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unit 2 galaxies.docx

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PCS 181
Margaret Buckby

Galaxies Types and Classifications The identification of other galaxies or independent stellar systems goes back to 1924 when Edwin Hubble found Cepheid variables in the nearby galaxies Messier 33 and Messier 31 Application of the PeriodLuminosity Relation for these variables in the Magellanic Clouds established that these were objects outside the confines of the Milky Way and of sizes comparable to it More thorough studies show that galaxies exhibit a 13wide range of properties from giant galaxies of 10 solar masses and sizes over 6150000 pc in diameter to dwarf galaxies of 10 solar masses and about 1000 pc in size Dwarf galaxies are by far the most numerous It is estimated that about 100 billion galaxies in the universe are observable with existing telescopesHubble quickly realized that the vast majority of galaxies have only a small number of shapes A classification based on their optical appearance or morphology is limited to four fundamental types of galaxiesellipticals spirals irregulars and S0s The Hubble classification has proven to be immensely valuable to the study of galaxies Although originally based solely on optical appearance appearance is also closely correlated with other physical properties of galaxies Elliptical E galaxies Ellipticals also sometimes called earlytype galaxies were so named because they look like elliptical blobs of light In general they show no obvious structural features other than a smooth concentration of light to the center The decrease in surface brightness with distance may be expressed in different ways but one reasonable 2approximation is IrIar where I is a central brightness r is the distance from the center and a is a distance at which the brightness is onequarter of that in the center In other words the brightness roughly falls off as the inverse square of the distance from the center of the galaxy Many ellipticals are round but others are noticeably elongated or flattened If the long axis is measured to have a dimension of a and the perpendicular short axis is measured as b then an ellipticity can be defined as 10 1b a rounded to the nearest unit
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