Figure 22-25: Star Formation in the Whirlpool Galaxy The spiral galaxy M51 (called the Whirlpool) is a real-life example of the density-wave model illustrated in Figure 22-24. (a) This infrared image shows where dust has piled up as the material within M51 passes through its spiral arms. Radio images of M51 show that hydrogen gas also piles up in the same locations, thus beginning the formation of new stars. (b) By the time stars complete their formation process, their motion around the galaxy has swept them “downstream” of the positions of greatest dust density, just as depicted in Figure 22-24.

(a: NASA, JPL-Caltech, and R. Kennicutt [Univ. of Arizona]; b: DSS)