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It mainly grew from doppler measurements of the spectral line of hydrogen at 21 cm, hydrogen which usually surrounds spiral galaxies and extend way farther than the "visible galaxy" (that is stars). If stars were leaving galaxies we would observe this in a lot of ways: flows of startstars leaving, for example, or a big correlation between the age of the galaxy and the number of the stars or its spacial extension. We would observe that the older galaxies tend to have less stars than similar younger galaxies.

It mainly grew from doppler measurements of the spectral line of hydrogen at 21 cm, hydrogen which usually surrounds spiral galaxies and extend way farther than the "visible galaxy" (that is stars). If stars were leaving galaxies we would observe this in a lot of ways: flows of start leaving, for example, or a big correlation between the age of the galaxy and the number of the stars or its spacial extension. We would observe that the older galaxies tend to have less stars than similar younger galaxies.

It mainly grew from doppler measurements of the spectral line of hydrogen at 21 cm, hydrogen which usually surrounds spiral galaxies and extend way farther than the "visible galaxy" (that is stars). If stars were leaving galaxies we would observe this in a lot of ways: flows of stars leaving, for example, or a big correlation between the age of the galaxy and the number of the stars or its spacial extension. We would observe that the older galaxies tend to have less stars than similar younger galaxies.

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It mainly grew from doppler measurements of the spectral line of hydrogen at 21 cm, hydrogen which usually surrounds spiral galaxies and extend way farther than the "visible galaxy" (that is stars). If stars were leaving galaxies we would observe this in a lot of ways: flows of start leaving, for example, or a big correlation between the age of the galaxy and the number of the stars or its spacial extension. We would observe that the older galaxies tend to have less stars than similar younger galaxies.