The bipolar Butterfly Nebula NGC 6302
The spherical Abell 39 Nebula
For the first time, a team of astronomers based in Germany has detected the presence of magnetic fields in the central stars of four planetary nebulae. Planetary nebulae are expanding gas shells that remain after Sun-like stars eject their outer layers at the end of their lifetimes. It is a long-standing and unsolved mystery why 80% of all planetary nebulae are not spherical. Theories suggest that magnetic fields play a role in shaping planetary nebulae. The team, led by Stefan Jordan, has now discovered the first direct clue that magnetic fields might indeed create these remarkable shapes.
Planetary nebulae are expanding gas shells that are ejected by Sun-like stars at the end of their lifetimes. Sun-like stars spend most of their lifetime burning hydrogen into helium. At the end of this hydrogen fusion phase, these stars increase their diameter by about a factor of 100 and become “red giant stars”. At the end of the red giant phase, the outer layers of the star are blown away. The ejected gas continues to expand out from the remaining central star, which later evolves into a “white dwarf” when all nuclear fusion has ceased. Astronomers believe that a planetary nebula forms when a fast stellar wind that comes from the central star catches up a slower wind produced earlier when the star ejected most of its outer layers. At the boundary between the two winds, a shock occurs that produces the visible dense shell characteristic of planetary nebulae. The gas shell is excited and lighted up by the light emitted by the hot central star. The light from the central star is able to light up the planetary nebula for some 10 000 years.
The observed shapes of planetary nebulae are very puzzling: most of them (about 80%) are bipolar or elliptical rather than spherically symmetric. This complexity has lead to beautiful and amazing images obtained with modern telescopes. The pictures below compare planetary nebulae with bipolar (left) and spherical (right) shapes.
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