Assertion: nebula | /r/HasProperty | formed
Facets:
Source sentences:
text: The structure of the bright central region is a result of the fast stellar wind from the central star interacting with the material expelled when the nebula was formed.
text: The nebula was formed about 4,000 years ago, when the central star, a red giant, ejected a shell of ionized gas in the final stage of its evolution before becoming a white dwarf.
text: These nebulae are formed when evolved giant stars eject their outer envelopes, exposing the hot core of the star, which then ionizes the surrounding cloud of expelled material with ultraviolet light.