Red Giants

Red giants are very bright stars which are typically between 0.25 and 8 solar masses. They have evolved away from the main sequence, with radii between approximately 20 and 100 times that of the Sun (see Figure 1). They inhabit a region of the colour magnitude diagram above and to the right of the main sequence meaning that as well as being brighter than a main sequence star of similar mass, they are also quite red indicating 'surface' temperatures of less than 4,000 - 5,000 K. An example of a nearby and well-known red giant star is Arcturus in the constellation of Bootes. It's thought that this is what our Sun will become in approximately another 5 billion years. Stars more massive than approximately 8 solar masses are thought to undergo a more extreme set of events, becoming supergiants and eventually exploding as supernovae.

 

Figure 1: Artist's impression of a red giant.
Credit: Oona Räisänen (https://commons.wikimedia.org/wiki/File%3ASun_red_giant.svg)