Portal:Astronomy/Picture/11 July 2005
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Planetary nebulae are formed when small to medium mass stars gently eject their outer layers once they have exhausted all the hydrogen and helium which has been burning in nuclear fusion reactions during the star's lifetime. Our Sun will become a planetary nebula in about 5 billion years' time. The Hubble Space Telescope has revealed more intricate detail and structure in planetary nebulae than astronomers had previously thought possible.