Galaxies can be divided into two main types. Early-type galaxies are compact and consist mainly of old stars, and their radio emission is primarily associated with the activity of the central black hole. In contrast, late-type galaxies have characteristic spiral arms and a large population of young stars, which largely account for their radio emission. Even among early-type galaxies, however, we observe significant differences. Some of them emit radio waves from regions much larger than the galaxy itself, while others emit only from a small region around the center.
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