| 单词 | planetary nebula |
| 释义 | planetary nebulanoun Astronomy.an expanding shell of thin ionized gas that is ejected from and surrounds a hot, dying star of about the same mass as the sun; the gas absorbs ultraviolet radiation from the central star and reemits it as visible light by the process of fluorescence. Origin of planetary nebulaFirst recorded in 1850–55; so named for its resemblance to the planets Uranus and Neptune Words nearby planetary nebulaplanet, plane table, planetarian, planetarium, planetary, planetary nebula, planetary precession, planetesimal, planetesimal hypothesis, planet gear, planetoid Dictionary.com UnabridgedBased on the Random House Unabridged Dictionary, © Random House, Inc. 2020 British Dictionary definitions for planetary nebulaplanetary nebula nounan expanding shell of gas surrounding a dying star, formed from matter ejected from the star's outer layers; the gas is ionized by the remaining hot stellar core, emitting light in the process Word Origin for planetary nebulaC18: named from its (occasional) resemblance to a planetary disc Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012 Scientific definitions for planetary nebulaplanetary nebula [ plăn′ĭ-tĕr′ē ] A nebula consisting of a rapidly expanding shell of glowing gas, mostly hydrogen, ejected from a red giant upon its collapse into a white dwarf. Ultraviolet radiation from the hot, luminous white dwarf ionizes the expanding gas and causes it to glow. The nebula disappears once the cooling dwarf can no longer ionize it, and its material eventually returns to the interstellar medium. See more at white dwarf. The American Heritage® Science Dictionary Copyright © 2011. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved. |
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