Figure 13-15 A Spiral Galaxy Known as the Southern Pinwheel Galaxy, M83 lies in the southern constellation Hydra about 15 million ly from Earth. (a) This visible-light image clearly shows the spiral arms. The presence of young stars and glowing, energized hydrogen indicates that star formation takes place in spiral arms. (b) This radio view at a wavelength of 21 cm shows the emission from interstellar hydrogen gas (H I). Note that essentially the same pattern of spiral arms is traced out in this image as in the visible-light photograph. (c) M83 has a much smoother appearance in this near-infrared view. This shows that cooler stars, which emit strongly in the infrared, are spread more uniformly across the galaxy’s disk. Note the elongated bar shape of the central bulge.