Extragalactic nebula

…In the past, they were lumped together as nebulae due to their appearance, but in 1925, E. Hubble identified Cepheid variable stars in three nebulae, including the Andromeda Galaxy, and when their distances were estimated, it became clear that some of the nebulae were outside the Milky Way, the huge collection of stars that surrounds our solar system, and were of the same size as the Milky Way. For this reason, they are called extragalactic nebulae and are distinguished from intragalactic nebulae such as emission nebulae, diffuse nebulae, and planetary nebulae. They have also been called island universes or microcosms to describe the large-scale, densely packed star system. …

*Some of the terminology that refers to "extragalactic nebula" is listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

…かつてはその見かけによって〈星雲nebula〉として一括されていたが，1925年にE.ハッブルによって〈アンドロメダ星雲〉を含む3個の〈星雲〉にセファイド型変光星が同定され，その距離が推算されるに及んで，星雲の一部がわれわれの太陽系を包む巨大な恒星の集りである銀河系の外にあって，銀河系と同等の規模をもつことが明らかになった。このため，銀河系外星雲extragalactic nebulaと呼んで輝線星雲，散光星雲，惑星状星雲などの銀河系内星雲と区別される。また，その大規模に密集した恒星系のようすを表して，島宇宙とか小宇宙と呼んだこともある。…

※「extragalactic nebula」について言及している用語解説の一部を掲載しています。

出典｜株式会社平凡社世界大百科事典 第２版について | 情報