Subgiare ✅

So the next time you look at Procyon or Polaris, take a moment to appreciate the subgiant. It is a star in the middle of its greatest transformation, a stellar butterfly halfway out of its cocoon. And one day, far in the future, our own Sun will enter that phase, marking the beginning of the end for the solar system.

But hydrogen is a finite resource. Once the core turns mostly into helium (which requires higher temperatures to fuse), fusion slows down. Gravity wins the tug-of-war for a moment, and the core contracts. This contraction raises the temperature and pressure in a thin shell around the core, igniting hydrogen fusion there . subgiare

The exact speed at which a star moves through the subgiant phase tells us about its metallicity (the abundance of elements heavier than helium). A star with more metals moves through the subgiant phase faster because the opacity of its outer layers changes. This, in turn, affects whether the star will eventually blow off its envelope to form a planetary nebula or explode as a supernova. So the next time you look at Procyon