Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer. Spiral galaxies viewed face-on display a grand design , with graceful spiral arms traced by bright star clusters and glowing stellar nurseries. Notably, the edge-on perspective of these galaxies allows a measurement of their galactic rotation speed using the Doppler effect . Plotting rotation speed versus distance from the center determines a galaxy's gravitational mass and historically led to premier evidence for mysterious Dark Matter .
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