<P> A mechanism for formation of most planetary nebulae is thought to be the following : at the end of the star 's life , during the red - giant phase , the outer layers of the star are expelled by strong stellar winds . After most of the red giant 's atmosphere is dissipated , the ultraviolet radiation of the hot luminous core , called a planetary nebula nucleus ( PNN ) , ionizes the outer layers earlier ejected from the star . Absorbed ultraviolet light energises the shell of nebulous gas around the central star , causing it to appear as a brightly coloured planetary nebula . </P>
<P> A mechanism for formation of most planetary nebulae is thought to be the following : at the end of the star 's life , during the red - giant phase , the outer layers of the star are expelled by strong stellar winds . After most of the red giant 's atmosphere is dissipated , the ultraviolet radiation of the hot luminous core , called a planetary nebula nucleus ( PNN ) , ionizes the outer layers earlier ejected from the star . Absorbed ultraviolet light energises the shell of nebulous gas around the central star , causing it to appear as a brightly coloured planetary nebula . </P>
what is at the center of a planetary nebula
what is at the center of a planetary nebula
the hot luminous core , called a planetary nebula nucleus ( PNN )
the hot luminous core , called a planetary nebula nucleus ( PNN )