Short answers, pulled from the story.
Pollux was the first giant star to be identified. This bright orange star in the constellation of the Twins was classified as a giant by Ejnar Hertzsprung in 1905 after he noticed it was far more luminous than the Sun despite having similar surface temperatures.
Ejnar Hertzsprung coined the terms giant and dwarf in 1905. He used these terms to distinguish stars of vastly different sizes that shared the same spectral color after observing that Pollux and other stars were far more luminous than the Sun.
The giant phase lasts for roughly 10% of a Sun-like star's life. This period of instability and growth begins when the core hydrogen is depleted and ends when the star moves on to its final chapters.
The helium flash occurs when the core temperature reaches 108 Kelvin. This explosive fusion process begins when helium starts to fuse into carbon and oxygen via the triple-alpha process in stars with a degenerate helium core.
Some Type O giants are more than a hundred thousand times as luminous as the Sun. These high-mass stars with masses above about 8 solar masses develop core-helium burning before the core becomes degenerate and eventually become a supernova.
RR Lyrae variables have periods less than a day while W Virginis variables have periods of 10 to 20 days. These yellow giants are high-luminosity stars that are generally unstable and serve as essential tools for mapping the cosmos.