ROBERT GENDLER / SCIENCE PHOTO LIBRARY ROBERT GENDLER / SCIENCE PHOTO LIBRARY
Helix nebula (NGC 7293), optical image. This is a planetary nebula, a series of shells of gas cast off from a Sun-like star towards the end of its life. The outer layers of the star expand as the core contracts, and they are cast off into space as the core becomes unstable and pulsates. This exposes the very hot core, which emits radiation that ionises the shells of gas and makes them glow. After this phase, the core cools and no longer emits the radiation needed to ionise the gas. At this point, the star becomes a white dwarf. The Helix nebula lies around 650 light years from Earth in the constellation Aquarius.
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