extragalactic H II region

extragalactic H II region

(eks-tră-gă-lak -tik) A cloud of hot ionized gas that is isolated in extragalactic space and is believed to consist of intergalactic gas that has begun to condense into stars and will probably become a dwarf irregular galaxy. Extragalactic H II regions were identified in 1970. They have no old stars, and a low metallicity indicates that some (like the compact galaxy IZw18) are less than 108 years old. These are the only ‘galaxies' significantly different in age from our Galaxy.
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Yet it is here, only 10 miles from downtown Boston, that I finally made some sense of the galaxy zoo known as the Virgo Cluster; first tracked down an extragalactic H II region (NGC 604 in Triangulum); became acquainted not only with the two most spectacular comets of my life but with other, far more elusive ones; knocked off many of the Messier objects that I never got around to observing before; winnowed a warped planetary nebula (10th-magnitude NGC 2438 in Puppis) from the glow of a major thoroughfare; watched Saturn's rings turn razor thin as Earth crossed their plane for the first time since I became an amateur astronomer.