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Types of Galaxies
Active Galactic Nuclei
Black Holes
Black-Hole in M87 (2019, 2021 Update)
Seyfert Galaxies
Radio Galaxies
Extremely Red Objects (ERO)
Formation and Evolution of Galaxy
Theory of Spiral Arm Formation
The Milky Way (+ 2022 Update)
Black Hole at the Milky Way Center
Dark Matter in the Milky Way

Types of Galaxies


Galaxies are systems of stars, gas and dust (see for example the Sombrero galaxy in Figure 05-01a). They exist in a wide variety of shapes and sizes. The simplest classification scheme, which was devised by Edwin Hubble, recognizes 4 basic types - elliptical, spiral, barred spiral, and irregular and arranges them in a sequence called the "tuning fork" diagram (see Figure 05-01b).

Elliptical galaxies are denotes by the letter E followed by the number from 0 to 7 to indicate the degree of flattening of the observed elliptical shape. An E0 galaxy appears spherical, where as an E7 galaxy is markedly flattened. The viewing angle adds some complications into this kind of classification, an elongated ellipsoid would appear spherical if seen "end-on".

Figure 05-01a Composition
[view large image]

Small ellipticals are "dwarf" systems denoted by "dE". The giant ellipticals are designated as "cD". This class of galaxies usually does not contain much interstellar matter.

galaxy types

Spiral galaxies, denote by S, have a central nucleus surrounded by a flattened disc with the stars, gas, and dust organized into a pattern of spiral arms. They are categorized according to the size of the nuclear bulge, the tightness of the spiral pattern, and the degree of "patchiness" in their arms. S0 is the transitional type called lenticular galaxy. An "Sa" galaxy has a large central nucleus and tightly wound, relatively smooth, arms; an "Sb" galaxy has a somewhat smaller nucleus and less tight arms that often contain conspicuous HII regions and clusters of hot young stars; and an "Sc" galaxy has a relatively small nucleus and loosely wound "knotty" arms dominated by numerous HII regions and youthful clumps of stars. In barred spirals, denoted by "SB", the arms emerge from the ends of what looks like a rigid bar of luminous matter that straddles the nucleus.

Irregular galaxies, which have no obvious nucleus or ordered structure, are denoted by "Irr" and are broadly subdivided into "Irr I" and "Irr II". Irr I galaxies display evidence of recent or ongoing star formation (e.g., OB associations (young stars) and HII regions (luminous nebulas)); Irr II galaxies have a disturbed appearance, and their shapes seem to have been distorted by violent internal activity or by collisions or close encounters with other galaxies.

Figure 05-01b Samples of Galaxy Types [view large image]

galaxy types The classification for the spirals is further subdivided into five luminosity classes: from I (most luminous) to V (least luminous). Figure 05-01b shows some real images for the different types of galaxies; while Figure 05-01c is a schematic diagram showing the side view of the elliptical galaxies and top view of the spiral galaxies. It is believed that a galaxy's type is determined by the amount of angular momentum it contains and the rate at which star formation has proceeded. Elliptical galaxies, and the spheroidal Population II halos of spirals, show little net systematic rotation. Their individual member stars and globular clusters move around their centers in random directions.

Figure 05-01c Types of Galaxies

galaxy merger Where the overall angular momentum was small, and star formation proceeded rapidly (thereby mopping up most of the gas early on in the evolutionary process), the end result would be an elliptical dominated by older stars and containing little, if any, gas. Where the angular momentum was greater, the result would be a more flattened system. Where star formation proceeded relatively slowly, the gaseous component would settle into a flattened disclike distribution. The first generation of stars would form within the spheroidal system and the later generations within the flattened disc as observed in the spiral and lenticular galaxies. Dwarf galaxies are much smaller than ordinary galaxies. Because of their size, they have relatively low gravity and matter can escape from them more easily. This property, combined with the fact that dwarf galaxies are the most common type of galaxy in the universe, makes them very important in understanding how the universe was seeded with various elements billions of years ago, when galaxies were forming. Recently in 2005, it is suggested that merger of gas clouds may also played a role in creating different galaxy type. Where a large galaxy was formed by the merger of many small gas clouds, it prevented the formation of disk structure and developed to a large elliptical galaxy (see Figure 05-01d).

Figure 05-01d Development Pathways [view large image]

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