A disc is a component of disc galaxies, such as spiral galaxies, or lenticular galaxies. ... This article is about the astronomical object. ... The interstellar medium (or ISM) is the name astronomers give to the tenuous gas and dust that pervade interstellar space. ... In astronomy, a bulge is a huge, tightly packed group of stars. ... The galactic halo is a region of space surrounding spiral galaxies, including our galaxy, the Milky Way. ... The Globular Cluster M80 in the constellation Scorpius is located about 28,000 light years from the Sun and contains hundreds of thousands of stars. ...


Spiral galaxies are named for the (usually two-armed) spiral structures that extend from the bulge into the disk. The spiral arms are sites of ongoing star formation and are brighter than the surrounding disk because of the young, hot OB stars that inhabit them. Roughly half 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. Our own Milky Way has long been believed to be a barred spiral, although the bar itself is difficult to observe from our position within the Galactic disk. The most convincing evidence for its existence comes from a recent survey, performed by the Spitzer Space Telescope, of stars in the Galactic center.[2] This article does not cite any references or sources. ... OB stars are hot, massive stars stars which form in loosely organized groups called OB associations. ... NGC 1300, viewed nearly face-on. ... The Spitzer Space Telescope (formerly the Space Infrared Telescope Facility [SIRTF]) is an infrared space observatory, the fourth and final of NASAs Great Observatories. ...


Together with irregulars, spiral galaxies make up approximately 70% of galaxies in the local Universe.[3] They are mostly found in low-density regions and are rare in the centers of galaxy clusters.[4] This article or section does not adequately cite its references or sources. ...

Contents

Structure

Spiral galaxies consist of several distinct components:

The relative importance, in terms of mass, brightness and size, of the different components varies from galaxy to galaxy. STARS can mean: Shock Trauma Air Rescue Society Special Tactics And Rescue Service, a fictional task force that appears in Capcoms Resident Evil video game franchise. ... The interstellar medium (or ISM) is the name astronomers give to the tenuous gas and dust that pervade interstellar space. ... In astronomy, a bulge is a huge, tightly packed group of stars. ... The giant elliptical galaxy ESO 325-G004. ... The galactic halo is a region of space surrounding spiral galaxies, including our galaxy, the Milky Way. ... A globular cluster is a spherical bundle of stars (star cluster) that orbits a galaxy as a satellite. ... Top: artists conception of a supermassive black hole tearing apart a star. ...


Spiral arms

Spiral arms are regions of stars that extend from the center of spiral and barred 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. Sa and SBa galaxies, for instance, have tightly wrapped arms, whereas Sc and SBc galaxies have very "loose" arms (with reference to the Hubble sequence). Either way, spiral arms contain a great many young, blue stars (due to the high mass density and the high rate of star formation), which make the arms so remarkable. STARS can mean: Shock Trauma Air Rescue Society Special Tactics And Rescue Service, a fictional task force that appears in Capcoms Resident Evil video game franchise. ... An unbarred spiral galaxy is a galaxy with out a central bar, or one that is not a barred spiral galaxy. ... NGC 1300, viewed nearly face-on. ... This article is about a celestial body. ...


Galactic bulge

A bulge is a huge, tightly packed group of stars. The term commonly refers to the central group of stars found in most spiral galaxies. This article is about the astronomical object. ...


Using the Hubble classification, the bulge of Sa galaxies is usually composed of population II stars, that is old, red stars with low metal content. Further, the bulge of Sa and SBa galaxies tends to be large. In contrast, the bulges of Sc and SBc galaxies are a great deal smaller, and are composed of young, blue, Population I stars. Bulges have similar properties to those of elliptical galaxies (scaled down to lower mass and luminosity).


Many bulges are thought to host a supermassive black hole at their center. Such black holes have never been directly observed, but many indirect proofs exist. In our own galaxy, for instance, the object called Sagittarius A* is believed to be a supermassive black hole. Top: artists conception of a supermassive black hole tearing apart a star. ... Sagittarius A* (pronounced A-star) is a bright and very compact source of radio emission at the center of the Milky Way Galaxy, part of a larger astronomical feature at that location (Sagittarius A). ...


Galactic spheroid

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 centre), or in a spheroidal galactic bulge around the galactic core. The galactic plane is the plane in which the majority of a flattened galaxys mass lies. ... Two bodies with a slight difference in mass orbiting around a common barycenter. ... The Galactic Center is the rotational center of the Milky Way galaxy. ... In mathematics, a spheroid is a quadric surface in three dimensions obtained by rotating an ellipse about one of its principal axes. ... In astronomy, a bulge is a huge, tightly packed group of stars. ...


However, some stars inhabit a spheroidal halo or galactic spheroid. The orbital behaviour of these stars is disputed, but they may describe 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, the Sagittarius Dwarf Elliptical 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. Prograde motion is the motion of a planetary body in a direction similar to that of other bodies within its system, and is sometimes called direct motion, especially in astrology. ... For the science fiction novella by William Shunn, see Inclination (novella). ... Galaxy mergers can happen when two (or more) galaxies collide. ... The Sagittarius Dwarf Elliptical Galaxy (Sag DEG) is a satellite galaxy of the Milky Way Galaxy. ...


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 the galactic disc (but similar to those in the galactic bulge). The galactic halo also contains many globular clusters. Interstellar cloud is the generic name given to accumulations of gas and dust in our galaxy. ... Stars can be grouped into two general types called Population I and Population II. The criteria for classification include space velocity, location in the galaxy, age, chemical composition, and differences in distribution on the Hertzsprung-Russell diagram. ... The globular cluster M80. ... The Globular Cluster M80 in the constellation Scorpius is located about 28,000 light years from the Sun and contains hundreds of thousands of stars. ...


The motion of halo stars does bring them through the disc on occasion, and a number of small red dwarf stars 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 centre of the galaxy—if they do so at all—these stars often display unusually high proper motion. For the type of star, see Red dwarf. ... Sol redirects here. ... Kapteyns Star (also known as GJ 191, HD 33793 or CD -45 1841) is a class M0 subdwarf discovered by Jacobus Kapteyn in 1897. ... Groombridge 1830 is a star. ... The proper motion of a star is the motion of the position of the star in the sky (the change in direction in which we see it, as opposed to the radial velocity) after eliminating the improper motions of the stars, which affect their measured coordinates but are not real...


Origin of the spiral structure

The pioneer of studies of the rotation of the Galaxy and the formation of the spiral arms was Bertil Lindblad in 1925 . He realised that the idea of stars arranged permanently in a spiral shape was untenable due to the "winding dilemma". Since the angular speed of rotation of the galactic disk varies with distance from the centre of the galaxy, a radial arm (like a spoke) would quickly become curved as the galaxy rotates. The arm would, after a few galactic rotations, become increasingly curved and wind around the galaxy ever tighter. Or, the stars on the outermost edge of the galaxy would have to move faster than those near the center, as the galaxy rotates. Neither behaviour is observed. 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 a density wave, it gets squeezed and makes new stars, some of which are short-lived blue stars that light the arms. Bertil Lindblad (Örebro November 26, 1895 – Saltsjöbaden [outside Stockholm] June 25, 1965) was a Swedish astronomer. ... Bertil Lindblad (Örebro November 26, 1895 – Saltsjöbaden [outside Stockholm] June 25, 1965) was a Swedish astronomer. ...

Explanation of spiral galaxy arms.
Explanation of spiral galaxy arms.

This idea was developed into density wave theory by C. C. Lin and Frank Shu in 1964.[5] They suggested that the spiral arms were manifestations of spiral density waves, 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 the galaxy's gas and stars. As the compression wave goes through, it triggers star formation on the leading edge of the spiral arms. They assumed that the stars travel in elliptical orbits and that the sizes as well as the orientations of their orbits are slightly-varying from each other, 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. It is clear that the elliptical orbits come close together in certain areas to give the effect of arms. Image File history File links No higher resolution available. ... Image File history File links No higher resolution available. ... 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 spiral arm structure of certain galaxies. ... Frank Shu Frank Shu (born in Kunming, China), is an astrophysicist, author and professor of astronomy at the University of California, Berkeley and the university president of the National Tsing Hua University. ...


Alternative hypotheses that have been proposed involve waves of star formation moving about the galaxy, also called the stochastic self-propagating star formation model or SSPSF model. This model proposes that star formation propagates via the action of shock waves produced by stellar winds and supernovae that compose the interstellar medium. The arms appear brighter because there are more young stars (hence more massive, bright stars). These massive, bright stars also die out quickly, which would leave just the (darker) background stellar distribution behind the waves, hence making the waves visible. The SSPSF (““stochastic self-propagating star formation) model of star formation was proposed by Mueller & Arnett [1] in 1976, generalized afterward by Gerola & Seiden [2] in 1978 and Gerola, Seiden, & Schulman [3 in 1980. ... Star formation is the process by which dense parts of molecular clouds collapse into a ball of plasma to form a star. ...


The different hypothesis do not have to be mutually-exclusive, as they may explain different types of spiral arms.


While stars, therefore, do not remain forever in the position that we now see them in, they also do not follow the arms. The arms simply appear to pass through the stars as the stars travel in their orbits.


Recent results suggest that the orientation of the spin axis of spiral galaxies is not a chance result, but instead they are preferentially aligned along the surface of cosmic voids.[6] That is, spiral galaxies tend to be oriented at a high angle of inclination relative to the large-scale structure of the surroundings. They have been described as lining up like "beads on a string," with their axis of rotation following the filaments around the edges of the voids.[7] In astronomy, voids are the empty spaces between filaments, the largest-scale structures in the Universe that contain very few, or no, galaxies. ... In astronomy, filaments are one of the largest known structures in the Universe, thread-like structures with a typical length of 70 to 150 megaparsec that form the boundaries between large voids in the universe. ...


Spiral nebulae

“Spiral nebula” is an old term for a spiral galaxy. Until the early 20th century, most astronomers believed that objects like the Whirlpool Galaxy were just one more form of nebula that were within our own Galaxy. In 1926, Edwin Hubble[8] finally proved that the spiral nebulae were, in fact, stellar systems independent of our own Milky Way and the term “spiral nebula” has since fallen into disuse. The Whirlpool Galaxy (also known as Messier 51a, M51a, or NGC 5194) is an interacting[4] grand-design[5] spiral galaxy located at a distance of approximately 23 million light-years in the constellation Canes Venatici. ... The Triangulum Emission Nebula NGC 604 The Pillars of Creation from the Eagle Nebula For other uses, see Nebula (disambiguation). ... For other uses, see Milky Way (disambiguation). ... Year 1926 (MCMXXVI) was a common year starting on Friday (link will display the full calendar) of the Gregorian calendar. ... Edwin Powell Hubble (November 20, 1889 – September 28, 1953) was an American astronomer. ...


Examples

The Triangulum Galaxy (also known as M33 or NGC 598) is a spiral galaxy about 2. ... The Whirlpool Galaxy (also known as M51 or NGC 5194) is a classic spiral galaxy located in the Canes Venatici constellation. ... For other uses, see Milky Way (disambiguation). ... The Andromeda Galaxy (IPA: , also known as Messier 31, M31, or NGC 224; older texts often called it the Great Andromeda Nebula) is a spiral galaxy approximately 2. ... The Sunflower Galaxy (also known as Spiral Galaxy M63, Messier Object 63, Messier 63, M63, or NGC 5055) is a spiral galaxy in the Canes Venatici constellation. ...

See also

Components

It has been proposed below that Disc (galaxy) be renamed and moved to galactic disc. ... In astronomy, a bulge is a huge, tightly packed group of stars. ... The galactic halo is a region of space surrounding spiral galaxies, including our galaxy, the Milky Way. ... The Structure of the Galaxy The galactic corona (not to be confused with the suns corona), also called the dark matter halo, is a region of space surrounding the galactic halo of spiral galaxies, including the Milky Way Galaxy that consist mostly of dark matter. ...

Classification

Astronomers classify galaxies based on their overall shape (elliptical, spiral or barred spiral) and further by the specific properties of the individual galaxy (for example degree of ellipse, number of spirals or definition of bar). ... It has been suggested that this article or section be merged with Galaxy classification. ... It has been suggested that disc (galaxy) be merged into this article or section. ... An active galaxy is a galaxy where a significant fraction of the energy output is not emitted by the normal components of a galaxy: stars, dust and interstellar gas. ... NGC 1300, viewed nearly face-on. ... A dwarf galaxy is a small galaxy composed of up to several billion stars, a small number compared to our own Milky Ways 200-400 billion stars. ... Dwarf elliptical galaxies, or dEs, are elliptical galaxies that are much smaller than others, classified as dE. They are quite common in galaxy groups and clusters, and are usually companions to other galaxies. ... Dwarf spheroidal galaxy (dSph) is a term in astronomy applied to the nine low luminosity dwarf elliptical galaxies that are companions to the Milky Way and to the similar systems that are companions to the Andromeda Galaxy M31. ... The giant elliptical galaxy ESO 325-G004. ... An intermediate spiral galaxy is a galaxy that is in between the classifications of a barred spiral galaxy and an unbarred spiral galaxy. ... This article or section does not adequately cite its references or sources. ... The Spindle Galaxy (NGC 5866), a lenticular galaxy in the Draco constellation. ... A ring galaxy is a galaxy with a ring-like appearance. ... The Antennae Galaxies are an example of a very high starburst galaxy occurring from the collision of NGC 4038/NGC 4039. ... Seyfert galaxies are spiral or irregular galaxies containing an extremely bright nucleus, most likely caused by a supermassive black hole, that can sometimes outshine the surrounding galaxy. ... An unbarred spiral galaxy is a galaxy with out a central bar, or one that is not a barred spiral galaxy. ...

Other

The anisotropy of the star density in the night sky makes the galactic coordinate system very useful for coordinating surveys, both those which require high densities of stars (at low galactic latitudes) and those which require a low density of stars (at high galactic latitudes) Many galaxies, including the Milky... In astrophysics, the questions of galaxy formation and evolution are: How, from a homogeneous universe, did we obtain the very heterogeneous one we live in? How did galaxies form? How do galaxies change over time? A spectacular head-on collision between two galaxies is seen in this NASA Hubble Space... Galaxy groups and clusters are super-structures in the spread of galaxies of the cosmos. ... List of galaxies: Abell 1835 IR1916 AM 0644-741 Andromeda Galaxy (M31/NGC 224) Andromeda I Andromeda II Andromeda III Aquarius Dwarf Barnards Galaxy (NGC 6822) Black Eye Galaxy (M64/NGC 4826) Bodes Galaxy (M81/NGC 3031) Canis Major Dwarf Galaxy Carina Dwarf Centaurus A Galaxy Draco Dwarf Fornax... The reader should be aware that there are certain unavoidable difficulties with this list. ... Timeline of galaxies, clusters of galaxies, and large-scale structure of the cosmos 964 - Al Sufi, a Persian astronomer makes the first preserved recording of the Large Magellanic Cloud. ... In astronomy, the Tully-Fisher relation, published by astronomers R. Brent Tully and J. Richard Fisher in 1977, is a standard candle that measures the distance to rotating spiral galaxies by the width of the galaxys spectral lines. ...

References

  1. ^ Hubble, E. P. (1936). The Realm of the Nebulae. New Haven: Yale University Press. ISBN 36018182. 
  2. ^ Benjamin, R. A. et al (September 2005). "First GLIMPSE Results on the Stellar Structure of the Galaxy." (fee required). The Astrophysical Journal 630 (2): L149-L152. Retrieved on 2007-09-21. 
  3. ^ Loveday, J. (February 1996). "The APM Bright Galaxy Catalogue.". Monthly Notices of the Royal Astronomical Society 278 (4): 1025-1048. Retrieved on 2007-09-15. 
  4. ^ Dressler, A. (March 1980). "Galaxy morphology in rich clusters - Implications for the formation and evolution of galaxies.". The Astrophysical Journal 236: 351-365. Retrieved on 2007-09-15. 
  5. ^ Lin, C. C.; Shu, F. H. (August 1964). "On the spiral structure of disk galaxies.". The Astrophysical Journal 140: 646-655. Retrieved on 2007-09-26. 
  6. ^ Trujillo, I.; Carretero, C.; Patiri, S.G. (2006). "Detection of the Effect of Cosmological Large-Scale Structure on the Orientation of Galaxies". The Astrophysical Journal 640 (2): L111–L114. 
  7. ^ Alder, Robert (2006). Galaxies like necklace beads. Astronomy magazine. Retrieved on 2006-08-10.
  8. ^ Hubble, E. P. (May 1926). "A spiral nebula as a stellar system: Messier 33.". The Astrophysical Journal 63: 236-274. Retrieved on 2007-09-21. 

Edwin Powell Hubble (November 20, 1889 – September 28, 1953) was an American astronomer. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 264th day of the year (265th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 258th day of the year (259th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 258th day of the year (259th in leap years) in the Gregorian calendar. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 269th day of the year (270th in leap years) in the Gregorian calendar. ... Year 2006 (MMVI) was a common year starting on Sunday of the Gregorian calendar. ... is the 222nd day of the year (223rd in leap years) in the Gregorian calendar. ... Edwin Powell Hubble (November 20, 1889 – September 28, 1953) was an American astronomer. ... Year 2007 (MMVII) is the current year, a common year starting on Monday of the Gregorian calendar and the AD/CE era in the 21st century. ... is the 264th day of the year (265th in leap years) in the Gregorian calendar. ...

External links

Wikimedia Commons has media related to:
Category:Spiral galaxies

  Results from FactBites:
 
Spiral galaxy - Wikipedia, the free encyclopedia (483 words)
Spiral Galaxy M74 presents a face-on view of its spiral arms.
Spiral galaxies are so named due to the bright arms of star formation within the disk that extend—roughly logarithmically—from the bulge.
The disks of spiral galaxies tend to be surrounded by large spheroid halos of Population II stars, many of which are concentrated in globular clusters that orbit the galactic center.
Barred spiral galaxy - Wikipedia, the free encyclopedia (698 words)
A barred spiral galaxy is a spiral galaxy with a band of bright stars emerging from the center and running across the middle of the galaxy.
Spiral arms appear to emerge from the ends of the "bar" in these galaxies, whereas they appear to emerge directly from the core in ordinary spiral galaxies.
When observing a distant spiral galaxy with a rotational axis perpendicular to the line of site, or one that appears "edge-on" to the observer, the shape of the bulge can be easily observed, and therefore quickly classified as either a barred spiral or a regular spiral.
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