Grand design spiral galaxy

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A Spitzer Space Telescope image of Messier 81, a grand design spiral. Ssc2003-06c.jpg
A Spitzer Space Telescope image of Messier 81, a grand design spiral.

A grand design spiral galaxy is a type of spiral galaxy with prominent and well-defined spiral arms, as opposed to multi-arm and flocculent spirals which have subtler structural features. The spiral arms of a grand design galaxy extend clearly around the galaxy through many radians and can be observed over a large fraction of the galaxy's radius. As of 2002, approximately 10 percent of all currently known spiral galaxies are classified as grand design type spirals, [1] including M51, M74, M81, M83, and M101.

Spiral galaxy galaxy having a number of arms of younger stars that spiral out from the centre containing older ones

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 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.

Flocculent spiral galaxy Patchy galaxy with discontinuous spiral arms

A flocculent spiral galaxy is a type of spiral galaxy. Unlike the well-defined spiral architecture of a grand design spiral galaxy, flocculent galaxies are patchy, with discontinuous spiral arms. Self-propagating star formation is the apparent explanation for the structure of flocculent spirals. Approximately 30% of spirals are flocculent, 10% are grand design, and the rest are referred to as "multi-armed". The multiple-arm type is sometimes grouped into the flocculent category.

Radian SI derived unit of angle

The radian is the SI unit for measuring angles, and is the standard unit of angular measure used in many areas of mathematics. The length of an arc of a unit circle is numerically equal to the measurement in radians of the angle that it subtends; one radian is just under 57.3 degrees. The unit was formerly an SI supplementary unit, but this category was abolished in 1995 and the radian is now considered an SI derived unit.

Origin of structure

Density wave theory is the preferred explanation for the well-defined structure of grand design spirals. [2] According to this theory, the spiral arms are created inside density waves that turn around the galaxy at different speeds from the stars in the galaxy's disk. Stars and gas are clumped in these dense regions due to gravitational attraction towards the dense material, though their location in the spiral arm may not be permanent. When they come close to the spiral arm, they are pulled toward the dense material by the force of gravity; and as they travel through the arm, they are slowed from exiting by the same gravitational pull. This causes the gas in particular to clump in the dense regions, which encourages gas clouds to collapse, producing star formation.

Density wave theory theory to explain the spiral arm structure of spiral galaxies

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 the idea of long-lived quasistatic spiral structure(QSSS hypothesis). In this hypothesis, the spiral pattern rotates in a particular angular frequency, whereas the stars in the galactic disk are orbiting at a different speed depending their distance to the galaxy center. The presence of spiral density waves in galaxies has implications on the star formation, since the gas orbiting around the galaxy may be compressed and form shock periodically. Theoretically, the formation of 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.

Star formation is the process by which dense regions within molecular clouds in interstellar space, sometimes referred to as "stellar nurseries" or "star-forming regions", collapse and form stars. As a branch of astronomy, star formation includes the study of the interstellar medium (ISM) and giant molecular clouds (GMC) as precursors to the star formation process, and the study of protostars and young stellar objects as its immediate products. It is closely related to planet formation, another branch of astronomy. Star formation theory, as well as accounting for the formation of a single star, must also account for the statistics of binary stars and the initial mass function. Most stars do not form in isolation but as part of a group of stars referred as star clusters or stellar associations.

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Galaxy Gravitationally bound astronomical structure

A galaxy is a gravitationally bound system of stars, stellar remnants, interstellar gas, dust, and dark matter. The word galaxy is derived from the Greek galaxias (γαλαξίας), literally "milky", a reference to the Milky Way. Galaxies range in size from dwarfs with just a few hundred million stars to giants with one hundred trillion stars, each orbiting its galaxy's center of mass.

Molecular cloud type of interstellar cloud

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Lenticular galaxy Type of galaxy intermediate between an elliptical and a spiral galaxy

A lenticular galaxy is a type of galaxy intermediate between an elliptical and a spiral galaxy in galaxy morphological classification schemes. It contains a large-scale disc but does not have large-scale spiral arms. Lenticular galaxies are disc galaxies that have used up or lost most of their interstellar matter and therefore have very little ongoing star formation. They may, however, retain significant dust in their disks. As a result, they consist mainly of aging stars. Despite the morphological differences, lenticular and elliptical galaxies share common properties like spectral features and scaling relations. Both can be considered early-type galaxies that are passively evolving, at least in the local part of the Universe. Connecting the E galaxies with the S0 galaxies are the ES galaxies with intermediate-scale discs.

Bulge (astronomy) A tightly packed group of stars within a larger formation

In astronomy, a bulge is a tightly packed group of stars within a larger formation. The term almost exclusively refers to the central group of stars found in most spiral galaxies. Bulges were historically thought to be elliptical galaxies that happened to have a disk of stars around them, but high-resolution images using the Hubble Space Telescope have revealed that many bulges lie at the heart of a spiral galaxy. It is now thought that there are at least two types of bulges: bulges that are like ellipticals and bulges that are like spiral galaxies.

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Barred spiral galaxy spiral galaxy with a central bar-shaped structure composed of stars

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Interacting galaxy individual galaxy whose gravitational field disturbs another galaxy

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Galactic tide Tidal force experienced by objects subject to the gravitational field of a galaxy

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SSPSF model

The SSPSF model of star formation was proposed by Mueller & Arnett in 1976, generalized afterward by Gerola & Seiden in 1978 and Gerola, Seiden, & Schulman in 1980. This model proposes that star formation propagates via the action of shock waves produced by stellar winds and supernovae traversing the gas that composes the interstellar medium.

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  1. Mihos, Chris (2002-01-11), Spiral Structure , retrieved 2007-05-30
  2. Masters, Karen (September 2002), What is the Origin of Spiral Structure in Galaxies, archived from the original on 2007-06-09, retrieved 2007-05-30