Galaxy

:This article is about celestial bodies. For alternate meanings, see galaxy (disambiguation).

Types of galaxies

Galaxies come in three main types: ellipticals, spirals, and irregulars. A slightly more extensive description of galaxy types is given by the Hubble sequence which serves well for many purposes. While the Hubble sequence does encompass all galaxies, it is entirely based upon visual morphological type. Hence it may miss the importance of certain characteristics of galaxies such as star formation rate.

Related Topics:
Ellipticals - Spirals - Irregulars - Hubble sequence

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Our own galaxy, the Milky Way, sometimes simply called the Galaxy (with uppercase), is a large disk-shaped barred spiral galaxy about 30 kiloparsecs or 100,000 light years in diameter and 3,000 light years in thickness. It contains about 3x1011 stars and has a total mass of about 6x1011 times the mass of the Sun.

Related Topics:
Milky Way - Barred spiral galaxy - 30 kiloparsecs - Light year

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In spiral galaxies, the spiral arms have the shape of approximate logarithmic spirals, a pattern that can be theoretically shown to result from a disturbance in a uniformly rotating mass of stars.

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Like the stars, the spiral arms also rotate around the center, but they do so with constant angular velocity. That means that stars pass in and out of spiral arms. The spiral arms are thought to be areas of high density or density waves. As stars move into an arm, they slow down, thus creating a higher density; this is akin to a "wave" of slowdowns moving along a highway full of moving cars.

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The arms are visible because the high density facilitates star formation and they therefore harbor many bright and young stars.

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A new set of galaxies, classified as Ultra Compact Dwarf Galaxies, were discovered in 2003 by Michael Drinkwater of the University of Queensland.

Related Topics:
Ultra Compact Dwarf Galaxies - Michael Drinkwater - University of Queensland

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