Each day a different image or photograph of our fascinating universe is featured, along with a brief explanation written by a professional astronomer. The Hubble Double Bubble Planetary Nebula is bubbling over with excitement. The hot gas expands into the surrounding interstellar medium in a fashion similar to the inflation of hot air balloons . A supersonic shock-wave can form at the boundary, causing newly excited gas there to shine as electrons recombine with resident elements . In the above picture , colors are assigned according to the energy of the recombinant radiation.
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