An Etymological Dictionary of Astronomy and Astrophysics
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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, → Hadar.

→ star.

Same as → blue horizontal branch star.

→ blue; → horizontal; → branch; → star.

Same as → blue horizontal branch star.

→ blue; → horizontal; → branch; → star.

A → post-AGB star that undergoes a last → thermal pulse when it is already on the → white dwarf  → cooling track. The thermal pulse will expand the hot central star, whereby hydrogen will be ingested into the → helium burning shell. This will temporarily return the star to the → AGB phase it has previously left.

→ born; → again; → asymptotic giant branch; → star.

Same as → extreme horizontal branch star.

→ extreme horizontal branch star.

Same as → extreme horizontal branch star.

→ extreme horizontal branch star.

Same as → field horizontal branch star.

→ field; → horizontal; → branch; → star.

A collective designation for massive O and B stars.

→ O star; → B star.

Same as → red horizontal branch star.

→ red; → horizontal; → branch; → star.

Same as → red horizontal branch star.

→ red; → horizontal; → branch; → star.

Same as → supra-horizontal branch star.

→ supra-; → horizontal; → branch; → star.

A member of a class of → B stars that are situated along the → main sequence with → spectral types ranging from B2 to B9 and masses from 3 to 7 → solar masses. In the → H-R diagram the SPB group lies below → beta Cephei variables, which are more massive. SPBs show light and line-profile variations that are multi-periodic with periods of the order of days. This variability is understood in terms of non-radial → stellar pulsations, and their → oscillation modes are high-order → g modes. Theoretical models attribute the pulsational nature of SPBs to the → kappa mechanism, acting in the metal → opacity bump at 2 x 10⁵ K. Their g-mode pulsations penetrate deep into the stellar interior, making these objects very promising for → asteroseismology. Several oscillation modes are excited simultaneously, resulting in periodicities on time scales of the order of months or even years. The prototype of this group is 53 Per. First introduced as a distinct class by Waelkens (1991, A&A 246, 453).

→ slow; → pulsating; → B star.