Final answer:
The red subgiant stage of a star involves the star expanding, increasing in luminosity, and cooling, which makes the star appear redder as it moves towards the red giant domain on the H-R diagram.
Step-by-step explanation:
The red subgiant stage of a star is characterized by an increase in luminosity and expansion of its outer layers, after leaving the main-sequence phase. This stage is a transition period where a star expands and moves toward the red-giant domain on the Hertzsprung-Russell (H-R) diagram. During this stage, the core contracts while the star's outer layers expand, causing the star to become more luminous and cooler, resulting in the star appearing redder, though more often as orange or orange-red. This marks a brief period of increased brightness before the star enters the red giant phase. Such stars are classified based on their structure and behaviour on an H-R diagram, which plots stars' luminosity against surface temperature.