  When stars depart from the main-sequence, various changes occur including the
loss of angular momentum owing to changes in the stellar interior and the
impact of stellar winds. These processes affect the amount of outer atmospheric
heating and emission as revealed by observations in the UV and X-ray spectral
regimes. From a theoretical perspective, both magnetic and acoustic energy
generation are affected as indicated by detailed theoretical simulations. Here,
I will summarize selected observational and theoretical results, including
recent work for Beta Hydri (G2~IV), a star constituting a prime example and
proxy for the future Sun.
