<Li> As gas clouds move into the density wave, the local mass density increases . Since the criteria for cloud collapse (the Jeans instability) depends on density, a higher density makes it more likely for clouds to collapse and form stars . </Li> <Li> As the compression wave goes through, it triggers star formation on the leading edge of the spiral arms . </Li> <Li> As clouds get swept up by the spiral arms, they collide with one another and drive shock waves through the gas, which in turn causes the gas to collapse and form stars . </Li> <P> The arms appear brighter because there are more young stars (hence more massive, bright stars). These massive, bright stars also die out quickly, which would leave just the darker background stellar distribution behind the waves, hence making the waves visible . </P>

What shape of galaxy has a very bright nucleus with two or more curved arms