Galaxies emit all kinds of electromagnetic radiation, from x-rays to radio waves. From this "light," astronomers get a clear picture of what these galaxies look like. But very distant galaxies pose a special problem. Light from these galaxies travels great distances (billions of light-years) to reach Earth. During its journey, the light is "stretched" due to the expansion of space. As a result, much of the light from the most distant galaxies is no longer visible, but has been shifted to the infrared where present instruments are less sensitive. The only light now in the visible region of the spectrum comes from regions where hot, young stars reside. These stars emit mostly ultraviolet light. But because this light has been stretched, it appears as visible light by the time it reaches Earth. Studying these distant galaxies is like trying to put together a puzzle with some of the pieces missing. So, astronomers are studying nearby galaxies in ultraviolet light to compare their shapes with those of their distant relatives.
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