effect is not unlike that of a hazy day, when buildings a few blocks away are easy to see while distant mountains seem to disappear.
    M17 is similar to other gas clouds, such as the Orion Nebula —but it has its own mysteries. Hydrogen gas cools very quickly in nebulae, so any brightly glowing region must have a source of continuous heat. The heat usually comes from a cluster of hot stars, but M17 seems to lack such a heat source. Moreover, nebulae as large as M17 tend to be the sites of star formation; surprisingly, M17 shows no evidence of young stars.
    Even early observers of M17 noted that it was seemingly devoid of stars. Few of the stars in this image are actually associated with the nebula; many are hundreds or