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.
an 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
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.
noun, Astronomy. 1. the small component of the precession of the equinoxes contributed by the motion of the ecliptic, the change in orientation of the plane of the earth’s orbit being produced by the gravitational attraction of the planets.
[plan-i-tes-uh-muh l] /ˌplæn ɪˈtɛs ə məl/ Astronomy noun 1. one of the small celestial bodies that, according to one theory (planetesimal hypothesis) were fused together to form the of the solar system. adjective 2. of or relating to a planetesimal or planetesimals. planetesimal (plān’ĭ-těs’ə-məl) Any of innumerable small bodies of accreted gas and dust thought […]
noun, Machinery. 1. any of the gears in an epicyclic train surrounding and engaging with the sun gear.
[plan-i-tol-uh-jee] /ˌplæn ɪˈtɒl ə dʒi/ noun 1. the branch of astronomy that deals with the physical features of the . /ˌplænɪˈtɒlədʒɪ/ noun 1. (astronomy) the study of the origin, composition, and distribution of matter in the planets