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. A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter Galaxy morphological classification is a system used by Astronomers to divide galaxies into groups based on their visual appearance 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. The Hubble sequence is a morphological classification scheme for Galaxies invented by Edwin Hubble in 1936 Spiral galaxies consist of a flat, rotating disk of stars, gas and dust, and a central concentration of stars known as the bulge. A disc is a component of Disc galaxies, such as Spiral galaxies, or Lenticular galaxies. A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth In Astronomy, a bulge is a tightly packed group of Stars The term almost exclusively refers to the central group of stars found in most spiral galaxies. These are surrounded by a much fainter halo of stars, many of which reside in globular clusters. A globular cluster is a spherical collection of Stars that orbits a galactic core as a Satellite.

Spiral galaxies are named for the (usually two-armed) spiral structures that extend from the bulge into the disk. In Mathematics, a spiral is a Curve which emanates from a central point getting progressively farther away as it revolves around the point 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. OB stars are hot massive Stars of spectral types O or B which form in loosely organized groups called OB associations They are short lived and 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. A barred spiral galaxy is a Spiral galaxy with a central bar-shaped structure composed of Stars Bars are found in approximately half of all spiral galaxies The most convincing evidence for its existence comes from a recent survey, performed by the Spitzer Space Telescope, of stars in the Galactic center. The Spitzer Space Telescope (formerly the Space Infrared Telescope Facility, SIRTF) is an Infrared Space observatory. ^[2]

Together with irregulars, spiral galaxies make up approximately 70% of galaxies in the local Universe. An irregular galaxy is a Galaxy that does not fall into any of the regular classes of the Hubble sequence. ^[3] They are mostly found in low-density regions and are rare in the centers of galaxy clusters. ^[4]

Contents 1 Structure 1.1 Spiral arms 1.2 Galactic bulge 1.3 Galactic spheroid 2 Origin of the spiral structure 2.1 Density waves model 2.1.1 Historical theory of Lin and Shu 2.1.2 Star formation caused by density waves 2.2 Shock waves model 2.3 More younger stars in spiral arms 2.4 Alignment of spin axis with cosmic voids 3 Spiral nebulae 4 Famous examples 5 See also 5.1 Components 5.2 Classification 5.3 Other 6 References 7 External links

Structure

Spiral galaxies consist of several distinct components:

• A flat, rotating disc of (mainly young) stars and interstellar matter
• A central stellar bulge of mainly older stars, which resembles an elliptical galaxy
• A near-spherical halo of stars, including many in globular clusters
• A supermassive black hole at the very center of the central bulge

The relative importance, in terms of mass, brightness and size, of the different components varies from galaxy to galaxy. A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth In Astronomy, a bulge is a tightly packed group of Stars The term almost exclusively refers to the central group of stars found in most spiral galaxies. An elliptical galaxy is a Galaxy belonging to one of the three main classes of galaxy originally described by Edwin Hubble (whose name was dedicated A globular cluster is a spherical collection of Stars that orbits a galactic core as a Satellite. A supermassive black hole is a Black hole with a Mass of an order of magnitude between 105 and 1

Spiral arms

Spiral arms are regions of stars that extend from the center of spiral and barred spiral galaxies. A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth An unbarred spiral galaxy is a galaxy without a central bar or one that is not a Barred spiral galaxy. A barred spiral galaxy is a Spiral galaxy with a central bar-shaped structure composed of Stars Bars are found in approximately half of all spiral galaxies A galaxy is a massive gravitationally bound system consisting of Stars an Interstellar medium of gas and dust, and Dark matter 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.

Galactic bulge

A bulge is a huge, tightly packed group of stars. In Astronomy, a bulge is a tightly packed group of Stars The term almost exclusively refers to the central group of stars found in most spiral galaxies. A star is a massive luminous ball of plasma. The nearest star to Earth is the Sun, which is the source of most of the Energy on Earth The term commonly refers to the central group of stars found in most spiral galaxies.

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. Some bulges have similar properties to those of elliptical galaxies (scaled down to lower mass and luminosity), and others simply appear as higher density centers of disks, with properties similar to disk galaxies.

Many bulges are thought to host a supermassive black hole at their center. A supermassive black hole is a Black hole with a Mass of an order of magnitude between 105 and 1 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. Sagittarius A* (pronounced "A-star" standard abbreviation Sgr A*) is a bright and very compact source of Radio emission at the center of

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 coordinate system is a Celestial coordinate system which is centered on the Sun and is aligned with the apparent center of the Milky Way galaxy In Physics, an orbit is the gravitationally curved path of one object around a point or another body for example the gravitational orbit of a planet around a star The Galactic Center is the rotational center of the Milky Way Galaxy. Equation A spheroid centered at the origin and rotated about the z axis is defined by the implicit equation \left(\frac{x}{a}\right^2+\left(\frac{y}{a}\right^2+\left(\frac{z}{b}\right^2 In Astronomy, a bulge is a tightly packed group of Stars The term almost exclusively refers to the central group of stars found in most spiral galaxies.

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. Direct motion is the motion of a Planetary body in a direction similar to that of other bodies within its system and is sometimes called prograde motion. Inclination in general is the Angle between a Reference plane and another plane or axis of direction 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. Galaxy mergers can occur when two (or more galaxies collide They are the most violent type of galaxy interaction. The Sagittarius Dwarf Elliptical Galaxy ( SagDEG) is an elliptically looped shaped 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). In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen In Astronomy and Physical cosmology, the metallicity of an object is the proportion of its matter made up of Chemical elements other than Hydrogen The galactic halo also contains many globular clusters. A globular cluster is a spherical collection of Stars that orbits a galactic core as a Satellite.

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. According to the Hertzsprung-Russell diagram, a red dwarf star is a small and relatively cool Star, of the Main sequence, either late K The Sun (Sol is the Star at the center of the Solar System. Kapteyn's 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 in the Constellation Ursa Major. It is a yellowish class G8 Subdwarf catalogued by Stephen Groombridge 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. The proper motion of a Star is the measurement of its change in position in the sky over time after Improper motions are accounted for

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. Bertil Lindblad ( Örebro November 26 1895 &ndash Saltsjöbaden (outside Stockholm) June 25 1965) was a Swedish 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. This is called the winding problem. 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.

There are two leading hypotheses or models for the spiral structures of galaxies:

• Star formation caused by density waves in the galactic disk of the galaxy. Density wave theory or the Lin-Shu density wave theory is a theory proposed by C A disc is a component of Disc galaxies, such as Spiral galaxies, or Lenticular galaxies.
• The SSPSF model - Star formation caused by shock waves in the interstellar medium. The SSPSF (““stochastic self-propagating star formation model of star formation was proposed by Mueller & Arnett in 1976, generalized afterward by Gerola & Seiden in 1978

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

Density waves model

See also: Density wave theory

Bertil Lindblad proposed that the arms represent regions of enhanced density (density waves) that rotate more slowly than the galaxy’s stars and gas. Density wave theory or the Lin-Shu density wave theory is a theory proposed by C Bertil Lindblad ( Örebro November 26 1895 &ndash Saltsjöbaden (outside Stockholm) June 25 1965) was a Swedish 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.

This idea was developed into density wave theory by C. Density wave theory or the Lin-Shu density wave theory is a theory proposed by C C. Lin and Frank Shu in 1964. Frank Shu (born in Kunming, China) is an Astrophysicist, Author and Professor of Astronomy at the University ^[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.

Historical theory of Lin and Shu

The first acceptable theory for the spiral structure was devised by C. C. Lin and Frank Shu in 1964.

• 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 centre. 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. 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.

Star formation caused by density waves

The following hypotheses exist for star formation caused by density waves:

• As gas clouds move into the density wave, the local mass density increases. Since the criteria for cloud collapse (the Jeans instability) depends on density, a higher density makes it more likely for clouds to collapse and form stars. The Jeans instability causes the collapse of interstellar gas clouds and subsequent star formation

• As the compression wave goes through, it triggers star formation on the leading edge of the spiral arms.

• As clouds get swept up by the spiral arms, they collide with one another and drive shock waves through the gas, which in turn causes the gas to collapse and form stars. For the music album by Converter see Shock Front For the 1977 horror film see Shock Waves A shock wave (also called

Shock waves model

Main article: SSPSF model

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. The SSPSF (““stochastic self-propagating star formation model of star formation was proposed by Mueller & Arnett in 1976, generalized afterward by Gerola & Seiden in 1978 The SSPSF (““stochastic self-propagating star formation model of star formation was proposed by Mueller & Arnett in 1976, generalized afterward by Gerola & Seiden in 1978 This model proposes that star formation propagates via the action of shock waves produced by stellar winds and supernovae that compose the interstellar medium. Star Formation is the process by which dense parts of Molecular clouds collapse into a ball of plasma to form a Star.

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

More younger stars in spiral arms

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.

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.

Alignment of spin axis with cosmic voids

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. In Astronomy, voids are the empty spaces between filaments, the largest-scale structures in the Universe, that contain very few or no galaxies ^[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. In Physical cosmology, filaments are the largest known structures in the Universe, thread-like structures with a typical length of 50 to 80 ''h''-1 ^[7]

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 Milky Way galaxy. The Whirlpool Galaxy (also known as Messier 51a, M51a, or NGC 5194) is an interacting Spiral galaxy located at a distance of A nebula (from Latin: "mist" pl nebulae or nebulæ, with ligature or nebulas) is an Interstellar cloud of The Milky Way (a translation of the Latin Via Lactea, in turn derived from the Greek Γαλαξίας (Galaxias sometimes referred to simply The idea that they might instead be other galaxies, independent of the Milky Way, was the subject of The Great Debate of 1920, between Heber Curtis and Harvard-based Harlow Shapley. This is about the infamous discussion of astronomy For the Dream Theater song about Stem cell research, see The Great Debate (song. Year 1920 ( MCMXX) was a Leap year starting on Thursday (link will display 1920 of the Gregorian calendar Heber Doust Curtis ( June 27 1872 – January 9 1942) was an American Astronomer. Harlow Shapley ( November 2 1885 &ndash October 20 1972) was an American Astronomer. In 1926, Edwin Hubble^[8] observed Cepheid variables in several spiral nebulae, including the Andromeda Galaxy, proving that they are, in fact, entire galaxies outside our own. 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. A Cepheid variable (pron ˈse-f(ē-id or ˈsē-f(ē-id or Cepheid is a member of a particular class of Variable stars notable for a fairly tight correlation The Andromeda Galaxy (ænˈdrɒmədə also known as Messier 31, M31, or NGC 224; often referred to as the Great Andromeda The term “spiral nebula” has since fallen into disuse.

Famous examples

Triangulum Whirlpool

Milky Way Andromeda

Sunflower

See also

Components

Galactic disk Bulge (astronomy)

Galactic halo

Galactic corona

Classification

Galaxy color-magnitude diagram Galaxy morphological classification Hubble sequence Disc galaxy Active galaxy Barred spiral galaxy

Dwarf galaxy Dwarf elliptical galaxy Dwarf spheroidal galaxy Elliptical galaxy Intermediate spiral galaxy Irregular galaxy

Lenticular galaxy Ring galaxy Starburst galaxy Seyfert galaxy Unbarred spiral galaxy

Other

Galactic coordinate system Galaxy formation and evolution Groups and clusters of galaxies

List of galaxies List of nearest galaxies

Timeline of galaxies, clusters of galaxies, and large scale structure Tully-Fisher relation

References

External links

• Giudice, G. F. ; Mollerach, S. ; Roulet, E. (1994). "Can EROS/MACHO be detecting the galactic spheroid instead of the galactic halo?". Physical Review D 50: 2406–2413. doi:10.1103/PhysRevD.50.2406. A digital object identifier ( DOI) is a permanent identifier given to an Electronic document.  
• Stephens, Tim. "AEGIS survey reveals new principle governing galaxy formation and evolution", UC Santa Cruz, March 6, 2007. Retrieved on 2006-05-24. Year 2006 ( MMVI) was a Common year starting on Sunday of the Gregorian calendar. Events 1218 - The Fifth Crusade leaves Acre for Egypt. 1276 - Magnus Ladulås is crowned  
• On Spiral Nebulae, Adriaan van Maanen, et al This article deals with the view that the nebulae are associated with the Milky Way.
• Spiral Galaxies @ SEDS Messier pages
• SpiralZoom.com, an educational website about Spiral Galaxies and other spiral formations found in nature. For high school & general audience.
• Spiral Structure explained

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