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definition | A region in the interstellar medium where processes of star formation are going on or have occurred in the past. |
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definition | An small irregular galaxy undergoing violent star formation activity. These objects appear blue by reason of containing clusters of hot, massive stars which ionize the surrounding interstellar gas. They are chemically unevolved since their metallicity is only 1/3 to 1/30 of the solar value. |
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definition | A type of ultra-cool brown dwarf with an effective temperature lower than 500 K. Near infrared spectra of these objects show deep absorption bands of H_{2}O and CH_4. So far only seven brown dwarf candidates belonging to this class have been found, all Y0 subtypes. These objects are very dim, with H magnitudes 19-23. The precise definition of the Y class requires new findings in the future about these objects. |
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definition | A white dwarf whose spectrum shows metal lines only; no H or He. |
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definition | A white dwarf whose spectrum shows carbon features of any kind. |
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definition | A main-sequence star, usually of spectral class B to F, whose spectrum shows bright emission lines superimposed on the normal absorption lines. The emission spectrum is explained by the presence of a circumstellar shell of gas surrounding the star at the equator. Shell stars are fast rotators. |