Living Under a Dying Star

A red giant over a desert world.

This is an excerpt from my post on Astrobites:

Title:  Habitable Zones of Post-Main Sequence Stars
Authors: Ramses Ramirez and Lisa Kaltenegger
First Author’s Institution: Cornell University
Status: Published in ApJ

In a billion years, Earth will be a desert planet, roasted by an ever brightening Sun.  All life on the surface will be extinct.  Four billion years after that, Earth may be swallowed up by the Sun as it expands into a red giant.  The Goldilocks zone that the Earth currently lives in, where it’s neither too hot nor too cold for liquid water to exist on Earth’ surface, is not a permanent region around the Sun.  Instead, the boundaries of a star’s habitable zone (HZ) evolve as the star does.  As a star in the main sequence (the core hydrogen burning phase) becomes older, it gradually becomes larger and brighter, which pushes the HZ around it farther out.  Once a star begins burning hydrogen in a shell around the core, and then later burning helium, things start changing much more drastically.  It is the HZ at this stage of a star’s life that the authors explored.