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...stars with surface temperatures typically of 25,000–50,000 K (although a few O-type stars with vastly greater temperatures have been described); lines of ionized helium appear in the spectra. Class B stars typically range from 10,000 K to 25,000 K and are also bluish white but show neutral helium lines. The surface temperatures of A-type stars range from 7,400 K to about 10,000 K; lines...
...I consists of younger stars, clusters, and associations—i.e., those that formed about 1,000,000 to 100,000,000 years ago. Certain stars, such as the very hot, blue-white O and B types (some of which are less than 1,000,000 years old), are designated as extreme Population I objects. All known Population I members occur near and in the arms of the Milky Way system and other...
The hot B-type stars, such as Epsilon Orionis, are characterized by lines of helium and of singly ionized oxygen, nitrogen, and neon. In very hot O-type stars, lines of ionized helium appear. Other prominent features include lines of doubly ionized nitrogen, oxygen, and carbon and of trebly ionized silicon, all of which require more energy to produce.
...the absorption lines of the nearby stars, whereas bright nebulae that emit their own light show characteristic emission lines quite unlike stars. The brightest reflection nebulae are illuminated by B-type stars that are very luminous but have temperatures lower than about 25,000 K, somewhat cooler than the O-type stars that would ionize the hydrogen in the gas and produce a diffuse emission...
...fact that their constituent stars are very much brighter than the stars constituting globular clusters. The most luminous stars in stellar associations are very young stars of spectral types O and B. They have absolute luminosities as bright as any star in the Galaxy—on the order of one million times the luminosity of the Sun. Such stars have very short lifetimes, only lasting a few...
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