Yellow giant

A giant star is a star with substantially larger radius and luminosity than a main-sequence (or dwarf) star of the same surface temperature. So they lie above the main sequence (luminosity class V in the Yerkes spectral classification) on the Hertzsprung–Russell diagram and correspond to luminosity classes II and III. The terms giant and dwarf were coined for stars of quite different luminosity despite similar temperature or spectral type by Ejnar Hertzsprung about 1905. Giant stars have radii up to a few hundred times the sun and luminosities between 10 and a few thousand times that of the Sun. Stars still more luminous than giants are referred to as supergiants and hypergiants. A hot, luminous main-sequence star may also be referred to as a giant, but any main sequence star is properly called a dwarf no matter how large and luminous it is. Giant stars with intermediate temperatures (spectral class G, F, and at least some A) are called yellow giants. They are far less numerous than red giants, partly because they only form from stars with somewhat higher masses, and partly because they spend less time in that phase of their lives. However, they are interesting because they include a number of important classes of variable stars. High luminosity yellow stars are generally unstable, leading to the instability strip on the HR diagram where the majority of stars are pulsating variables. The instability strip reaches from the main sequence up to hypergiant luminosities, but at the luminosities of giants there are several classes of variable stars: (1) RR Lyrae variables, pulsating horizontal branch class A (sometimes F) stars with periods less than a day and amplitudes of a magnitude of less; (2) W Virginis variables, more luminous pulsating variables also known as type II Cepheids, with periods of 10–20 days; (3) Type I Cepheid variables, more luminous still and mostly supergiants, with even longer periods; (4) Delta Scuti variables, rare subgiant or even main sequence stars. Yellow giants may be moderate mass stars evolving for the first time towards the red giant branch, or they may be more evolved stars on the horizontal branch. Evolution towards the red giant branch for the first time is very rapid while stars can spend much longer on the horizontal branch. Horizontal branch stars, with more heavy elements and lower mass, are more unstable. Examples: (1) Sigma Octantis (σ Octantis), an F-type giant and a Delta Scuti variable; (2) Alpha Aurigae Aa (α Aurigae Aa), a G-type giant, one of the stars making up Capella. [Giant star. Wikipedia]