Yellow supergiant

[4] Supergiants are identified in the Yerkes spectral classification by luminosities classes Ia and Ib, with intermediates such as Iab and Ia/ab sometimes being used.

[5] The neutral oxygen lines, such as the 777.3 nm triplet, have also been used since they are extremely sensitive to luminosity across a wide range of spectral types.

[12] Many yellow supergiants are in a region of the HR diagram known as the instability strip because their temperatures and luminosities cause them to be dynamically unstable.

Most yellow supergiants observed in the instability strip are Cepheid variables, named for δ Cephei, which pulsate with well-defined periods that are related to their luminosities.

Stellar models show that blue loops rely on particular chemical makeups and other assumptions, but they are most likely for stars of low red supergiant mass.

While cooling for the first time or when performing a sufficiently extended blue loop, yellow supergiants will cross the instability strip and pulsate as Classical Cepheid variables with periods around ten days and longer.

Such yellow stars may be given a supergiant luminosity class despite their low masses but assisted by luminous pulsation.

In the AGB thermal pulses from the helium-fusing shell of stars may cause a blue loop across the instability strip.

Such stars will pulsate as W Virginis variables and again may be classified as relatively low luminosity yellow supergiants.

These stars have masses lower than the sun, but luminosities that can be 10,000 L☉ or higher, so they will become yellow supergiants for a short time.

They may be post-AGB stars reignited by a late helium shell flash, or they could be formed from white dwarf mergers.

If these are confirmed then an explanation must be found for how a star of moderate mass still with a helium core would cause a core-collapse supernova.