Knowra O-type star O-type star An O-type star is a very hot, massive star whose spectrum shows prominent ionized-helium absorption lines. It is the hottest spectral class of ordinary stars.
Stellar spectral classification : A system for classifying stars by the absorption lines in their spectra, especially those sensitive to temperature. O-type stars are defined by their distinctive spectral lines within this temperature-based sequence.
Molecular cloud : A cold, dense interstellar cloud composed mainly of molecular hydrogen, where stars can form. O-type stars begin when dense regions of molecular clouds collapse.
Ionizing radiation : Radiation energetic enough to remove electrons from atoms or molecules. O-type stars supply much of the ionizing radiation in young stellar environments.
B-type star : A hot, blue-white stellar spectral class cooler than O type, with prominent neutral-helium lines. B-type stars are cooler, and their spectra show a different balance of helium ionization.
He II spectral line : A spectral feature produced by singly ionized helium, whose absorption lines mark very hot stellar atmospheres. Prominent He II absorption is a defining signature of O-type stars.
Star formation : The process by which dense regions of interstellar gas and dust collapse into stars. It explains how O-type stars gather mass before reaching the main sequence.
H-alpha line : A red hydrogen spectral line emitted when an electron falls from the third to the second energy level. Ionized gas powered by O stars often emits H-alpha light used to trace recent star formation.
Wolf–Rayet star : A hot, massive evolved star with broad emission lines formed in a dense, fast stellar wind. Some massive stars evolve from O-type spectra into the emission-dominated Wolf–Rayet phase.
Stellar atmosphere : The outer gaseous layers of a star, where much of its observed radiation and spectral lines form. Its high temperature and ionization state produce the characteristic O-star spectrum.
Initial mass function : A statistical description of the distribution of stellar masses at the time stars form. Its steep decline at high masses explains why O-type stars are rare.
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