ROBERT GENDLER & JIM MISTI / SCIENCE PHOTO LIBRARY ROBERT GENDLER & JIM MISTI / SCIENCE PHOTO LIBRARY
Helix nebula (NGC 7293). 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, radiation from which ionises the shells of gas, causing them to glow. Most stars pass through a planetary nebula phase but they are short lived, lasting only around 10,000 years before the core cools so much that it no longer emits the radiation needed to ionise the gas. The star becomes a white dwarf. The Helix nebula lies around 650 light years from Earth in the constellation Aquarius.
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