かつては惑星をもった1つの恒星と考えられていたが、現在では膨張する電離ガスに包まれた高温の星であるとされている星雲で、そのガスは星の発する強力な放射線によって蛍光性の光を放っている
(a nebula that was once thought to be a star with its planets but is now thought to be a very hot star surrounded by an expanding envelope of ionized gases that emit a fluorescent glow because of intense radiation from the star)
a planet called the Jovian planet
a nebula that has an undetermined form, called indeterminate nebula
a nebula called the Orion nebula
any star around which a planetary system revolves