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A star of spectral type B, whose spectrum is marked by absorption lines of hydrogen;
also known as B-type star. B-type optical spectra are characterized by the presence
of neutral He lines (mainly He I 4471 Å) at about type B9. He I strengthens up to
about B2, then decreases. Ionized helium (mainly He II 4541 Å) first appears at about
B0. Most metallic lines are absent or weak, except some absorption lines for the higher
ionization states of silicon, oxygen, carbon, and magnesium. These are hot stars with
effective temperatures ranging from about 10,000 K at B9 to nearly 30,000 K at B0.
They are between 3 and 20 solar masses. Some famous examples include Rigel, Achernar,
and Hadar.
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