This image demonstrates how astronomers use radio radiation to penetrate obscuring interstellar dust and reveal the inner workings of a nebula. The cloud of ionized hydrogen gas  (represented here as red) surrounds the Trapezium Cluster at the heart of Orion’s Great Nebula. Yellow, green, and blue represent progressively cooler gas. Without radio waves to penetrate the dust, astronomers could not easily determine the location and size of this cloud. 
    The red ionized hydrogen gas is called an H II region. The presence of such a region tells astronomers that hot, young stars are nearby. Ultraviolet radiation from the stars heats hydrogen atoms in sur- rounding gas clouds, causing them to lose their outer electrons and