The Traditional Approach: Turbulence and Gravitational Instability
Many modern astronomers accept that galaxies must form from roughly uniform clouds of gas. Different authors use different values for the density of such initial clouds, their estimates usually being based on ‘reasonable physical arguments’, but such estimates are always in the range 10−24 g cm−3 to 10−27g cm−3 (the lower limit is the observed mean density within a cluster of galaxies, and the upper the observed mean density within our own galaxy).
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