Other Galaxies Galaxies are organized systems thousands to hundreds of thousands of light years across made of tens of millions to trillions of stars sometimes mixed with gas and dust all held together by their mutual gravity. There is not some unfortunate astronomer counting up all the stars in the galaxies. You can quickly get an estimate of the number of stars in a galaxy by dividing the total luminosity of the galaxy by a typical star's luminosity. A more accurate value would result if you use the galaxy's luminosity function (a table of the proportion of stars of a given luminosity). Or you could divide the total mass of the galaxy by a typical star's mass (or use the mass function to get the proportions right). The distances between galaxies are large and are often measured in megaparsecs. A megaparsec is one million parsecs (or about 3.3 million light years). For instance, the distance between the Milky Way and the closest large galaxy, the Andromeda Galaxy, is about 0.899 megaparsecs.
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