Giant star

[2] The terms giant and dwarf were coined for stars of quite different luminosity despite similar temperature or spectral type (namely K and M) by Ejnar Hertzsprung in 1905[3] or 1906.

[5] A star becomes a giant after all the hydrogen available for fusion at its core has been depleted and, as a result, leaves the main sequence.

This causes the outer layers to expand even further and generates a strong convective zone that brings heavy elements to the surface in a process called the first dredge-up.

According to stellar evolution theory, no star of such low mass can have evolved to that stage within the age of the Universe.

As with the earlier collapse of the helium core, this starts convection in the outer layers, triggers a second dredge-up, and causes a dramatic increase in size and luminosity.

Stars only remain on the AGB for around a million years, becoming increasingly unstable until they exhaust their fuel, go through a planetary nebula phase, and then become a carbon–oxygen white dwarf.

They start core-helium burning before the core becomes degenerate and develop smoothly into red supergiants without a strong increase in luminosity.

Classification is complex and difficult with small differences between luminosity classes and a continuous range of intermediate forms.

For most of their lifetimes, such stars have their interior thoroughly mixed by convection and so they can continue fusing hydrogen for a time in excess of 1012 years, much longer than the current age of the Universe.

Subgiants are an entirely separate spectroscopic luminosity class (IV) from giants, but share many features with them.

Although some subgiants are simply over-luminous main-sequence stars due to chemical variation or age, others are a distinct evolutionary track towards true giants.

Examples: Bright giants are stars of luminosity class II in the Yerkes spectral classification.

They are the most obvious grouping of stars after the main sequence on most HR diagrams, although white dwarfs are more numerous but far less luminous.

It is thought that some post-AGB stars experiencing a late thermal pulse can become peculiar[clarification needed] blue giants.