gas and dust. Studies of this material indicate that it is slowing down, most likely because of these collisions. Astronomers think that the bright yellow and green regions inside the faint blue shell are heavier elementsΓÇö including knots of sulfur and oxygenΓÇöcreated in the inner layers of the original star. As the blue exterior has slowed, the heavier materials have caught up with, and are now breaking through, it. This movement of heavy elements can actually be observed: Over several years, individual yellow and green knots appear, brighten, and fade as the heavy elements from deep within the supernova catch up to, and move through, the lighter blue shell.
