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11 - Chemical evolution in other sorts of galaxies

Published online by Cambridge University Press:  05 June 2012

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Summary

As we are used to call the appearance of the heavens, where it is surrounded with a bright zone, the Milky Way, it may not be amiss to point out some other very remarkable Nebulae which cannot well be less, but are probably much larger than our own system; and, being also extended, the inhabitants of the planets that attend the stars which compose them must likewise perceive the same phenomena. For which reason they may also be called milky-ways by way of distinction.

William Herschel, ‘On the Construction of the Heavens’, Phil. Trans., LXXV (1785), 213.

Dwarf galaxies

Introduction

Nearby dwarf galaxies are classified in four main types: (i) dwarf irregular (dIrr) galaxies are the most common type by number, and are usually unstructured gasrich systems with varying levels of star formation occurring in a haphazard manner across the galaxy. (ii) Blue compact dwarf (BCD) or H II galaxies are gas-rich systems dominated by very active star formation and resembling giant H II regions found in large galaxies. They appear to be forming stars at a rate which they can only maintain for a short period. (iii) Dwarf spheroidal (dSph) galaxies usually have no gas in their centre down to very low limits. Their stellar distribution is similar to that of globular clusters, although less centrally concentrated, but a detailed study of their HR diagrams often reveals that several distinct bursts of star formation have occurred in the past.

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Publisher: Cambridge University Press
Print publication year: 2009

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