Above is the electron probability density distribution for the atomic state: n=2, l=0, m=0. The basic morphology is a sphere within a larger, concentric sphere.
      Two-shell planetary nebulae such as Shapley 1 (shown above) are fairly common. The shells are usually concentric.
Source: H.E. White, Physical Review, vol. 37, pg. 1423, 1931. Source: H.E. White, Physical Review, vol. 37, pg. 1423, 1931.



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